test: use eslint to fix var->const/let

Manually fix issues that eslint --fix couldn't do automatically.

PR-URL: https://github.com/nodejs/node/pull/10685
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

730 files changed, 3390 insertions, 3371 deletions

diff --git a/test/addons/buffer-free-callback/test.js b/test/addons/buffer-free-callback/test.js
index 8dbc2d5772..d25130a905 100644
--- a/test/addons/buffer-free-callback/test.js
+++ b/test/addons/buffer-free-callback/test.js
@@ -5,8 +5,8 @@ const common = require('../../common');
 const binding = require(`./build/${common.buildType}/binding`);
 
 function check(size, alignment, offset) {
-  var buf = binding.alloc(size, alignment, offset);
-  var slice = buf.slice(size >>> 1);
+  let buf = binding.alloc(size, alignment, offset);
+  let slice = buf.slice(size >>> 1);
 
   buf = null;
   binding.check(slice);
diff --git a/test/addons/load-long-path/test.js b/test/addons/load-long-path/test.js
index ea0783331b..73d85a75a0 100644
--- a/test/addons/load-long-path/test.js
+++ b/test/addons/load-long-path/test.js
@@ -14,9 +14,9 @@ common.refreshTmpDir();
 // make a path that is more than 260 chars long.
 // Any given folder cannot have a name longer than 260 characters,
 // so create 10 nested folders each with 30 character long names.
-var addonDestinationDir = path.resolve(common.tmpDir);
+let addonDestinationDir = path.resolve(common.tmpDir);
 
-for (var i = 0; i < 10; i++) {
+for (let i = 0; i < 10; i++) {
   addonDestinationDir = path.join(addonDestinationDir, 'x'.repeat(30));
   fs.mkdirSync(addonDestinationDir);
 }
@@ -28,7 +28,7 @@ const addonPath = path.join(__dirname,
 const addonDestinationPath = path.join(addonDestinationDir, 'binding.node');
 
 // Copy binary to long path destination
-var contents = fs.readFileSync(addonPath);
+const contents = fs.readFileSync(addonPath);
 fs.writeFileSync(addonDestinationPath, contents);
 
 // Attempt to load at long path destination
diff --git a/test/addons/make-callback-recurse/test.js b/test/addons/make-callback-recurse/test.js
index 0145a142e4..46b1327d7d 100644
--- a/test/addons/make-callback-recurse/test.js
+++ b/test/addons/make-callback-recurse/test.js
@@ -65,7 +65,7 @@ assert.throws(function() {
   results.push(2);
 
   setImmediate(common.mustCall(function() {
-    for (var i = 0; i < results.length; i++) {
+    for (let i = 0; i < results.length; i++) {
       assert.strictEqual(results[i], i,
                          `verifyExecutionOrder(${arg}) results: ${results}`);
     }
diff --git a/test/addons/repl-domain-abort/test.js b/test/addons/repl-domain-abort/test.js
index 1f30f0f9be..95536f9971 100644
--- a/test/addons/repl-domain-abort/test.js
+++ b/test/addons/repl-domain-abort/test.js
@@ -4,27 +4,27 @@ const assert = require('assert');
 const repl = require('repl');
 const stream = require('stream');
 const path = require('path');
-var buildType = process.config.target_defaults.default_configuration;
-var buildPath = path.join(__dirname, 'build', buildType, 'binding');
+const buildType = process.config.target_defaults.default_configuration;
+let buildPath = path.join(__dirname, 'build', buildType, 'binding');
 // On Windows, escape backslashes in the path before passing it to REPL.
 if (common.isWindows)
   buildPath = buildPath.replace(/\\/g, '/');
-var cb_ran = false;
+let cb_ran = false;
 
 process.on('exit', function() {
   assert(cb_ran);
   console.log('ok');
 });
 
-var lines = [
+const lines = [
   // This line shouldn't cause an assertion error.
   'require(\'' + buildPath + '\')' +
   // Log output to double check callback ran.
   '.method(function() { console.log(\'cb_ran\'); });',
 ];
 
-var dInput = new stream.Readable();
-var dOutput = new stream.Writable();
+const dInput = new stream.Readable();
+const dOutput = new stream.Writable();
 
 dInput._read = function _read(size) {
   while (lines.length > 0 && this.push(lines.shift()));
@@ -38,7 +38,7 @@ dOutput._write = function _write(chunk, encoding, cb) {
   cb();
 };
 
-var options = {
+const options = {
   input: dInput,
   output: dOutput,
   terminal: false,
diff --git a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-at-max.js b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-at-max.js
index 2ce695852e..5dd727f8ff 100644
--- a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-at-max.js
+++ b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-at-max.js
@@ -14,8 +14,9 @@ if (!common.enoughTestMem) {
   return;
 }
 
+let buf;
 try {
-  var buf = Buffer.allocUnsafe(kStringMaxLength);
+  buf = Buffer.allocUnsafe(kStringMaxLength);
 } catch (e) {
   // If the exception is not due to memory confinement then rethrow it.
   if (e.message !== 'Array buffer allocation failed') throw (e);
diff --git a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-ascii.js b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-ascii.js
index 4be525d79f..2466c8ec42 100644
--- a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-ascii.js
+++ b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-ascii.js
@@ -14,8 +14,9 @@ if (!common.enoughTestMem) {
 // v8::String::kMaxLength defined in v8.h
 const kStringMaxLength = process.binding('buffer').kStringMaxLength;
 
+let buf;
 try {
-  var buf = Buffer.allocUnsafe(kStringMaxLength + 1);
+  buf = Buffer.allocUnsafe(kStringMaxLength + 1);
 } catch (e) {
   // If the exception is not due to memory confinement then rethrow it.
   if (e.message !== 'Array buffer allocation failed') throw (e);
diff --git a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-base64.js b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-base64.js
index 47426bf635..16f0f5392e 100644
--- a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-base64.js
+++ b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-base64.js
@@ -14,8 +14,9 @@ if (!common.enoughTestMem) {
 // v8::String::kMaxLength defined in v8.h
 const kStringMaxLength = process.binding('buffer').kStringMaxLength;
 
+let buf;
 try {
-  var buf = Buffer.allocUnsafe(kStringMaxLength + 1);
+  buf = Buffer.allocUnsafe(kStringMaxLength + 1);
 } catch (e) {
   // If the exception is not due to memory confinement then rethrow it.
   if (e.message !== 'Array buffer allocation failed') throw (e);
diff --git a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-binary.js b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-binary.js
index 51324a3f33..1028706b1d 100644
--- a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-binary.js
+++ b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-binary.js
@@ -14,8 +14,9 @@ if (!common.enoughTestMem) {
 // v8::String::kMaxLength defined in v8.h
 const kStringMaxLength = process.binding('buffer').kStringMaxLength;
 
+let buf;
 try {
-  var buf = Buffer.allocUnsafe(kStringMaxLength + 1);
+  buf = Buffer.allocUnsafe(kStringMaxLength + 1);
 } catch (e) {
   // If the exception is not due to memory confinement then rethrow it.
   if (e.message !== 'Array buffer allocation failed') throw (e);
@@ -33,7 +34,7 @@ assert.throws(function() {
   buf.toString('latin1');
 }, /"toString\(\)" failed/);
 
-var maxString = buf.toString('latin1', 1);
+let maxString = buf.toString('latin1', 1);
 assert.strictEqual(maxString.length, kStringMaxLength);
 // Free the memory early instead of at the end of the next assignment
 maxString = undefined;
diff --git a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-hex.js b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-hex.js
index ae6966d054..caf6112011 100644
--- a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-hex.js
+++ b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-hex.js
@@ -14,8 +14,9 @@ if (!common.enoughTestMem) {
 // v8::String::kMaxLength defined in v8.h
 const kStringMaxLength = process.binding('buffer').kStringMaxLength;
 
+let buf;
 try {
-  var buf = Buffer.allocUnsafe(kStringMaxLength + 1);
+  buf = Buffer.allocUnsafe(kStringMaxLength + 1);
 } catch (e) {
   // If the exception is not due to memory confinement then rethrow it.
   if (e.message !== 'Array buffer allocation failed') throw (e);
diff --git a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-utf8.js b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-utf8.js
index cea175aa93..4d3199a033 100644
--- a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-utf8.js
+++ b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-1-utf8.js
@@ -14,8 +14,9 @@ if (!common.enoughTestMem) {
 // v8::String::kMaxLength defined in v8.h
 const kStringMaxLength = process.binding('buffer').kStringMaxLength;
 
+let buf;
 try {
-  var buf = Buffer.allocUnsafe(kStringMaxLength + 1);
+  buf = Buffer.allocUnsafe(kStringMaxLength + 1);
 } catch (e) {
   // If the exception is not due to memory confinement then rethrow it.
   if (e.message !== 'Array buffer allocation failed') throw (e);
diff --git a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-2.js b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-2.js
index 0fab606e46..786d0b6798 100644
--- a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-2.js
+++ b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-2.js
@@ -14,8 +14,9 @@ if (!common.enoughTestMem) {
 // v8::String::kMaxLength defined in v8.h
 const kStringMaxLength = process.binding('buffer').kStringMaxLength;
 
+let buf;
 try {
-  var buf = Buffer.allocUnsafe(kStringMaxLength + 2);
+  buf = Buffer.allocUnsafe(kStringMaxLength + 2);
 } catch (e) {
   // If the exception is not due to memory confinement then rethrow it.
   if (e.message !== 'Array buffer allocation failed') throw (e);
diff --git a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max.js b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max.js
index f984179525..36b0d4174a 100644
--- a/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max.js
+++ b/test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max.js
@@ -14,8 +14,9 @@ if (!common.enoughTestMem) {
 // v8::String::kMaxLength defined in v8.h
 const kStringMaxLength = process.binding('buffer').kStringMaxLength;
 
+let buf;
 try {
-  var buf = Buffer.allocUnsafe(kStringMaxLength * 2 + 2);
+  buf = Buffer.allocUnsafe(kStringMaxLength * 2 + 2);
 } catch (e) {
   // If the exception is not due to memory confinement then rethrow it.
   if (e.message !== 'Array buffer allocation failed') throw (e);
diff --git a/test/common.js b/test/common.js
index 75fcc58379..699fa5991d 100644
--- a/test/common.js
+++ b/test/common.js
@@ -212,7 +212,7 @@ exports.hasIPv6 = Object.keys(ifaces).some(function(name) {
  * the process aborts.
  */
 exports.childShouldThrowAndAbort = function() {
-  var testCmd = '';
+  let testCmd = '';
   if (!exports.isWindows) {
     // Do not create core files, as it can take a lot of disk space on
     // continuous testing and developers' machines
diff --git a/test/debugger/helper-debugger-repl.js b/test/debugger/helper-debugger-repl.js
index d6c3e5ca9e..38c65a03fa 100644
--- a/test/debugger/helper-debugger-repl.js
+++ b/test/debugger/helper-debugger-repl.js
@@ -4,12 +4,12 @@ const assert = require('assert');
 const spawn = require('child_process').spawn;
 
 process.env.NODE_DEBUGGER_TIMEOUT = 2000;
-var port = common.PORT;
+const port = common.PORT;
 
-var child;
-var buffer = '';
-var expected = [];
-var quit;
+let child;
+let buffer = '';
+const expected = [];
+let quit;
 
 function startDebugger(scriptToDebug) {
   scriptToDebug = process.env.NODE_DEBUGGER_TEST_SCRIPT ||
@@ -34,23 +34,23 @@ function startDebugger(scriptToDebug) {
     console.log(line);
     assert.ok(expected.length > 0, 'Got unexpected line: ' + line);
 
-    var expectedLine = expected[0].lines.shift();
+    const expectedLine = expected[0].lines.shift();
     assert.ok(line.match(expectedLine) !== null, line + ' != ' + expectedLine);
 
     if (expected[0].lines.length === 0) {
-      var callback = expected[0].callback;
+      const callback = expected[0].callback;
       expected.shift();
       callback && callback();
     }
   });
 
-  var childClosed = false;
+  let childClosed = false;
   child.on('close', function(code) {
     assert(!code);
     childClosed = true;
   });
 
-  var quitCalled = false;
+  let quitCalled = false;
   quit = function() {
     if (quitCalled || childClosed) return;
     quitCalled = true;
@@ -60,7 +60,7 @@ function startDebugger(scriptToDebug) {
 
   setTimeout(function() {
     console.error('dying badly buffer=%j', buffer);
-    var err = 'Timeout';
+    let err = 'Timeout';
     if (expected.length > 0 && expected[0].lines) {
       err = err + '. Expected: ' + expected[0].lines.shift();
     }
@@ -95,7 +95,7 @@ function addTest(input, output) {
       child.stdin.write(expected[0].input + '\n');
 
       if (!expected[0].lines) {
-        var callback = expected[0].callback;
+        const callback = expected[0].callback;
         expected.shift();
 
         callback && callback();
@@ -107,17 +107,17 @@ function addTest(input, output) {
   expected.push({input: input, lines: output, callback: next});
 }
 
-var handshakeLines = [
+const handshakeLines = [
   /listening on /,
   /connecting.* ok/
 ];
 
-var initialBreakLines = [
+const initialBreakLines = [
   /break in .*:1/,
   /1/, /2/, /3/
 ];
 
-var initialLines = handshakeLines.concat(initialBreakLines);
+const initialLines = handshakeLines.concat(initialBreakLines);
 
 // Process initial lines
 addTest(null, initialLines);
diff --git a/test/debugger/test-debugger-repl-restart.js b/test/debugger/test-debugger-repl-restart.js
index 7dbef4288a..f5de9f5ee9 100644
--- a/test/debugger/test-debugger-repl-restart.js
+++ b/test/debugger/test-debugger-repl-restart.js
@@ -3,13 +3,13 @@ require('../common');
 const repl = require('./helper-debugger-repl.js');
 
 repl.startDebugger('breakpoints.js');
-var linesWithBreakpoint = [
+const linesWithBreakpoint = [
   /1/, /2/, /3/, /4/, /5/, /\* 6/
 ];
 // We slice here, because addTest will change the given array.
 repl.addTest('sb(6)', linesWithBreakpoint.slice());
 
-var initialLines = repl.initialLines.slice();
+const initialLines = repl.initialLines.slice();
 initialLines.splice(2, 0, /Restoring/, /Warning/);
 
 // Restart the debugged script
diff --git a/test/debugger/test-debugger-repl-term.js b/test/debugger/test-debugger-repl-term.js
index 8c44372f68..4932df12e7 100644
--- a/test/debugger/test-debugger-repl-term.js
+++ b/test/debugger/test-debugger-repl-term.js
@@ -6,7 +6,7 @@ const repl = require('./helper-debugger-repl.js');
 
 repl.startDebugger('breakpoints.js');
 
-var addTest = repl.addTest;
+const addTest = repl.addTest;
 
 // next
 addTest('n', [
diff --git a/test/debugger/test-debugger-repl-utf8.js b/test/debugger/test-debugger-repl-utf8.js
index 1af75840f5..9021415888 100644
--- a/test/debugger/test-debugger-repl-utf8.js
+++ b/test/debugger/test-debugger-repl-utf8.js
@@ -1,6 +1,6 @@
 'use strict';
 const common = require('../common');
-var script = common.fixturesDir + '/breakpoints_utf8.js';
+const script = common.fixturesDir + '/breakpoints_utf8.js';
 process.env.NODE_DEBUGGER_TEST_SCRIPT = script;
 
 require('./test-debugger-repl.js');
diff --git a/test/debugger/test-debugger-repl.js b/test/debugger/test-debugger-repl.js
index 0725348e56..32dbfa22c0 100644
--- a/test/debugger/test-debugger-repl.js
+++ b/test/debugger/test-debugger-repl.js
@@ -4,7 +4,7 @@ const repl = require('./helper-debugger-repl.js');
 
 repl.startDebugger('breakpoints.js');
 
-var addTest = repl.addTest;
+const addTest = repl.addTest;
 
 // Next
 addTest('n', [
diff --git a/test/gc/test-http-client-connaborted.js b/test/gc/test-http-client-connaborted.js
index 071baf738e..6ff2a282d8 100644
--- a/test/gc/test-http-client-connaborted.js
+++ b/test/gc/test-http-client-connaborted.js
@@ -18,7 +18,7 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var server = http.createServer(serverHandler);
+const server = http.createServer(serverHandler);
 server.listen(0, getall);
 
 function getall() {
@@ -31,7 +31,7 @@ function getall() {
       statusLater();
     }
 
-    var req = http.get({
+    const req = http.get({
       hostname: 'localhost',
       pathname: '/',
       port: server.address().port
@@ -44,14 +44,14 @@ function getall() {
   setImmediate(getall);
 }
 
-for (var i = 0; i < 10; i++)
+for (let i = 0; i < 10; i++)
   getall();
 
 function afterGC() {
   countGC++;
 }
 
-var timer;
+let timer;
 function statusLater() {
   global.gc();
   if (timer) clearTimeout(timer);
diff --git a/test/gc/test-http-client-onerror.js b/test/gc/test-http-client-onerror.js
index a2ef0fa4be..b17ab428a2 100644
--- a/test/gc/test-http-client-onerror.js
+++ b/test/gc/test-http-client-onerror.js
@@ -20,7 +20,7 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var server = http.createServer(serverHandler);
+const server = http.createServer(serverHandler);
 server.listen(0, runTest);
 
 function getall() {
@@ -37,7 +37,7 @@ function getall() {
       throw er;
     }
 
-    var req = http.get({
+    const req = http.get({
       hostname: 'localhost',
       pathname: '/',
       port: server.address().port
@@ -51,7 +51,7 @@ function getall() {
 }
 
 function runTest() {
-  for (var i = 0; i < 10; i++)
+  for (let i = 0; i < 10; i++)
     getall();
 }
 
@@ -59,7 +59,7 @@ function afterGC() {
   countGC++;
 }
 
-var timer;
+let timer;
 function statusLater() {
   global.gc();
   if (timer) clearTimeout(timer);
diff --git a/test/gc/test-http-client-timeout.js b/test/gc/test-http-client-timeout.js
index 27baf837ca..fb4e8ba0f7 100644
--- a/test/gc/test-http-client-timeout.js
+++ b/test/gc/test-http-client-timeout.js
@@ -22,7 +22,7 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var server = http.createServer(serverHandler);
+const server = http.createServer(serverHandler);
 server.listen(0, getall);
 
 function getall() {
@@ -36,7 +36,7 @@ function getall() {
       statusLater();
     }
 
-    var req = http.get({
+    const req = http.get({
       hostname: 'localhost',
       pathname: '/',
       port: server.address().port
@@ -53,14 +53,14 @@ function getall() {
   setImmediate(getall);
 }
 
-for (var i = 0; i < 10; i++)
+for (let i = 0; i < 10; i++)
   getall();
 
 function afterGC() {
   countGC++;
 }
 
-var timer;
+let timer;
 function statusLater() {
   global.gc();
   if (timer) clearTimeout(timer);
diff --git a/test/gc/test-http-client.js b/test/gc/test-http-client.js
index 06bd08b265..f81cab0b09 100644
--- a/test/gc/test-http-client.js
+++ b/test/gc/test-http-client.js
@@ -18,7 +18,7 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var server = http.createServer(serverHandler);
+const server = http.createServer(serverHandler);
 server.listen(0, getall);
 
 
@@ -34,7 +34,7 @@ function getall() {
       res.on('end', global.gc);
     }
 
-    var req = http.get({
+    const req = http.get({
       hostname: 'localhost',
       pathname: '/',
       port: server.address().port
@@ -47,7 +47,7 @@ function getall() {
   setImmediate(getall);
 }
 
-for (var i = 0; i < 10; i++)
+for (let i = 0; i < 10; i++)
   getall();
 
 function afterGC() {
diff --git a/test/gc/test-net-timeout.js b/test/gc/test-net-timeout.js
index 2d9bc134a3..c797bee41c 100644
--- a/test/gc/test-net-timeout.js
+++ b/test/gc/test-net-timeout.js
@@ -7,14 +7,13 @@ require('../common');
 function serverHandler(sock) {
   sock.setTimeout(120000);
   sock.resume();
-  var timer;
   sock.on('close', function() {
     clearTimeout(timer);
   });
   sock.on('error', function(err) {
     assert.strictEqual(err.code, 'ECONNRESET');
   });
-  timer = setTimeout(function() {
+  const timer = setTimeout(function() {
     sock.end('hello\n');
   }, 100);
 }
@@ -29,7 +28,7 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var server = net.createServer(serverHandler);
+const server = net.createServer(serverHandler);
 server.listen(0, getall);
 
 function getall() {
@@ -50,7 +49,7 @@ function getall() {
   setImmediate(getall);
 }
 
-for (var i = 0; i < 10; i++)
+for (let i = 0; i < 10; i++)
   getall();
 
 function afterGC() {
diff --git a/test/inspector/inspector-helper.js b/test/inspector/inspector-helper.js
index 105d9b358b..9db9613184 100644
--- a/test/inspector/inspector-helper.js
+++ b/test/inspector/inspector-helper.js
@@ -110,7 +110,7 @@ function makeBufferingDataCallback(dataCallback) {
       buffer = Buffer.alloc(0);
     else
       buffer = Buffer.from(lines.pop(), 'utf8');
-    for (var line of lines)
+    for (const line of lines)
       dataCallback(line);
   };
 }
@@ -222,7 +222,7 @@ TestSession.prototype.sendInspectorCommands = function(commands) {
 };
 
 TestSession.prototype.createCallbackWithTimeout_ = function(message) {
-  var promise = new Promise((resolve) => {
+  const promise = new Promise((resolve) => {
     this.enqueue((callback) => {
       const timeoutId = timeout(message);
       resolve(() => {
diff --git a/test/internet/test-dgram-multicast-multi-process.js b/test/internet/test-dgram-multicast-multi-process.js
index de6bf874ed..46f47a5982 100644
--- a/test/internet/test-dgram-multicast-multi-process.js
+++ b/test/internet/test-dgram-multicast-multi-process.js
@@ -13,6 +13,7 @@ const messages = [
 ];
 const workers = {};
 const listeners = 3;
+let listening, sendSocket, done, timer, dead;
 
 
 // Skip test in FreeBSD jails.
@@ -76,10 +77,10 @@ function launchChildProcess(index) {
         Object.keys(workers).forEach(function(pid) {
           const worker = workers[pid];
 
-          var count = 0;
+          let count = 0;
 
           worker.messagesReceived.forEach(function(buf) {
-            for (var i = 0; i < messages.length; ++i) {
+            for (let i = 0; i < messages.length; ++i) {
               if (buf.toString() === messages[i].toString()) {
                 count++;
                 break;
@@ -110,13 +111,13 @@ function killChildren(children) {
 }
 
 if (process.argv[2] !== 'child') {
-  var listening = 0;
-  var dead = 0;
-  var i = 0;
-  var done = 0;
+  listening = 0;
+  dead = 0;
+  let i = 0;
+  done = 0;
 
   // Exit the test if it doesn't succeed within TIMEOUT.
-  var timer = setTimeout(function() {
+  timer = setTimeout(function() {
     console.error('[PARENT] Responses were not received within %d ms.',
                   TIMEOUT);
     console.error('[PARENT] Fail');
@@ -127,11 +128,11 @@ if (process.argv[2] !== 'child') {
   }, TIMEOUT);
 
   // Launch child processes.
-  for (var x = 0; x < listeners; x++) {
+  for (let x = 0; x < listeners; x++) {
     launchChildProcess(x);
   }
 
-  var sendSocket = dgram.createSocket('udp4');
+  sendSocket = dgram.createSocket('udp4');
 
   // The socket is actually created async now.
   sendSocket.on('listening', function() {
diff --git a/test/internet/test-dgram-send-cb-quelches-error.js b/test/internet/test-dgram-send-cb-quelches-error.js
index e20c8e1070..abcc36859b 100644
--- a/test/internet/test-dgram-send-cb-quelches-error.js
+++ b/test/internet/test-dgram-send-cb-quelches-error.js
@@ -1,12 +1,12 @@
 'use strict';
 const common = require('../common');
-var mustCall = common.mustCall;
+const mustCall = common.mustCall;
 const assert = require('assert');
 const dgram = require('dgram');
 const dns = require('dns');
 
-var socket = dgram.createSocket('udp4');
-var buffer = Buffer.from('gary busey');
+const socket = dgram.createSocket('udp4');
+const buffer = Buffer.from('gary busey');
 
 dns.setServers([]);
 
diff --git a/test/internet/test-dns-cares-domains.js b/test/internet/test-dns-cares-domains.js
index 61ac679274..5328d7e3d9 100644
--- a/test/internet/test-dns-cares-domains.js
+++ b/test/internet/test-dns-cares-domains.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const dns = require('dns');
 const domain = require('domain');
 
-var methods = [
+const methods = [
   'resolve4',
   'resolve6',
   'resolveCname',
@@ -18,7 +18,7 @@ var methods = [
 ];
 
 methods.forEach(function(method) {
-  var d = domain.create();
+  const d = domain.create();
   d.run(function() {
     dns[method]('google.com', function() {
       assert.strictEqual(process.domain, d, method + ' retains domain');
diff --git a/test/internet/test-dns.js b/test/internet/test-dns.js
index be3f9b0469..8a857c030a 100644
--- a/test/internet/test-dns.js
+++ b/test/internet/test-dns.js
@@ -15,7 +15,7 @@ const queue = [];
 
 function TEST(f) {
   function next() {
-    var f = queue.shift();
+    const f = queue.shift();
     if (f) {
       running = true;
       console.log(f.name);
@@ -44,7 +44,7 @@ function checkWrap(req) {
 
 
 TEST(function test_reverse_bogus(done) {
-  var error;
+  let error;
 
   try {
     dns.reverse('bogus ip', function() {
@@ -61,12 +61,12 @@ TEST(function test_reverse_bogus(done) {
 });
 
 TEST(function test_resolve4_ttl(done) {
-  var req = dns.resolve4('google.com', { ttl: true }, function(err, result) {
+  const req = dns.resolve4('google.com', { ttl: true }, function(err, result) {
     assert.ifError(err);
     assert.ok(result.length > 0);
 
-    for (var i = 0; i < result.length; i++) {
-      var item = result[i];
+    for (let i = 0; i < result.length; i++) {
+      const item = result[i];
       assert.ok(item);
       assert.strictEqual(typeof item, 'object');
       assert.strictEqual(typeof item.ttl, 'number');
@@ -82,12 +82,12 @@ TEST(function test_resolve4_ttl(done) {
 });
 
 TEST(function test_resolve6_ttl(done) {
-  var req = dns.resolve6('google.com', { ttl: true }, function(err, result) {
+  const req = dns.resolve6('google.com', { ttl: true }, function(err, result) {
     assert.ifError(err);
     assert.ok(result.length > 0);
 
-    for (var i = 0; i < result.length; i++) {
-      var item = result[i];
+    for (let i = 0; i < result.length; i++) {
+      const item = result[i];
       assert.ok(item);
       assert.strictEqual(typeof item, 'object');
       assert.strictEqual(typeof item.ttl, 'number');
@@ -103,12 +103,12 @@ TEST(function test_resolve6_ttl(done) {
 });
 
 TEST(function test_resolveMx(done) {
-  var req = dns.resolveMx('gmail.com', function(err, result) {
+  const req = dns.resolveMx('gmail.com', function(err, result) {
     assert.ifError(err);
     assert.ok(result.length > 0);
 
-    for (var i = 0; i < result.length; i++) {
-      var item = result[i];
+    for (let i = 0; i < result.length; i++) {
+      const item = result[i];
       assert.ok(item);
       assert.strictEqual(typeof item, 'object');
 
@@ -125,7 +125,7 @@ TEST(function test_resolveMx(done) {
 });
 
 TEST(function test_resolveMx_failure(done) {
-  var req = dns.resolveMx('something.invalid', function(err, result) {
+  const req = dns.resolveMx('something.invalid', function(err, result) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, 'ENOTFOUND');
 
@@ -138,12 +138,12 @@ TEST(function test_resolveMx_failure(done) {
 });
 
 TEST(function test_resolveNs(done) {
-  var req = dns.resolveNs('rackspace.com', function(err, names) {
+  const req = dns.resolveNs('rackspace.com', function(err, names) {
     assert.ifError(err);
     assert.ok(names.length > 0);
 
-    for (var i = 0; i < names.length; i++) {
-      var name = names[i];
+    for (let i = 0; i < names.length; i++) {
+      const name = names[i];
       assert.ok(name);
       assert.strictEqual(typeof name, 'string');
     }
@@ -155,7 +155,7 @@ TEST(function test_resolveNs(done) {
 });
 
 TEST(function test_resolveNs_failure(done) {
-  var req = dns.resolveNs('something.invalid', function(err, result) {
+  const req = dns.resolveNs('something.invalid', function(err, result) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, 'ENOTFOUND');
 
@@ -168,12 +168,12 @@ TEST(function test_resolveNs_failure(done) {
 });
 
 TEST(function test_resolveSrv(done) {
-  var req = dns.resolveSrv('_jabber._tcp.google.com', function(err, result) {
+  const req = dns.resolveSrv('_jabber._tcp.google.com', function(err, result) {
     assert.ifError(err);
     assert.ok(result.length > 0);
 
-    for (var i = 0; i < result.length; i++) {
-      var item = result[i];
+    for (let i = 0; i < result.length; i++) {
+      const item = result[i];
       assert.ok(item);
       assert.strictEqual(typeof item, 'object');
 
@@ -192,7 +192,7 @@ TEST(function test_resolveSrv(done) {
 });
 
 TEST(function test_resolveSrv_failure(done) {
-  var req = dns.resolveSrv('something.invalid', function(err, result) {
+  const req = dns.resolveSrv('something.invalid', function(err, result) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, 'ENOTFOUND');
 
@@ -205,12 +205,12 @@ TEST(function test_resolveSrv_failure(done) {
 });
 
 TEST(function test_resolvePtr(done) {
-  var req = dns.resolvePtr('8.8.8.8.in-addr.arpa', function(err, result) {
+  const req = dns.resolvePtr('8.8.8.8.in-addr.arpa', function(err, result) {
     assert.ifError(err);
     assert.ok(result.length > 0);
 
-    for (var i = 0; i < result.length; i++) {
-      var item = result[i];
+    for (let i = 0; i < result.length; i++) {
+      const item = result[i];
       assert.ok(item);
       assert.strictEqual(typeof item, 'string');
     }
@@ -222,7 +222,7 @@ TEST(function test_resolvePtr(done) {
 });
 
 TEST(function test_resolvePtr_failure(done) {
-  var req = dns.resolvePtr('something.invalid', function(err, result) {
+  const req = dns.resolvePtr('something.invalid', function(err, result) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, 'ENOTFOUND');
 
@@ -235,12 +235,12 @@ TEST(function test_resolvePtr_failure(done) {
 });
 
 TEST(function test_resolveNaptr(done) {
-  var req = dns.resolveNaptr('sip2sip.info', function(err, result) {
+  const req = dns.resolveNaptr('sip2sip.info', function(err, result) {
     assert.ifError(err);
     assert.ok(result.length > 0);
 
-    for (var i = 0; i < result.length; i++) {
-      var item = result[i];
+    for (let i = 0; i < result.length; i++) {
+      const item = result[i];
       assert.ok(item);
       assert.strictEqual(typeof item, 'object');
 
@@ -259,7 +259,7 @@ TEST(function test_resolveNaptr(done) {
 });
 
 TEST(function test_resolveNaptr_failure(done) {
-  var req = dns.resolveNaptr('something.invalid', function(err, result) {
+  const req = dns.resolveNaptr('something.invalid', function(err, result) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, 'ENOTFOUND');
 
@@ -272,7 +272,7 @@ TEST(function test_resolveNaptr_failure(done) {
 });
 
 TEST(function test_resolveSoa(done) {
-  var req = dns.resolveSoa('nodejs.org', function(err, result) {
+  const req = dns.resolveSoa('nodejs.org', function(err, result) {
     assert.ifError(err);
     assert.ok(result);
     assert.strictEqual(typeof result, 'object');
@@ -305,7 +305,7 @@ TEST(function test_resolveSoa(done) {
 });
 
 TEST(function test_resolveSoa_failure(done) {
-  var req = dns.resolveSoa('something.invalid', function(err, result) {
+  const req = dns.resolveSoa('something.invalid', function(err, result) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, 'ENOTFOUND');
 
@@ -318,12 +318,12 @@ TEST(function test_resolveSoa_failure(done) {
 });
 
 TEST(function test_resolveCname(done) {
-  var req = dns.resolveCname('www.microsoft.com', function(err, names) {
+  const req = dns.resolveCname('www.microsoft.com', function(err, names) {
     assert.ifError(err);
     assert.ok(names.length > 0);
 
-    for (var i = 0; i < names.length; i++) {
-      var name = names[i];
+    for (let i = 0; i < names.length; i++) {
+      const name = names[i];
       assert.ok(name);
       assert.strictEqual(typeof name, 'string');
     }
@@ -335,7 +335,7 @@ TEST(function test_resolveCname(done) {
 });
 
 TEST(function test_resolveCname_failure(done) {
-  var req = dns.resolveCname('something.invalid', function(err, result) {
+  const req = dns.resolveCname('something.invalid', function(err, result) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, 'ENOTFOUND');
 
@@ -349,7 +349,7 @@ TEST(function test_resolveCname_failure(done) {
 
 
 TEST(function test_resolveTxt(done) {
-  var req = dns.resolveTxt('google.com', function(err, records) {
+  const req = dns.resolveTxt('google.com', function(err, records) {
     assert.ifError(err);
     assert.strictEqual(records.length, 1);
     assert.ok(util.isArray(records[0]));
@@ -361,7 +361,7 @@ TEST(function test_resolveTxt(done) {
 });
 
 TEST(function test_resolveTxt_failure(done) {
-  var req = dns.resolveTxt('something.invalid', function(err, result) {
+  const req = dns.resolveTxt('something.invalid', function(err, result) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, 'ENOTFOUND');
 
@@ -375,7 +375,7 @@ TEST(function test_resolveTxt_failure(done) {
 
 
 TEST(function test_lookup_failure(done) {
-  var req = dns.lookup('does.not.exist', 4, function(err, ip, family) {
+  const req = dns.lookup('does.not.exist', 4, function(err, ip, family) {
     assert.ok(err instanceof Error);
     assert.strictEqual(err.errno, dns.NOTFOUND);
     assert.strictEqual(err.errno, 'ENOTFOUND');
@@ -390,7 +390,7 @@ TEST(function test_lookup_failure(done) {
 
 
 TEST(function test_lookup_null(done) {
-  var req = dns.lookup(null, function(err, ip, family) {
+  const req = dns.lookup(null, function(err, ip, family) {
     assert.ifError(err);
     assert.strictEqual(ip, null);
     assert.strictEqual(family, 4);
@@ -403,7 +403,7 @@ TEST(function test_lookup_null(done) {
 
 
 TEST(function test_lookup_ip_all(done) {
-  var req = dns.lookup('127.0.0.1', {all: true}, function(err, ips, family) {
+  const req = dns.lookup('127.0.0.1', {all: true}, function(err, ips, family) {
     assert.ifError(err);
     assert.ok(Array.isArray(ips));
     assert.ok(ips.length > 0);
@@ -418,7 +418,7 @@ TEST(function test_lookup_ip_all(done) {
 
 
 TEST(function test_lookup_null_all(done) {
-  var req = dns.lookup(null, {all: true}, function(err, ips, family) {
+  const req = dns.lookup(null, {all: true}, function(err, ips, family) {
     assert.ifError(err);
     assert.ok(Array.isArray(ips));
     assert.strictEqual(ips.length, 0);
@@ -431,7 +431,7 @@ TEST(function test_lookup_null_all(done) {
 
 
 TEST(function test_lookup_all_mixed(done) {
-  var req = dns.lookup('www.google.com', {all: true}, function(err, ips) {
+  const req = dns.lookup('www.google.com', {all: true}, function(err, ips) {
     assert.ifError(err);
     assert.ok(Array.isArray(ips));
     assert.ok(ips.length > 0);
@@ -453,7 +453,7 @@ TEST(function test_lookup_all_mixed(done) {
 
 
 TEST(function test_lookupservice_invalid(done) {
-  var req = dns.lookupService('1.2.3.4', 80, function(err, host, service) {
+  const req = dns.lookupService('1.2.3.4', 80, function(err, host, service) {
     assert(err instanceof Error);
     assert.strictEqual(err.code, 'ENOTFOUND');
     assert.ok(/1\.2\.3\.4/.test(err.message));
@@ -466,7 +466,7 @@ TEST(function test_lookupservice_invalid(done) {
 
 
 TEST(function test_reverse_failure(done) {
-  var req = dns.reverse('0.0.0.0', function(err) {
+  const req = dns.reverse('0.0.0.0', function(err) {
     assert(err instanceof Error);
     assert.strictEqual(err.code, 'ENOTFOUND');  // Silly error code...
     assert.strictEqual(err.hostname, '0.0.0.0');
@@ -480,7 +480,7 @@ TEST(function test_reverse_failure(done) {
 
 
 TEST(function test_lookup_failure(done) {
-  var req = dns.lookup('nosuchhostimsure', function(err) {
+  const req = dns.lookup('nosuchhostimsure', function(err) {
     assert(err instanceof Error);
     assert.strictEqual(err.code, 'ENOTFOUND');  // Silly error code...
     assert.strictEqual(err.hostname, 'nosuchhostimsure');
@@ -494,7 +494,7 @@ TEST(function test_lookup_failure(done) {
 
 
 TEST(function test_resolve_failure(done) {
-  var req = dns.resolve4('nosuchhostimsure', function(err) {
+  const req = dns.resolve4('nosuchhostimsure', function(err) {
     assert(err instanceof Error);
 
     switch (err.code) {
@@ -516,12 +516,12 @@ TEST(function test_resolve_failure(done) {
 });
 
 
-var getaddrinfoCallbackCalled = false;
+let getaddrinfoCallbackCalled = false;
 
 console.log('looking up nodejs.org...');
 
-var cares = process.binding('cares_wrap');
-var req = new cares.GetAddrInfoReqWrap();
+const cares = process.binding('cares_wrap');
+const req = new cares.GetAddrInfoReqWrap();
 cares.getaddrinfo(req, 'nodejs.org', 4);
 
 req.oncomplete = function(err, domains) {
diff --git a/test/internet/test-net-connect-timeout.js b/test/internet/test-net-connect-timeout.js
index e379f41467..95e25614ff 100644
--- a/test/internet/test-net-connect-timeout.js
+++ b/test/internet/test-net-connect-timeout.js
@@ -7,21 +7,21 @@ const common = require('../common');
 const net = require('net');
 const assert = require('assert');
 
-var start = new Date();
+const start = new Date();
 
-var T = 100;
+const T = 100;
 
 // 192.0.2.1 is part of subnet assigned as "TEST-NET" in RFC 5737.
 // For use solely in documentation and example source code.
 // In short, it should be unreachable.
 // In practice, it's a network black hole.
-var socket = net.createConnection(9999, '192.0.2.1');
+const socket = net.createConnection(9999, '192.0.2.1');
 
 socket.setTimeout(T);
 
 socket.on('timeout', common.mustCall(function() {
   console.error('timeout');
-  var now = new Date();
+  const now = new Date();
   assert.ok(now - start < T + 500);
   socket.destroy();
 }));
diff --git a/test/internet/test-net-connect-unref.js b/test/internet/test-net-connect-unref.js
index fa45cc711e..f3be840984 100644
--- a/test/internet/test-net-connect-unref.js
+++ b/test/internet/test-net-connect-unref.js
@@ -2,10 +2,9 @@
 const common = require('../common');
 const net = require('net');
 
-var client;
-var TIMEOUT = 10 * 1000;
+const TIMEOUT = 10 * 1000;
 
-client = net.createConnection(53, '8.8.8.8', function() {
+const client = net.createConnection(53, '8.8.8.8', function() {
   client.unref();
 });
 
diff --git a/test/internet/test-tls-connnect-melissadata.js b/test/internet/test-tls-connnect-melissadata.js
index 3c6c205d6e..f57b897099 100644
--- a/test/internet/test-tls-connnect-melissadata.js
+++ b/test/internet/test-tls-connnect-melissadata.js
@@ -9,7 +9,7 @@ if (!common.hasCrypto) {
 }
 
 const tls = require('tls');
-var socket = tls.connect(443, 'address.melissadata.net', function() {
+const socket = tls.connect(443, 'address.melissadata.net', function() {
   socket.resume();
   socket.destroy();
 });
diff --git a/test/known_issues/test-url-parse-conformance.js b/test/known_issues/test-url-parse-conformance.js
index d70a322d6b..f7fcec821c 100644
--- a/test/known_issues/test-url-parse-conformance.js
+++ b/test/known_issues/test-url-parse-conformance.js
@@ -9,14 +9,14 @@ const path = require('path');
 
 const tests = require(path.join(common.fixturesDir, 'url-tests.json'));
 
-var failed = 0;
-var attempted = 0;
+let failed = 0;
+let attempted = 0;
 
 tests.forEach((test) => {
   attempted++;
   // Skip comments
   if (typeof test === 'string') return;
-  var parsed;
+  let parsed;
 
   try {
     // Attempt to parse
@@ -28,7 +28,7 @@ tests.forEach((test) => {
     } else {
       // Test was not supposed to fail, so we're good so far. Now
       // check the results of the parse.
-      var username, password;
+      let username, password;
       try {
         assert.strictEqual(test.href, parsed.href);
         assert.strictEqual(test.protocol, parsed.protocol);
diff --git a/test/message/eval_messages.js b/test/message/eval_messages.js
index 188311851c..d50df9081b 100644
--- a/test/message/eval_messages.js
+++ b/test/message/eval_messages.js
@@ -5,16 +5,16 @@ require('../common');
 const spawn = require('child_process').spawn;
 
 function run(cmd, strict, cb) {
-  var args = [];
+  const args = [];
   if (strict) args.push('--use_strict');
   args.push('-pe', cmd);
-  var child = spawn(process.execPath, args);
+  const child = spawn(process.execPath, args);
   child.stdout.pipe(process.stdout);
   child.stderr.pipe(process.stdout);
   child.on('close', cb);
 }
 
-var queue =
+const queue =
   [ 'with(this){__filename}',
     '42',
     'throw new Error("hello")',
@@ -22,7 +22,7 @@ var queue =
     'var ______________________________________________; throw 10' ];
 
 function go() {
-  var c = queue.shift();
+  const c = queue.shift();
   if (!c) return console.log('done');
   run(c, false, function() {
     run(c, true, go);
diff --git a/test/message/max_tick_depth.js b/test/message/max_tick_depth.js
index 1d2ec175c5..c433c0bab3 100644
--- a/test/message/max_tick_depth.js
+++ b/test/message/max_tick_depth.js
@@ -2,7 +2,7 @@
 require('../common');
 
 process.maxTickDepth = 10;
-var i = 20;
+let i = 20;
 process.nextTick(function f() {
   console.error('tick %d', i);
   if (i-- > 0)
diff --git a/test/message/stdin_messages.js b/test/message/stdin_messages.js
index 4759e2b0d5..b32004c329 100644
--- a/test/message/stdin_messages.js
+++ b/test/message/stdin_messages.js
@@ -5,17 +5,17 @@ require('../common');
 const spawn = require('child_process').spawn;
 
 function run(cmd, strict, cb) {
-  var args = [];
+  const args = [];
   if (strict) args.push('--use_strict');
   args.push('-p');
-  var child = spawn(process.execPath, args);
+  const child = spawn(process.execPath, args);
   child.stdout.pipe(process.stdout);
   child.stderr.pipe(process.stdout);
   child.stdin.end(cmd);
   child.on('close', cb);
 }
 
-var queue =
+const queue =
   [ 'with(this){__filename}',
     '42',
     'throw new Error("hello")',
@@ -23,7 +23,7 @@ var queue =
     'var ______________________________________________; throw 10' ];
 
 function go() {
-  var c = queue.shift();
+  const c = queue.shift();
   if (!c) return console.log('done');
   run(c, false, function() {
     run(c, true, go);
diff --git a/test/parallel/test-assert-typedarray-deepequal.js b/test/parallel/test-assert-typedarray-deepequal.js
index 2e6c88848a..f5ecb274a8 100644
--- a/test/parallel/test-assert-typedarray-deepequal.js
+++ b/test/parallel/test-assert-typedarray-deepequal.js
@@ -5,7 +5,7 @@ const assert = require('assert');
 const a = require('assert');
 
 function makeBlock(f) {
-  var args = Array.prototype.slice.call(arguments, 1);
+  const args = Array.prototype.slice.call(arguments, 1);
   return function() {
     return f.apply(this, args);
   };
diff --git a/test/parallel/test-assert.js b/test/parallel/test-assert.js
index ba0b74f2c5..e00f044942 100644
--- a/test/parallel/test-assert.js
+++ b/test/parallel/test-assert.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const a = require('assert');
 
 function makeBlock(f) {
-  var args = Array.prototype.slice.call(arguments, 1);
+  const args = Array.prototype.slice.call(arguments, 1);
   return function() {
     return f.apply(this, args);
   };
@@ -120,8 +120,8 @@ assert.throws(makeBlock(a.deepEqual, {a: 4}, {a: 4, b: true}),
 assert.doesNotThrow(makeBlock(a.deepEqual, ['a'], {0: 'a'}));
 //(although not necessarily the same order),
 assert.doesNotThrow(makeBlock(a.deepEqual, {a: 4, b: '1'}, {b: '1', a: 4}));
-var a1 = [1, 2, 3];
-var a2 = [1, 2, 3];
+const a1 = [1, 2, 3];
+const a2 = [1, 2, 3];
 a1.a = 'test';
 a1.b = true;
 a2.b = true;
@@ -131,7 +131,7 @@ assert.throws(makeBlock(a.deepEqual, Object.keys(a1), Object.keys(a2)),
 assert.doesNotThrow(makeBlock(a.deepEqual, a1, a2));
 
 // having an identical prototype property
-var nbRoot = {
+const nbRoot = {
   toString: function() { return this.first + ' ' + this.last; }
 };
 
@@ -149,8 +149,8 @@ function nameBuilder2(first, last) {
 }
 nameBuilder2.prototype = nbRoot;
 
-var nb1 = new nameBuilder('Ryan', 'Dahl');
-var nb2 = new nameBuilder2('Ryan', 'Dahl');
+const nb1 = new nameBuilder('Ryan', 'Dahl');
+let nb2 = new nameBuilder2('Ryan', 'Dahl');
 
 assert.doesNotThrow(makeBlock(a.deepEqual, nb1, nb2));
 
@@ -267,8 +267,8 @@ function Constructor2(first, last) {
   this.last = last;
 }
 
-var obj1 = new Constructor1('Ryan', 'Dahl');
-var obj2 = new Constructor2('Ryan', 'Dahl');
+const obj1 = new Constructor1('Ryan', 'Dahl');
+let obj2 = new Constructor2('Ryan', 'Dahl');
 
 assert.throws(makeBlock(a.deepStrictEqual, obj1, obj2), a.AssertionError);
 
@@ -285,7 +285,7 @@ assert.throws(makeBlock(assert.deepStrictEqual, true, 1),
 assert.throws(makeBlock(assert.deepStrictEqual, Symbol(), Symbol()),
               a.AssertionError);
 
-var s = Symbol();
+const s = Symbol();
 assert.doesNotThrow(makeBlock(assert.deepStrictEqual, s, s));
 
 
@@ -326,7 +326,7 @@ assert.throws(makeBlock(thrower, a.AssertionError));
 assert.throws(makeBlock(thrower, TypeError));
 
 // when passing a type, only catch errors of the appropriate type
-var threw = false;
+let threw = false;
 try {
   a.throws(makeBlock(thrower, TypeError), a.AssertionError);
 } catch (e) {
@@ -394,10 +394,11 @@ a.throws(makeBlock(thrower, TypeError), function(err) {
 // https://github.com/nodejs/node/issues/3188
 threw = false;
 
+let AnotherErrorType;
 try {
-  var ES6Error = class extends Error {};
+  const ES6Error = class extends Error {};
 
-  var AnotherErrorType = class extends Error {};
+  AnotherErrorType = class extends Error {};
 
   const functionThatThrows = function() {
     throw new AnotherErrorType('foo');
@@ -436,7 +437,7 @@ assert.ok(threw);
   a.throws(makeBlock(a.deepStrictEqual, d, e), /AssertionError/);
 }
 // GH-7178. Ensure reflexivity of deepEqual with `arguments` objects.
-var args = (function() { return arguments; })();
+const args = (function() { return arguments; })();
 a.throws(makeBlock(a.deepEqual, [], args));
 a.throws(makeBlock(a.deepEqual, args, []));
 
@@ -455,7 +456,7 @@ a.throws(makeBlock(a.deepEqual, args, []));
   a.doesNotThrow(makeBlock(a.deepEqual, someArgs, sameArgs));
 }
 
-var circular = {y: 1};
+const circular = {y: 1};
 circular.x = circular;
 
 function testAssertionMessage(actual, expected) {
@@ -520,7 +521,7 @@ try {
 
 // Verify that throws() and doesNotThrow() throw on non-function block
 function testBlockTypeError(method, block) {
-  var threw = true;
+  let threw = true;
 
   try {
     method(block);
diff --git a/test/parallel/test-async-wrap-check-providers.js b/test/parallel/test-async-wrap-check-providers.js
index c574403eae..354534a6b3 100644
--- a/test/parallel/test-async-wrap-check-providers.js
+++ b/test/parallel/test-async-wrap-check-providers.js
@@ -28,7 +28,7 @@ keyList.splice(0, 1);
 // want to improve under https://github.com/nodejs/node/issues/5085.
 // strip out fs watch related parts for now
 if (common.isAix) {
-  for (var i = 0; i < keyList.length; i++) {
+  for (let i = 0; i < keyList.length; i++) {
     if ((keyList[i] === 'FSEVENTWRAP') || (keyList[i] === 'STATWATCHER')) {
       keyList.splice(i, 1);
     }
diff --git a/test/parallel/test-async-wrap-throw-from-callback.js b/test/parallel/test-async-wrap-throw-from-callback.js
index adc92c69d9..b1a51773e2 100644
--- a/test/parallel/test-async-wrap-throw-from-callback.js
+++ b/test/parallel/test-async-wrap-throw-from-callback.js
@@ -13,8 +13,8 @@ const domain = require('domain');
 const spawn = require('child_process').spawn;
 const callbacks = [ 'init', 'pre', 'post', 'destroy' ];
 const toCall = process.argv[2];
-var msgCalled = 0;
-var msgReceived = 0;
+let msgCalled = 0;
+let msgReceived = 0;
 
 function init() {
   if (toCall === 'init')
@@ -57,7 +57,7 @@ if (typeof process.argv[2] === 'string') {
     msgCalled++;
 
     const child = spawn(process.execPath, [__filename, item]);
-    var errstring = '';
+    let errstring = '';
 
     child.stderr.on('data', (data) => {
       errstring += data.toString();
diff --git a/test/parallel/test-async-wrap-uid.js b/test/parallel/test-async-wrap-uid.js
index 19cb01cfa8..f16388cfe7 100644
--- a/test/parallel/test-async-wrap-uid.js
+++ b/test/parallel/test-async-wrap-uid.js
@@ -6,7 +6,7 @@ const assert = require('assert');
 const async_wrap = process.binding('async_wrap');
 
 // Give the event loop time to clear out the final uv_close().
-var si_cntr = 3;
+let si_cntr = 3;
 process.on('beforeExit', () => {
   if (--si_cntr > 0) setImmediate(() => {});
 });
diff --git a/test/parallel/test-buffer-ascii.js b/test/parallel/test-buffer-ascii.js
index ea125283f2..cf5b529183 100644
--- a/test/parallel/test-buffer-ascii.js
+++ b/test/parallel/test-buffer-ascii.js
@@ -16,7 +16,7 @@ const expected = 'Cb\u0000\u0019est, graphiquement, la rC)union ' +
 
 const buf = Buffer.from(input);
 
-for (var i = 0; i < expected.length; ++i) {
+for (let i = 0; i < expected.length; ++i) {
   assert.strictEqual(buf.slice(i).toString('ascii'), expected.slice(i));
 
   // Skip remainder of multi-byte sequence.
diff --git a/test/parallel/test-buffer-bytelength.js b/test/parallel/test-buffer-bytelength.js
index e31e514ffa..c30e06cc73 100644
--- a/test/parallel/test-buffer-bytelength.js
+++ b/test/parallel/test-buffer-bytelength.js
@@ -24,37 +24,37 @@ assert(ArrayBuffer.isView(Buffer.allocUnsafeSlow(10)));
 assert(ArrayBuffer.isView(Buffer.from('')));
 
 // buffer
-var incomplete = Buffer.from([0xe4, 0xb8, 0xad, 0xe6, 0x96]);
+const incomplete = Buffer.from([0xe4, 0xb8, 0xad, 0xe6, 0x96]);
 assert.strictEqual(Buffer.byteLength(incomplete), 5);
-var ascii = Buffer.from('abc');
+const ascii = Buffer.from('abc');
 assert.strictEqual(Buffer.byteLength(ascii), 3);
 
 // ArrayBuffer
-var buffer = new ArrayBuffer(8);
+const buffer = new ArrayBuffer(8);
 assert.strictEqual(Buffer.byteLength(buffer), 8);
 
 // TypedArray
-var int8 = new Int8Array(8);
+const int8 = new Int8Array(8);
 assert.strictEqual(Buffer.byteLength(int8), 8);
-var uint8 = new Uint8Array(8);
+const uint8 = new Uint8Array(8);
 assert.strictEqual(Buffer.byteLength(uint8), 8);
-var uintc8 = new Uint8ClampedArray(2);
+const uintc8 = new Uint8ClampedArray(2);
 assert.strictEqual(Buffer.byteLength(uintc8), 2);
-var int16 = new Int16Array(8);
+const int16 = new Int16Array(8);
 assert.strictEqual(Buffer.byteLength(int16), 16);
-var uint16 = new Uint16Array(8);
+const uint16 = new Uint16Array(8);
 assert.strictEqual(Buffer.byteLength(uint16), 16);
-var int32 = new Int32Array(8);
+const int32 = new Int32Array(8);
 assert.strictEqual(Buffer.byteLength(int32), 32);
-var uint32 = new Uint32Array(8);
+const uint32 = new Uint32Array(8);
 assert.strictEqual(Buffer.byteLength(uint32), 32);
-var float32 = new Float32Array(8);
+const float32 = new Float32Array(8);
 assert.strictEqual(Buffer.byteLength(float32), 32);
-var float64 = new Float64Array(8);
+const float64 = new Float64Array(8);
 assert.strictEqual(Buffer.byteLength(float64), 64);
 
 // DataView
-var dv = new DataView(new ArrayBuffer(2));
+const dv = new DataView(new ArrayBuffer(2));
 assert.strictEqual(Buffer.byteLength(dv), 2);
 
 // special case: zero length string
diff --git a/test/parallel/test-buffer-concat.js b/test/parallel/test-buffer-concat.js
index 7c2c176a1a..44dc0ad21d 100644
--- a/test/parallel/test-buffer-concat.js
+++ b/test/parallel/test-buffer-concat.js
@@ -5,7 +5,7 @@ const assert = require('assert');
 const zero = [];
 const one = [ Buffer.from('asdf') ];
 const long = [];
-for (var i = 0; i < 10; i++) long.push(Buffer.from('asdf'));
+for (let i = 0; i < 10; i++) long.push(Buffer.from('asdf'));
 
 const flatZero = Buffer.concat(zero);
 const flatOne = Buffer.concat(one);
diff --git a/test/parallel/test-buffer-copy.js b/test/parallel/test-buffer-copy.js
index fef4acf7df..5737ed7480 100644
--- a/test/parallel/test-buffer-copy.js
+++ b/test/parallel/test-buffer-copy.js
@@ -5,7 +5,7 @@ const assert = require('assert');
 
 const b = Buffer.allocUnsafe(1024);
 const c = Buffer.allocUnsafe(512);
-var cntr = 0;
+let cntr = 0;
 
 {
   // copy 512 bytes, from 0 to 512.
diff --git a/test/parallel/test-buffer-fill.js b/test/parallel/test-buffer-fill.js
index 8cfeeabe91..06a9044bec 100644
--- a/test/parallel/test-buffer-fill.js
+++ b/test/parallel/test-buffer-fill.js
@@ -243,7 +243,7 @@ function writeToFill(string, offset, end, encoding) {
   // Convert "end" to "length" (which write understands).
   const length = end - offset < 0 ? 0 : end - offset;
 
-  var wasZero = false;
+  let wasZero = false;
   do {
     const written = buf2.write(string, offset, length, encoding);
     offset += written;
@@ -324,7 +324,7 @@ Buffer.alloc(8, '');
 {
   let elseWasLast = false;
   assert.throws(() => {
-    var ctr = 0;
+    let ctr = 0;
     const start = {
       [Symbol.toPrimitive]() {
         // We use this condition to get around the check in lib/buffer.js
@@ -357,7 +357,7 @@ assert.throws(() => {
 {
   let elseWasLast = false;
   assert.throws(() => {
-    var ctr = 0;
+    let ctr = 0;
     const end = {
       [Symbol.toPrimitive]() {
         // We use this condition to get around the check in lib/buffer.js
diff --git a/test/parallel/test-buffer-includes.js b/test/parallel/test-buffer-includes.js
index 15e1eedeb5..9b0aef0ad7 100644
--- a/test/parallel/test-buffer-includes.js
+++ b/test/parallel/test-buffer-includes.js
@@ -140,7 +140,7 @@ assert.strictEqual(
 
 
 // test usc2 encoding
-var twoByteString = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');
+let twoByteString = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');
 
 assert(twoByteString.includes('\u0395', 4, 'ucs2'));
 assert(twoByteString.includes('\u03a3', -4, 'ucs2'));
@@ -190,7 +190,7 @@ assert(!mixedByteStringUtf8.includes('\u0396'));
 
 // Test complex string includes algorithms. Only trigger for long strings.
 // Long string that isn't a simple repeat of a shorter string.
-var longString = 'A';
+let longString = 'A';
 for (let i = 66; i < 76; i++) {  // from 'B' to 'K'
   longString = longString + String.fromCharCode(i) + longString;
 }
@@ -198,7 +198,7 @@ for (let i = 66; i < 76; i++) {  // from 'B' to 'K'
 const longBufferString = Buffer.from(longString);
 
 // pattern of 15 chars, repeated every 16 chars in long
-var pattern = 'ABACABADABACABA';
+let pattern = 'ABACABADABACABA';
 for (let i = 0; i < longBufferString.length - pattern.length; i += 7) {
   const includes = longBufferString.includes(pattern, i);
   assert(includes, 'Long ABACABA...-string at index ' + i);
@@ -229,8 +229,8 @@ assert(!allCharsBufferUtf8.includes('notfound'));
 assert(!allCharsBufferUcs2.includes('notfound'));
 
 // Find substrings in Utf8.
-var lengths = [1, 3, 15];  // Single char, simple and complex.
-var indices = [0x5, 0x60, 0x400, 0x680, 0x7ee, 0xFF02, 0x16610, 0x2f77b];
+let lengths = [1, 3, 15];  // Single char, simple and complex.
+let indices = [0x5, 0x60, 0x400, 0x680, 0x7ee, 0xFF02, 0x16610, 0x2f77b];
 for (let lengthIndex = 0; lengthIndex < lengths.length; lengthIndex++) {
   for (let i = 0; i < indices.length; i++) {
     const index = indices[i];
diff --git a/test/parallel/test-buffer-indexof.js b/test/parallel/test-buffer-indexof.js
index 6aab628fe9..580a93ed66 100644
--- a/test/parallel/test-buffer-indexof.js
+++ b/test/parallel/test-buffer-indexof.js
@@ -191,7 +191,7 @@ assert.equal(Buffer.from('aaaa00a').indexOf('3030', 'hex'), 4);
   assert.equal(-1, twoByteString.indexOf('\u03a3', -2, 'ucs2'));
 }
 
-var mixedByteStringUcs2 =
+const mixedByteStringUcs2 =
     Buffer.from('\u039a\u0391abc\u03a3\u03a3\u0395', 'ucs2');
 assert.equal(6, mixedByteStringUcs2.indexOf('bc', 0, 'ucs2'));
 assert.equal(10, mixedByteStringUcs2.indexOf('\u03a3', 0, 'ucs2'));
@@ -226,7 +226,7 @@ assert.equal(
       6, twoByteString.indexOf('\u03a3\u0395', 0, 'ucs2'), 'Sigma Epsilon');
 }
 
-var mixedByteStringUtf8 = Buffer.from('\u039a\u0391abc\u03a3\u03a3\u0395');
+const mixedByteStringUtf8 = Buffer.from('\u039a\u0391abc\u03a3\u03a3\u0395');
 assert.equal(5, mixedByteStringUtf8.indexOf('bc'));
 assert.equal(5, mixedByteStringUtf8.indexOf('bc', 5));
 assert.equal(5, mixedByteStringUtf8.indexOf('bc', -8));
@@ -236,15 +236,15 @@ assert.equal(-1, mixedByteStringUtf8.indexOf('\u0396'));
 
 // Test complex string indexOf algorithms. Only trigger for long strings.
 // Long string that isn't a simple repeat of a shorter string.
-var longString = 'A';
+let longString = 'A';
 for (let i = 66; i < 76; i++) {  // from 'B' to 'K'
   longString = longString + String.fromCharCode(i) + longString;
 }
 
-var longBufferString = Buffer.from(longString);
+const longBufferString = Buffer.from(longString);
 
 // pattern of 15 chars, repeated every 16 chars in long
-var pattern = 'ABACABADABACABA';
+let pattern = 'ABACABADABACABA';
 for (let i = 0; i < longBufferString.length - pattern.length; i += 7) {
   const index = longBufferString.indexOf(pattern, i);
   assert.equal((i + 15) & ~0xf, index, 'Long ABACABA...-string at index ' + i);
@@ -260,17 +260,17 @@ assert.equal(
     1535, longBufferString.indexOf(pattern, 512), 'Long JABACABA..., Second J');
 
 // Search for a non-ASCII string in a pure ASCII string.
-var asciiString = Buffer.from(
+const asciiString = Buffer.from(
     'arglebargleglopglyfarglebargleglopglyfarglebargleglopglyf');
 assert.equal(-1, asciiString.indexOf('\x2061'));
 assert.equal(3, asciiString.indexOf('leb', 0));
 
 // Search in string containing many non-ASCII chars.
-var allCodePoints = [];
+const allCodePoints = [];
 for (let i = 0; i < 65536; i++) allCodePoints[i] = i;
-var allCharsString = String.fromCharCode.apply(String, allCodePoints);
-var allCharsBufferUtf8 = Buffer.from(allCharsString);
-var allCharsBufferUcs2 = Buffer.from(allCharsString, 'ucs2');
+const allCharsString = String.fromCharCode.apply(String, allCodePoints);
+const allCharsBufferUtf8 = Buffer.from(allCharsString);
+const allCharsBufferUcs2 = Buffer.from(allCharsString, 'ucs2');
 
 // Search for string long enough to trigger complex search with ASCII pattern
 // and UC16 subject.
@@ -307,10 +307,10 @@ assert.strictEqual(Buffer.from('aaaaa').indexOf('b', 'ucs2'), -1);
         length = 4 * length;
       }
 
-      var patternBufferUtf8 = allCharsBufferUtf8.slice(index, index + length);
+      const patternBufferUtf8 = allCharsBufferUtf8.slice(index, index + length);
       assert.equal(index, allCharsBufferUtf8.indexOf(patternBufferUtf8));
 
-      var patternStringUtf8 = patternBufferUtf8.toString();
+      const patternStringUtf8 = patternBufferUtf8.toString();
       assert.equal(index, allCharsBufferUtf8.indexOf(patternStringUtf8));
     }
   }
@@ -325,12 +325,12 @@ assert.strictEqual(Buffer.from('aaaaa').indexOf('b', 'ucs2'), -1);
       const index = indices[i] * 2;
       const length = lengths[lengthIndex];
 
-      var patternBufferUcs2 =
+      const patternBufferUcs2 =
           allCharsBufferUcs2.slice(index, index + length);
       assert.equal(
           index, allCharsBufferUcs2.indexOf(patternBufferUcs2, 0, 'ucs2'));
 
-      var patternStringUcs2 = patternBufferUcs2.toString('ucs2');
+      const patternStringUcs2 = patternBufferUcs2.toString('ucs2');
       assert.equal(
           index, allCharsBufferUcs2.indexOf(patternStringUcs2, 0, 'ucs2'));
     }
@@ -434,7 +434,7 @@ assert.strictEqual(buf_bc.lastIndexOf('你好', 5, 'binary'), -1);
 assert.strictEqual(buf_bc.lastIndexOf(Buffer.from('你好'), 7), -1);
 
 // Test lastIndexOf on a longer buffer:
-var bufferString = new Buffer('a man a plan a canal panama');
+const bufferString = new Buffer('a man a plan a canal panama');
 assert.equal(15, bufferString.lastIndexOf('canal'));
 assert.equal(21, bufferString.lastIndexOf('panama'));
 assert.equal(0, bufferString.lastIndexOf('a man a plan a canal panama'));
@@ -485,16 +485,17 @@ assert.equal(511, longBufferString.lastIndexOf(pattern, 1534));
 
 // countBits returns the number of bits in the binary reprsentation of n.
 function countBits(n) {
-  for (var count = 0; n > 0; count++) {
+  let count;
+  for (count = 0; n > 0; count++) {
     n = n & (n - 1); // remove top bit
   }
   return count;
 }
-var parts = [];
-for (var i = 0; i < 1000000; i++) {
+const parts = [];
+for (let i = 0; i < 1000000; i++) {
   parts.push((countBits(i) % 2 === 0) ? 'yolo' : 'swag');
 }
-var reallyLong = new Buffer(parts.join(' '));
+const reallyLong = new Buffer(parts.join(' '));
 assert.equal('yolo swag swag yolo', reallyLong.slice(0, 19).toString());
 
 // Expensive reverse searches. Stress test lastIndexOf:
diff --git a/test/parallel/test-buffer-inspect.js b/test/parallel/test-buffer-inspect.js
index 4fed249207..41e8197da6 100644
--- a/test/parallel/test-buffer-inspect.js
+++ b/test/parallel/test-buffer-inspect.js
@@ -6,13 +6,13 @@ const buffer = require('buffer');
 
 buffer.INSPECT_MAX_BYTES = 2;
 
-var b = Buffer.allocUnsafe(4);
+let b = Buffer.allocUnsafe(4);
 b.fill('1234');
 
-var s = buffer.SlowBuffer(4);
+let s = buffer.SlowBuffer(4);
 s.fill('1234');
 
-var expected = '<Buffer 31 32 ... >';
+let expected = '<Buffer 31 32 ... >';
 
 assert.strictEqual(util.inspect(b), expected);
 assert.strictEqual(util.inspect(s), expected);
diff --git a/test/parallel/test-buffer-iterator.js b/test/parallel/test-buffer-iterator.js
index 7a25682ebc..3120ff1d2a 100644
--- a/test/parallel/test-buffer-iterator.js
+++ b/test/parallel/test-buffer-iterator.js
@@ -3,8 +3,8 @@ require('../common');
 const assert = require('assert');
 
 const buffer = Buffer.from([1, 2, 3, 4, 5]);
-var arr;
-var b;
+let arr;
+let b;
 
 // buffers should be iterable
 
diff --git a/test/parallel/test-buffer-swap.js b/test/parallel/test-buffer-swap.js
index f72b638f75..bcb636edb1 100644
--- a/test/parallel/test-buffer-swap.js
+++ b/test/parallel/test-buffer-swap.js
@@ -45,80 +45,80 @@ assert.deepStrictEqual(buf3_64, Buffer.from([0x01, 0x02, 0x0a, 0x09, 0x08, 0x07,
                                              0x0f, 0x10]));
 
 // Force use of native code (Buffer size above threshold limit for js impl)
-var buf4A = new Uint32Array(256).fill(0x04030201);
-var buf4 = Buffer.from(buf4A.buffer, buf4A.byteOffset);
-var buf5A = new Uint32Array(256).fill(0x03040102);
-var buf5 = Buffer.from(buf5A.buffer, buf5A.byteOffset);
+const buf4A = new Uint32Array(256).fill(0x04030201);
+const buf4 = Buffer.from(buf4A.buffer, buf4A.byteOffset);
+const buf5A = new Uint32Array(256).fill(0x03040102);
+const buf5 = Buffer.from(buf5A.buffer, buf5A.byteOffset);
 
 buf4.swap16();
 assert.deepStrictEqual(buf4, buf5);
 
-var buf6A = new Uint32Array(256).fill(0x04030201);
-var buf6 = Buffer.from(buf6A.buffer);
-var bu7A = new Uint32Array(256).fill(0x01020304);
-var buf7 = Buffer.from(bu7A.buffer, bu7A.byteOffset);
+const buf6A = new Uint32Array(256).fill(0x04030201);
+const buf6 = Buffer.from(buf6A.buffer);
+const bu7A = new Uint32Array(256).fill(0x01020304);
+const buf7 = Buffer.from(bu7A.buffer, bu7A.byteOffset);
 
 buf6.swap32();
 assert.deepStrictEqual(buf6, buf7);
 
-var buf8A = new Uint8Array(256 * 8);
-var buf9A = new Uint8Array(256 * 8);
+const buf8A = new Uint8Array(256 * 8);
+const buf9A = new Uint8Array(256 * 8);
 for (let i = 0; i < buf8A.length; i++) {
   buf8A[i] = i % 8;
   buf9A[buf9A.length - i - 1] = i % 8;
 }
-var buf8 = Buffer.from(buf8A.buffer, buf8A.byteOffset);
-var buf9 = Buffer.from(buf9A.buffer, buf9A.byteOffset);
+const buf8 = Buffer.from(buf8A.buffer, buf8A.byteOffset);
+const buf9 = Buffer.from(buf9A.buffer, buf9A.byteOffset);
 
 buf8.swap64();
 assert.deepStrictEqual(buf8, buf9);
 
 // Test native code with buffers that are not memory-aligned
-var buf10A = new Uint8Array(256 * 8);
-var buf11A = new Uint8Array(256 * 8 - 2);
+const buf10A = new Uint8Array(256 * 8);
+const buf11A = new Uint8Array(256 * 8 - 2);
 for (let i = 0; i < buf10A.length; i++) {
   buf10A[i] = i % 2;
 }
 for (let i = 1; i < buf11A.length; i++) {
   buf11A[buf11A.length - i] = (i + 1) % 2;
 }
-var buf10 = Buffer.from(buf10A.buffer, buf10A.byteOffset);
+const buf10 = Buffer.from(buf10A.buffer, buf10A.byteOffset);
 // 0|1 0|1 0|1...
-var buf11 = Buffer.from(buf11A.buffer, buf11A.byteOffset);
+const buf11 = Buffer.from(buf11A.buffer, buf11A.byteOffset);
 // 0|0 1|0 1|0...
 
 buf10.slice(1, buf10.length - 1).swap16();
 assert.deepStrictEqual(buf10.slice(0, buf11.length), buf11);
 
 
-var buf12A = new Uint8Array(256 * 8);
-var buf13A = new Uint8Array(256 * 8 - 4);
+const buf12A = new Uint8Array(256 * 8);
+const buf13A = new Uint8Array(256 * 8 - 4);
 for (let i = 0; i < buf12A.length; i++) {
   buf12A[i] = i % 4;
 }
 for (let i = 1; i < buf13A.length; i++) {
   buf13A[buf13A.length - i] = (i + 1) % 4;
 }
-var buf12 = Buffer.from(buf12A.buffer, buf12A.byteOffset);
+const buf12 = Buffer.from(buf12A.buffer, buf12A.byteOffset);
 // 0|1 2 3 0|1 2 3...
-var buf13 = Buffer.from(buf13A.buffer, buf13A.byteOffset);
+const buf13 = Buffer.from(buf13A.buffer, buf13A.byteOffset);
 // 0|0 3 2 1|0 3 2...
 
 buf12.slice(1, buf12.length - 3).swap32();
 assert.deepStrictEqual(buf12.slice(0, buf13.length), buf13);
 
 
-var buf14A = new Uint8Array(256 * 8);
-var buf15A = new Uint8Array(256 * 8 - 8);
+const buf14A = new Uint8Array(256 * 8);
+const buf15A = new Uint8Array(256 * 8 - 8);
 for (let i = 0; i < buf14A.length; i++) {
   buf14A[i] = i % 8;
 }
 for (let i = 1; i < buf15A.length; i++) {
   buf15A[buf15A.length - i] = (i + 1) % 8;
 }
-var buf14 = Buffer.from(buf14A.buffer, buf14A.byteOffset);
+const buf14 = Buffer.from(buf14A.buffer, buf14A.byteOffset);
 // 0|1 2 3 4 5 6 7 0|1 2 3 4...
-var buf15 = Buffer.from(buf15A.buffer, buf15A.byteOffset);
+const buf15 = Buffer.from(buf15A.buffer, buf15A.byteOffset);
 // 0|0 7 6 5 4 3 2 1|0 7 6 5...
 
 buf14.slice(1, buf14.length - 7).swap64();
diff --git a/test/parallel/test-child-process-constructor.js b/test/parallel/test-child-process-constructor.js
index 59277dcabe..d33c7ae3ac 100644
--- a/test/parallel/test-child-process-constructor.js
+++ b/test/parallel/test-child-process-constructor.js
@@ -3,11 +3,11 @@
 require('../common');
 const assert = require('assert');
 const child_process = require('child_process');
-var ChildProcess = child_process.ChildProcess;
+const ChildProcess = child_process.ChildProcess;
 assert.strictEqual(typeof ChildProcess, 'function');
 
 // test that we can call spawn
-var child = new ChildProcess();
+const child = new ChildProcess();
 child.spawn({
   file: process.execPath,
   args: ['--interactive'],
diff --git a/test/parallel/test-child-process-default-options.js b/test/parallel/test-child-process-default-options.js
index 2c60e274de..37e7ea7494 100644
--- a/test/parallel/test-child-process-default-options.js
+++ b/test/parallel/test-child-process-default-options.js
@@ -6,14 +6,14 @@ const spawn = require('child_process').spawn;
 
 process.env.HELLO = 'WORLD';
 
-var child;
+let child;
 if (common.isWindows) {
   child = spawn('cmd.exe', ['/c', 'set'], {});
 } else {
   child = spawn('/usr/bin/env', [], {});
 }
 
-var response = '';
+let response = '';
 
 child.stdout.setEncoding('utf8');
 
diff --git a/test/parallel/test-child-process-double-pipe.js b/test/parallel/test-child-process-double-pipe.js
index c5b7c093af..dac48a2db7 100644
--- a/test/parallel/test-child-process-double-pipe.js
+++ b/test/parallel/test-child-process-double-pipe.js
@@ -8,7 +8,7 @@ const spawn = require('child_process').spawn;
 // We're trying to reproduce:
 // $ echo "hello\nnode\nand\nworld" | grep o | sed s/o/a/
 
-var grep, sed, echo;
+let grep, sed, echo;
 
 if (common.isWindows) {
   grep = spawn('grep', ['--binary', 'o']),
@@ -82,7 +82,7 @@ grep.stdout.on('end', function(code) {
 });
 
 
-var result = '';
+let result = '';
 
 // print sed's output
 sed.stdout.on('data', function(data) {
diff --git a/test/parallel/test-child-process-env.js b/test/parallel/test-child-process-env.js
index cac82b9952..ad41f978e2 100644
--- a/test/parallel/test-child-process-env.js
+++ b/test/parallel/test-child-process-env.js
@@ -4,14 +4,14 @@ const assert = require('assert');
 
 const spawn = require('child_process').spawn;
 
-var env = {
+const env = {
   'HELLO': 'WORLD'
 };
 Object.setPrototypeOf(env, {
   'FOO': 'BAR'
 });
 
-var child;
+let child;
 if (common.isWindows) {
   child = spawn('cmd.exe', ['/c', 'set'], {env: env});
 } else {
@@ -19,7 +19,7 @@ if (common.isWindows) {
 }
 
 
-var response = '';
+let response = '';
 
 child.stdout.setEncoding('utf8');
 
diff --git a/test/parallel/test-child-process-exec-cwd.js b/test/parallel/test-child-process-exec-cwd.js
index 3b6e428a4a..95feb42314 100644
--- a/test/parallel/test-child-process-exec-cwd.js
+++ b/test/parallel/test-child-process-exec-cwd.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const exec = require('child_process').exec;
 
-var pwdcommand, dir;
+let pwdcommand, dir;
 
 if (common.isWindows) {
   pwdcommand = 'echo %cd%';
diff --git a/test/parallel/test-child-process-exec-stdout-stderr-data-string.js b/test/parallel/test-child-process-exec-stdout-stderr-data-string.js
index 8ab834c98f..472d31f16a 100644
--- a/test/parallel/test-child-process-exec-stdout-stderr-data-string.js
+++ b/test/parallel/test-child-process-exec-stdout-stderr-data-string.js
@@ -4,8 +4,8 @@ const common = require('../common');
 const assert = require('assert');
 const exec = require('child_process').exec;
 
-var stdoutCalls = 0;
-var stderrCalls = 0;
+let stdoutCalls = 0;
+let stderrCalls = 0;
 
 const command = common.isWindows ? 'dir' : 'ls';
 exec(command).stdout.on('data', (data) => {
diff --git a/test/parallel/test-child-process-exit-code.js b/test/parallel/test-child-process-exit-code.js
index 2bd1256fe2..9596a20222 100644
--- a/test/parallel/test-child-process-exit-code.js
+++ b/test/parallel/test-child-process-exit-code.js
@@ -4,17 +4,17 @@ const assert = require('assert');
 const spawn = require('child_process').spawn;
 const path = require('path');
 
-var exitScript = path.join(common.fixturesDir, 'exit.js');
-var exitChild = spawn(process.argv[0], [exitScript, 23]);
+const exitScript = path.join(common.fixturesDir, 'exit.js');
+const exitChild = spawn(process.argv[0], [exitScript, 23]);
 exitChild.on('exit', common.mustCall(function(code, signal) {
   assert.strictEqual(code, 23);
   assert.strictEqual(signal, null);
 }));
 
 
-var errorScript = path.join(common.fixturesDir,
-                            'child_process_should_emit_error.js');
-var errorChild = spawn(process.argv[0], [errorScript]);
+const errorScript = path.join(common.fixturesDir,
+                              'child_process_should_emit_error.js');
+const errorChild = spawn(process.argv[0], [errorScript]);
 errorChild.on('exit', common.mustCall(function(code, signal) {
   assert.ok(code !== 0);
   assert.strictEqual(signal, null);
diff --git a/test/parallel/test-child-process-fork-close.js b/test/parallel/test-child-process-fork-close.js
index d6e9675ba8..d8b1aa0bf3 100644
--- a/test/parallel/test-child-process-fork-close.js
+++ b/test/parallel/test-child-process-fork-close.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const fork = require('child_process').fork;
 
-var cp = fork(common.fixturesDir + '/child-process-message-and-exit.js');
+const cp = fork(common.fixturesDir + '/child-process-message-and-exit.js');
 
 let gotMessage = false;
 let gotExit = false;
diff --git a/test/parallel/test-child-process-fork-dgram.js b/test/parallel/test-child-process-fork-dgram.js
index b25cf07ef6..4447c54cae 100644
--- a/test/parallel/test-child-process-fork-dgram.js
+++ b/test/parallel/test-child-process-fork-dgram.js
@@ -37,8 +37,8 @@ if (process.argv[2] === 'child') {
 
   const msg = Buffer.from('Some bytes');
 
-  var childGotMessage = false;
-  var parentGotMessage = false;
+  let childGotMessage = false;
+  let parentGotMessage = false;
 
   parentServer.once('message', function(msg, rinfo) {
     parentGotMessage = true;
diff --git a/test/parallel/test-child-process-fork-exec-argv.js b/test/parallel/test-child-process-fork-exec-argv.js
index 2daef37b66..97d1e21a2f 100644
--- a/test/parallel/test-child-process-fork-exec-argv.js
+++ b/test/parallel/test-child-process-fork-exec-argv.js
@@ -2,8 +2,8 @@
 require('../common');
 const assert = require('assert');
 const child_process = require('child_process');
-var spawn = child_process.spawn;
-var fork = child_process.fork;
+const spawn = child_process.spawn;
+const fork = child_process.fork;
 
 if (process.argv[2] === 'fork') {
   process.stdout.write(JSON.stringify(process.execArgv), function() {
@@ -12,11 +12,11 @@ if (process.argv[2] === 'fork') {
 } else if (process.argv[2] === 'child') {
   fork(__filename, ['fork']);
 } else {
-  var execArgv = ['--stack-size=256'];
-  var args = [__filename, 'child', 'arg0'];
+  const execArgv = ['--stack-size=256'];
+  const args = [__filename, 'child', 'arg0'];
 
-  var child = spawn(process.execPath, execArgv.concat(args));
-  var out = '';
+  const child = spawn(process.execPath, execArgv.concat(args));
+  let out = '';
 
   child.stdout.on('data', function(chunk) {
     out += chunk;
diff --git a/test/parallel/test-child-process-fork-exec-path.js b/test/parallel/test-child-process-fork-exec-path.js
index e9938d8ba1..5af0184a21 100644
--- a/test/parallel/test-child-process-fork-exec-path.js
+++ b/test/parallel/test-child-process-fork-exec-path.js
@@ -3,9 +3,9 @@ const common = require('../common');
 const assert = require('assert');
 const fs = require('fs');
 const path = require('path');
-var msg = {test: 'this'};
-var nodePath = process.execPath;
-var copyPath = path.join(common.tmpDir, 'node-copy.exe');
+const msg = {test: 'this'};
+const nodePath = process.execPath;
+const copyPath = path.join(common.tmpDir, 'node-copy.exe');
 
 if (process.env.FORK) {
   assert(process.send);
@@ -23,7 +23,7 @@ if (process.env.FORK) {
   fs.chmodSync(copyPath, '0755');
 
   // slow but simple
-  var envCopy = JSON.parse(JSON.stringify(process.env));
+  const envCopy = JSON.parse(JSON.stringify(process.env));
   envCopy.FORK = 'true';
   const child = require('child_process').fork(__filename, {
     execPath: copyPath,
diff --git a/test/parallel/test-child-process-fork-net.js b/test/parallel/test-child-process-fork-net.js
index cbafcb13fb..c66b808ec0 100644
--- a/test/parallel/test-child-process-fork-net.js
+++ b/test/parallel/test-child-process-fork-net.js
@@ -19,7 +19,7 @@ ProgressTracker.prototype.check = function() {
 
 if (process.argv[2] === 'child') {
 
-  var serverScope;
+  let serverScope;
 
   process.on('message', function onServer(msg, server) {
     if (msg.what !== 'server') return;
@@ -58,17 +58,17 @@ if (process.argv[2] === 'child') {
   process.send({what: 'ready'});
 } else {
 
-  var child = fork(process.argv[1], ['child']);
+  const child = fork(process.argv[1], ['child']);
 
   child.on('exit', function() {
     console.log('CHILD: died');
   });
 
   // send net.Server to child and test by connecting
-  var testServer = function(callback) {
+  const testServer = function(callback) {
 
     // destroy server execute callback when done
-    var progress = new ProgressTracker(2, function() {
+    const progress = new ProgressTracker(2, function() {
       server.on('close', function() {
         console.log('PARENT: server closed');
         child.send({what: 'close'});
@@ -77,11 +77,11 @@ if (process.argv[2] === 'child') {
     });
 
     // we expect 4 connections and close events
-    var connections = new ProgressTracker(4, progress.done.bind(progress));
-    var closed = new ProgressTracker(4, progress.done.bind(progress));
+    const connections = new ProgressTracker(4, progress.done.bind(progress));
+    const closed = new ProgressTracker(4, progress.done.bind(progress));
 
     // create server and send it to child
-    var server = net.createServer();
+    const server = net.createServer();
     server.on('connection', function(socket) {
       console.log('PARENT: got connection');
       socket.destroy();
@@ -94,12 +94,12 @@ if (process.argv[2] === 'child') {
     server.listen(0);
 
     // handle client messages
-    var messageHandlers = function(msg) {
+    const messageHandlers = function(msg) {
 
       if (msg.what === 'listening') {
         // make connections
-        var socket;
-        for (var i = 0; i < 4; i++) {
+        let socket;
+        for (let i = 0; i < 4; i++) {
           socket = net.connect(server.address().port, function() {
             console.log('CLIENT: connected');
           });
@@ -122,11 +122,11 @@ if (process.argv[2] === 'child') {
   };
 
   // send net.Socket to child
-  var testSocket = function(callback) {
+  const testSocket = function(callback) {
 
     // create a new server and connect to it,
     // but the socket will be handled by the child
-    var server = net.createServer();
+    const server = net.createServer();
     server.on('connection', function(socket) {
       socket.on('close', function() {
         console.log('CLIENT: socket closed');
@@ -145,8 +145,8 @@ if (process.argv[2] === 'child') {
     // will have to do.
     server.listen(0, function() {
       console.error('testSocket, listening');
-      var connect = net.connect(server.address().port);
-      var store = '';
+      const connect = net.connect(server.address().port);
+      let store = '';
       connect.on('data', function(chunk) {
         store += chunk;
         console.log('CLIENT: got data');
@@ -160,8 +160,8 @@ if (process.argv[2] === 'child') {
   };
 
   // create server and send it to child
-  var serverSuccess = false;
-  var socketSuccess = false;
+  let serverSuccess = false;
+  let socketSuccess = false;
   child.on('message', function onReady(msg) {
     if (msg.what !== 'ready') return;
     child.removeListener('message', onReady);
diff --git a/test/parallel/test-child-process-fork-net2.js b/test/parallel/test-child-process-fork-net2.js
index cee5c0a4ca..6d20f41904 100644
--- a/test/parallel/test-child-process-fork-net2.js
+++ b/test/parallel/test-child-process-fork-net2.js
@@ -3,11 +3,11 @@ const common = require('../common');
 const assert = require('assert');
 const fork = require('child_process').fork;
 const net = require('net');
-var count = 12;
+const count = 12;
 
 if (process.argv[2] === 'child') {
-  var needEnd = [];
-  var id = process.argv[3];
+  const needEnd = [];
+  const id = process.argv[3];
 
   process.on('message', function(m, socket) {
     if (!socket) return;
@@ -60,11 +60,11 @@ if (process.argv[2] === 'child') {
 
 } else {
 
-  var child1 = fork(process.argv[1], ['child', '1']);
-  var child2 = fork(process.argv[1], ['child', '2']);
-  var child3 = fork(process.argv[1], ['child', '3']);
+  const child1 = fork(process.argv[1], ['child', '1']);
+  const child2 = fork(process.argv[1], ['child', '2']);
+  const child3 = fork(process.argv[1], ['child', '3']);
 
-  var server = net.createServer();
+  const server = net.createServer();
 
   let connected = 0;
   let closed = 0;
@@ -94,10 +94,10 @@ if (process.argv[2] === 'child') {
     }
   });
 
-  var disconnected = 0;
+  let disconnected = 0;
   server.on('listening', function() {
 
-    var j = count, client;
+    let j = count, client;
     while (j--) {
       client = net.connect(this.address().port, '127.0.0.1');
       client.on('error', function() {
@@ -112,7 +112,7 @@ if (process.argv[2] === 'child') {
     }
   });
 
-  var closeEmitted = false;
+  let closeEmitted = false;
   server.on('close', common.mustCall(function() {
     closeEmitted = true;
 
@@ -123,7 +123,7 @@ if (process.argv[2] === 'child') {
 
   server.listen(0, '127.0.0.1');
 
-  var closeServer = function() {
+  const closeServer = function() {
     server.close();
 
     setTimeout(function() {
diff --git a/test/parallel/test-child-process-fork-ref.js b/test/parallel/test-child-process-fork-ref.js
index 46eaab584f..1f8816c0cf 100644
--- a/test/parallel/test-child-process-fork-ref.js
+++ b/test/parallel/test-child-process-fork-ref.js
@@ -16,9 +16,10 @@ if (process.argv[2] === 'child') {
   });
 
 } else {
-  var child = fork(__filename, ['child'], {silent: true});
+  const child = fork(__filename, ['child'], {silent: true});
 
-  var ipc = [], stdout = '';
+  const ipc = [];
+  let stdout = '';
 
   child.on('message', function(msg) {
     ipc.push(msg);
diff --git a/test/parallel/test-child-process-fork-ref2.js b/test/parallel/test-child-process-fork-ref2.js
index 6d6c56b6b3..9f59aa344a 100644
--- a/test/parallel/test-child-process-fork-ref2.js
+++ b/test/parallel/test-child-process-fork-ref2.js
@@ -12,7 +12,7 @@ if (process.argv[2] === 'child') {
   }, 400);
 
 } else {
-  var child = fork(__filename, ['child']);
+  const child = fork(__filename, ['child']);
 
   child.on('disconnect', function() {
     console.log('parent -> disconnect');
diff --git a/test/parallel/test-child-process-fork-regr-gh-2847.js b/test/parallel/test-child-process-fork-regr-gh-2847.js
index d985b8b4ac..0bcf4bf9a1 100644
--- a/test/parallel/test-child-process-fork-regr-gh-2847.js
+++ b/test/parallel/test-child-process-fork-regr-gh-2847.js
@@ -14,7 +14,7 @@ if (!cluster.isMaster) {
   return;
 }
 
-var server = net.createServer(function(s) {
+const server = net.createServer(function(s) {
   if (common.isWindows) {
     s.on('error', function(err) {
       // Prevent possible ECONNRESET errors from popping up
@@ -26,10 +26,10 @@ var server = net.createServer(function(s) {
     s.destroy();
   }, 100);
 }).listen(0, function() {
-  var worker = cluster.fork();
+  const worker = cluster.fork();
 
   function send(callback) {
-    var s = net.connect(server.address().port, function() {
+    const s = net.connect(server.address().port, function() {
       worker.send({}, s, callback);
     });
 
diff --git a/test/parallel/test-child-process-fork.js b/test/parallel/test-child-process-fork.js
index a8548eff58..bfc68e41dc 100644
--- a/test/parallel/test-child-process-fork.js
+++ b/test/parallel/test-child-process-fork.js
@@ -2,9 +2,9 @@
 const common = require('../common');
 const assert = require('assert');
 const fork = require('child_process').fork;
-var args = ['foo', 'bar'];
+const args = ['foo', 'bar'];
 
-var n = fork(common.fixturesDir + '/child-process-spawn-node.js', args);
+const n = fork(common.fixturesDir + '/child-process-spawn-node.js', args);
 
 assert.strictEqual(n.channel, n._channel);
 assert.deepStrictEqual(args, ['foo', 'bar']);
diff --git a/test/parallel/test-child-process-internal.js b/test/parallel/test-child-process-internal.js
index 5d488f7839..d795b69d7e 100644
--- a/test/parallel/test-child-process-internal.js
+++ b/test/parallel/test-child-process-internal.js
@@ -3,9 +3,9 @@ const common = require('../common');
 const assert = require('assert');
 
 //messages
-var PREFIX = 'NODE_';
-var normal = {cmd: 'foo' + PREFIX};
-var internal = {cmd: PREFIX + 'bar'};
+const PREFIX = 'NODE_';
+const normal = {cmd: 'foo' + PREFIX};
+const internal = {cmd: PREFIX + 'bar'};
 
 if (process.argv[2] === 'child') {
   //send non-internal message containing PREFIX at a non prefix position
@@ -19,7 +19,7 @@ if (process.argv[2] === 'child') {
 } else {
 
   const fork = require('child_process').fork;
-  var child = fork(process.argv[1], ['child']);
+  const child = fork(process.argv[1], ['child']);
 
   child.once('message', common.mustCall(function(data) {
     assert.deepStrictEqual(data, normal);
diff --git a/test/parallel/test-child-process-send-returns-boolean.js b/test/parallel/test-child-process-send-returns-boolean.js
index 941a2da645..460c2b0022 100644
--- a/test/parallel/test-child-process-send-returns-boolean.js
+++ b/test/parallel/test-child-process-send-returns-boolean.js
@@ -15,7 +15,7 @@ assert.strictEqual(rv, true);
 
 const spawnOptions = { stdio: ['pipe', 'pipe', 'pipe', 'ipc'] };
 const s = spawn(process.execPath, [emptyFile], spawnOptions);
-var handle = null;
+let handle = null;
 s.on('exit', function() {
   handle.close();
 });
diff --git a/test/parallel/test-child-process-set-blocking.js b/test/parallel/test-child-process-set-blocking.js
index 5a542fa97e..54037aea69 100644
--- a/test/parallel/test-child-process-set-blocking.js
+++ b/test/parallel/test-child-process-set-blocking.js
@@ -3,9 +3,9 @@ const common = require('../common');
 const assert = require('assert');
 const ch = require('child_process');
 
-var SIZE = 100000;
+const SIZE = 100000;
 
-var cp = ch.spawn('python', ['-c', 'print ' + SIZE + ' * "C"'], {
+const cp = ch.spawn('python', ['-c', 'print ' + SIZE + ' * "C"'], {
   stdio: 'inherit'
 });
 
diff --git a/test/parallel/test-child-process-silent.js b/test/parallel/test-child-process-silent.js
index e087ae1cde..3bfa9f5f7d 100644
--- a/test/parallel/test-child-process-silent.js
+++ b/test/parallel/test-child-process-silent.js
@@ -32,15 +32,15 @@ if (process.argv[2] === 'pipe') {
   // testcase | start parent && child IPC test
 
   // testing: is stderr and stdout piped to parent
-  var args = [process.argv[1], 'parent'];
-  var parent = childProcess.spawn(process.execPath, args);
+  const args = [process.argv[1], 'parent'];
+  const parent = childProcess.spawn(process.execPath, args);
 
   //got any stderr or std data
-  var stdoutData = false;
+  let stdoutData = false;
   parent.stdout.on('data', function() {
     stdoutData = true;
   });
-  var stderrData = false;
+  let stderrData = false;
   parent.stdout.on('data', function() {
     stderrData = true;
   });
@@ -52,8 +52,8 @@ if (process.argv[2] === 'pipe') {
   child.stderr.pipe(process.stderr, {end: false});
   child.stdout.pipe(process.stdout, {end: false});
 
-  var childSending = false;
-  var childReciveing = false;
+  let childSending = false;
+  let childReciveing = false;
   child.on('message', function(message) {
     if (childSending === false) {
       childSending = (message === 'message from child');
diff --git a/test/parallel/test-child-process-spawn-typeerror.js b/test/parallel/test-child-process-spawn-typeerror.js
index a3ae3d36f5..400a9fca72 100644
--- a/test/parallel/test-child-process-spawn-typeerror.js
+++ b/test/parallel/test-child-process-spawn-typeerror.js
@@ -14,7 +14,7 @@ const invalidFileMsg =
 const empty = common.fixturesDir + '/empty.js';
 
 assert.throws(function() {
-  var child = spawn(invalidcmd, 'this is not an array');
+  const child = spawn(invalidcmd, 'this is not an array');
   child.on('error', common.fail);
 }, TypeError);
 
diff --git a/test/parallel/test-child-process-spawnsync-env.js b/test/parallel/test-child-process-spawnsync-env.js
index 05ef18bf72..6f5ea3dc83 100644
--- a/test/parallel/test-child-process-spawnsync-env.js
+++ b/test/parallel/test-child-process-spawnsync-env.js
@@ -6,8 +6,8 @@ const cp = require('child_process');
 if (process.argv[2] === 'child') {
   console.log(process.env.foo);
 } else {
-  var expected = 'bar';
-  var child = cp.spawnSync(process.execPath, [__filename, 'child'], {
+  const expected = 'bar';
+  const child = cp.spawnSync(process.execPath, [__filename, 'child'], {
     env: Object.assign(process.env, { foo: expected })
   });
 
diff --git a/test/parallel/test-child-process-spawnsync-input.js b/test/parallel/test-child-process-spawnsync-input.js
index 8c0464facd..035f757e51 100644
--- a/test/parallel/test-child-process-spawnsync-input.js
+++ b/test/parallel/test-child-process-spawnsync-input.js
@@ -17,7 +17,7 @@ const args = [
   `console.log("${msgOut}"); console.error("${msgErr}");`
 ];
 
-var ret;
+let ret;
 
 
 function checkSpawnSyncRet(ret) {
@@ -51,7 +51,7 @@ if (process.argv.indexOf('spawnchild') !== -1) {
 verifyBufOutput(spawnSync(process.execPath, [__filename, 'spawnchild', 1]));
 verifyBufOutput(spawnSync(process.execPath, [__filename, 'spawnchild', 2]));
 
-var options = {
+let options = {
   input: 1234
 };
 
diff --git a/test/parallel/test-child-process-spawnsync-timeout.js b/test/parallel/test-child-process-spawnsync-timeout.js
index 207315c531..abebe41e6e 100644
--- a/test/parallel/test-child-process-spawnsync-timeout.js
+++ b/test/parallel/test-child-process-spawnsync-timeout.js
@@ -4,8 +4,8 @@ const assert = require('assert');
 
 const spawnSync = require('child_process').spawnSync;
 
-var TIMER = 200;
-var SLEEP = 5000;
+const TIMER = 200;
+const SLEEP = 5000;
 
 switch (process.argv[2]) {
   case 'child':
@@ -15,12 +15,12 @@ switch (process.argv[2]) {
     }, SLEEP);
     break;
   default:
-    var start = Date.now();
-    var ret = spawnSync(process.execPath, [__filename, 'child'],
+    const start = Date.now();
+    const ret = spawnSync(process.execPath, [__filename, 'child'],
                         {timeout: TIMER});
     assert.strictEqual(ret.error.errno, 'ETIMEDOUT');
     console.log(ret);
-    var end = Date.now() - start;
+    const end = Date.now() - start;
     assert(end < SLEEP);
     assert(ret.status > 128 || ret.signal);
     break;
diff --git a/test/parallel/test-child-process-stdin-ipc.js b/test/parallel/test-child-process-stdin-ipc.js
index c4a8064680..2623e2deee 100644
--- a/test/parallel/test-child-process-stdin-ipc.js
+++ b/test/parallel/test-child-process-stdin-ipc.js
@@ -10,7 +10,7 @@ if (process.argv[2] === 'child') {
   return;
 }
 
-var proc = spawn(process.execPath, [__filename, 'child'], {
+const proc = spawn(process.execPath, [__filename, 'child'], {
   stdio: ['ipc', 'inherit', 'inherit']
 });
 
diff --git a/test/parallel/test-child-process-stdio-big-write-end.js b/test/parallel/test-child-process-stdio-big-write-end.js
index 4c9f4159d0..24991492f6 100644
--- a/test/parallel/test-child-process-stdio-big-write-end.js
+++ b/test/parallel/test-child-process-stdio-big-write-end.js
@@ -28,8 +28,9 @@ function parent() {
   });
 
   // Write until the buffer fills up.
+  let buf;
   do {
-    var buf = Buffer.alloc(BUFSIZE, '.');
+    buf = Buffer.alloc(BUFSIZE, '.');
     sent += BUFSIZE;
   } while (child.stdin.write(buf));
 
diff --git a/test/parallel/test-child-process-stdio-inherit.js b/test/parallel/test-child-process-stdio-inherit.js
index f7b992e4e7..d2b64aeb30 100644
--- a/test/parallel/test-child-process-stdio-inherit.js
+++ b/test/parallel/test-child-process-stdio-inherit.js
@@ -9,10 +9,10 @@ else
   grandparent();
 
 function grandparent() {
-  var child = spawn(process.execPath, [__filename, 'parent']);
+  const child = spawn(process.execPath, [__filename, 'parent']);
   child.stderr.pipe(process.stderr);
-  var output = '';
-  var input = 'asdfasdf';
+  let output = '';
+  const input = 'asdfasdf';
 
   child.stdout.on('data', function(chunk) {
     output += chunk;
diff --git a/test/parallel/test-cli-eval.js b/test/parallel/test-cli-eval.js
index 8db7ad3087..9a1714cb6e 100644
--- a/test/parallel/test-cli-eval.js
+++ b/test/parallel/test-cli-eval.js
@@ -14,7 +14,7 @@ const nodejs = '"' + process.execPath + '"';
 
 // replace \ by / because windows uses backslashes in paths, but they're still
 // interpreted as the escape character when put between quotes.
-var filename = __filename.replace(/\\/g, '/');
+const filename = __filename.replace(/\\/g, '/');
 
 // assert that nothing is written to stdout
 child.exec(nodejs + ' --eval 42',
@@ -32,7 +32,7 @@ child.exec(nodejs + ' --eval "console.error(42)"',
 
 // assert that the expected output is written to stdout
 ['--print', '-p -e', '-pe', '-p'].forEach(function(s) {
-  var cmd = nodejs + ' ' + s + ' ';
+  const cmd = nodejs + ' ' + s + ' ';
 
   child.exec(cmd + '42',
              function(err, stdout, stderr) {
diff --git a/test/parallel/test-cli-syntax.js b/test/parallel/test-cli-syntax.js
index 839e50d7d9..d7781eddff 100644
--- a/test/parallel/test-cli-syntax.js
+++ b/test/parallel/test-cli-syntax.js
@@ -5,10 +5,10 @@ const assert = require('assert');
 const spawnSync = require('child_process').spawnSync;
 const path = require('path');
 
-var node = process.execPath;
+const node = process.execPath;
 
 // test both sets of arguments that check syntax
-var syntaxArgs = [
+const syntaxArgs = [
   ['-c'],
   ['--check']
 ];
@@ -25,8 +25,8 @@ var syntaxArgs = [
 
   // loop each possible option, `-c` or `--check`
   syntaxArgs.forEach(function(args) {
-    var _args = args.concat(file);
-    var c = spawnSync(node, _args, {encoding: 'utf8'});
+    const _args = args.concat(file);
+    const c = spawnSync(node, _args, {encoding: 'utf8'});
 
     // no output should be produced
     assert.strictEqual(c.stdout, '', 'stdout produced');
@@ -46,14 +46,14 @@ var syntaxArgs = [
 
   // loop each possible option, `-c` or `--check`
   syntaxArgs.forEach(function(args) {
-    var _args = args.concat(file);
-    var c = spawnSync(node, _args, {encoding: 'utf8'});
+    const _args = args.concat(file);
+    const c = spawnSync(node, _args, {encoding: 'utf8'});
 
     // no stdout should be produced
     assert.strictEqual(c.stdout, '', 'stdout produced');
 
     // stderr should have a syntax error message
-    var match = c.stderr.match(/^SyntaxError: Unexpected identifier$/m);
+    const match = c.stderr.match(/^SyntaxError: Unexpected identifier$/m);
     assert(match, 'stderr incorrect');
 
     assert.strictEqual(c.status, 1, 'code == ' + c.status);
@@ -69,14 +69,14 @@ var syntaxArgs = [
 
   // loop each possible option, `-c` or `--check`
   syntaxArgs.forEach(function(args) {
-    var _args = args.concat(file);
-    var c = spawnSync(node, _args, {encoding: 'utf8'});
+    const _args = args.concat(file);
+    const c = spawnSync(node, _args, {encoding: 'utf8'});
 
     // no stdout should be produced
     assert.strictEqual(c.stdout, '', 'stdout produced');
 
     // stderr should have a module not found error message
-    var match = c.stderr.match(/^Error: Cannot find module/m);
+    const match = c.stderr.match(/^Error: Cannot find module/m);
     assert(match, 'stderr incorrect');
 
     assert.strictEqual(c.status, 1, 'code == ' + c.status);
diff --git a/test/parallel/test-cluster-basic.js b/test/parallel/test-cluster-basic.js
index 0ffdcf180b..92251b7324 100644
--- a/test/parallel/test-cluster-basic.js
+++ b/test/parallel/test-cluster-basic.js
@@ -56,7 +56,6 @@ if (cluster.isWorker) {
     }
   };
 
-  var worker;
   const stateNames = Object.keys(checks.worker.states);
 
   //Check events, states, and emit arguments
@@ -72,7 +71,7 @@ if (cluster.isWorker) {
       checks.cluster.equal[name] = worker === arguments[0];
 
       //Check state
-      var state = stateNames[index];
+      const state = stateNames[index];
       checks.worker.states[state] = (state === worker.state);
     }));
   });
@@ -86,7 +85,7 @@ if (cluster.isWorker) {
   cluster.on('exit', common.mustCall(() => {}));
 
   //Create worker
-  worker = cluster.fork();
+  const worker = cluster.fork();
   assert.strictEqual(worker.id, 1);
   assert(worker instanceof cluster.Worker,
          'the worker is not a instance of the Worker constructor');
diff --git a/test/parallel/test-cluster-bind-privileged-port.js b/test/parallel/test-cluster-bind-privileged-port.js
index 228e1f0f1e..3e062697fd 100644
--- a/test/parallel/test-cluster-bind-privileged-port.js
+++ b/test/parallel/test-cluster-bind-privileged-port.js
@@ -19,7 +19,7 @@ if (cluster.isMaster) {
     assert.strictEqual(exitCode, 0);
   }));
 } else {
-  var s = net.createServer(common.fail);
+  const s = net.createServer(common.fail);
   s.listen(42, common.fail.bind(null, 'listen should have failed'));
   s.on('error', common.mustCall((err) => {
     assert.strictEqual(err.code, 'EACCES');
diff --git a/test/parallel/test-cluster-dgram-1.js b/test/parallel/test-cluster-dgram-1.js
index 3b27fa8e20..2539199775 100644
--- a/test/parallel/test-cluster-dgram-1.js
+++ b/test/parallel/test-cluster-dgram-1.js
@@ -21,10 +21,10 @@ else
 
 
 function master() {
-  var listening = 0;
+  let listening = 0;
 
   // Fork 4 workers.
-  for (var i = 0; i < NUM_WORKERS; i++)
+  for (let i = 0; i < NUM_WORKERS; i++)
     cluster.fork();
 
   // Wait until all workers are listening.
@@ -35,7 +35,7 @@ function master() {
     // Start sending messages.
     const buf = Buffer.from('hello world');
     const socket = dgram.createSocket('udp4');
-    var sent = 0;
+    let sent = 0;
     doSend();
 
     function doSend() {
@@ -60,7 +60,7 @@ function master() {
   }
 
   function setupWorker(worker) {
-    var received = 0;
+    let received = 0;
 
     worker.on('message', common.mustCall((msg) => {
       received = msg.received;
@@ -75,10 +75,10 @@ function master() {
 
 
 function worker() {
-  var received = 0;
+  let received = 0;
 
   // Create udp socket and start listening.
-  var socket = dgram.createSocket('udp4');
+  const socket = dgram.createSocket('udp4');
 
   socket.on('message', common.mustCall((data, info) => {
     received++;
diff --git a/test/parallel/test-cluster-dgram-2.js b/test/parallel/test-cluster-dgram-2.js
index 863e0fa358..37aea29e1a 100644
--- a/test/parallel/test-cluster-dgram-2.js
+++ b/test/parallel/test-cluster-dgram-2.js
@@ -20,10 +20,10 @@ else
 
 
 function master() {
-  var received = 0;
+  let received = 0;
 
   // Start listening on a socket.
-  var socket = dgram.createSocket('udp4');
+  const socket = dgram.createSocket('udp4');
   socket.bind(common.PORT);
 
   // Disconnect workers when the expected number of messages have been
@@ -42,7 +42,7 @@ function master() {
   }, NUM_WORKERS * PACKETS_PER_WORKER));
 
   // Fork workers.
-  for (var i = 0; i < NUM_WORKERS; i++)
+  for (let i = 0; i < NUM_WORKERS; i++)
     cluster.fork();
 }
 
diff --git a/test/parallel/test-cluster-dgram-reuse.js b/test/parallel/test-cluster-dgram-reuse.js
index 27fc1be985..aed565a380 100644
--- a/test/parallel/test-cluster-dgram-reuse.js
+++ b/test/parallel/test-cluster-dgram-reuse.js
@@ -25,16 +25,16 @@ function next() {
 
   // Work around health check issue
   process.nextTick(() => {
-    for (var i = 0; i < sockets.length; i++)
+    for (let i = 0; i < sockets.length; i++)
       sockets[i].close(close);
   });
 }
 
-var waiting = 2;
+let waiting = 2;
 function close() {
   if (--waiting === 0)
     cluster.worker.disconnect();
 }
 
-for (var i = 0; i < 2; i++)
+for (let i = 0; i < 2; i++)
   dgram.createSocket({ type: 'udp4', reuseAddr: true }).bind(common.PORT, next);
diff --git a/test/parallel/test-cluster-disconnect-race.js b/test/parallel/test-cluster-disconnect-race.js
index e89b9e134f..09966cbab8 100644
--- a/test/parallel/test-cluster-disconnect-race.js
+++ b/test/parallel/test-cluster-disconnect-race.js
@@ -16,9 +16,9 @@ if (common.isWindows) {
 cluster.schedulingPolicy = cluster.SCHED_NONE;
 
 if (cluster.isMaster) {
-  var worker1, worker2;
+  let worker2;
 
-  worker1 = cluster.fork();
+  const worker1 = cluster.fork();
   worker1.on('message', common.mustCall(function() {
     worker2 = cluster.fork();
     worker1.disconnect();
@@ -32,7 +32,7 @@ if (cluster.isMaster) {
   return;
 }
 
-var server = net.createServer();
+const server = net.createServer();
 
 server.listen(common.PORT, function() {
   process.send('listening');
diff --git a/test/parallel/test-cluster-disconnect-suicide-race.js b/test/parallel/test-cluster-disconnect-suicide-race.js
index 6f23b23fd2..15d0700225 100644
--- a/test/parallel/test-cluster-disconnect-suicide-race.js
+++ b/test/parallel/test-cluster-disconnect-suicide-race.js
@@ -14,7 +14,7 @@ if (cluster.isMaster) {
   return cluster.fork();
 }
 
-var eventFired = false;
+let eventFired = false;
 
 cluster.worker.disconnect();
 
diff --git a/test/parallel/test-cluster-disconnect-with-no-workers.js b/test/parallel/test-cluster-disconnect-with-no-workers.js
index 4b38720d71..15c5898a7d 100644
--- a/test/parallel/test-cluster-disconnect-with-no-workers.js
+++ b/test/parallel/test-cluster-disconnect-with-no-workers.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const cluster = require('cluster');
 
-var disconnected;
+let disconnected;
 
 process.on('exit', function() {
   assert(disconnected);
diff --git a/test/parallel/test-cluster-disconnect.js b/test/parallel/test-cluster-disconnect.js
index 4609724130..0fc1611e2d 100644
--- a/test/parallel/test-cluster-disconnect.js
+++ b/test/parallel/test-cluster-disconnect.js
@@ -14,13 +14,13 @@ if (cluster.isWorker) {
   }).listen(common.PORT + 1, '127.0.0.1');
 
 } else if (cluster.isMaster) {
-  var servers = 2;
+  const servers = 2;
 
   // test a single TCP server
   const testConnection = function(port, cb) {
-    var socket = net.connect(port, '127.0.0.1', () => {
+    const socket = net.connect(port, '127.0.0.1', () => {
       // buffer result
-      var result = '';
+      let result = '';
       socket.on('data', common.mustCall((chunk) => { result += chunk; }));
 
       // check result
@@ -32,9 +32,9 @@ if (cluster.isWorker) {
 
   // test both servers created in the cluster
   const testCluster = function(cb) {
-    var done = 0;
+    let done = 0;
 
-    for (var i = 0, l = servers; i < l; i++) {
+    for (let i = 0, l = servers; i < l; i++) {
       testConnection(common.PORT + i, (success) => {
         assert.ok(success);
         done += 1;
@@ -47,10 +47,10 @@ if (cluster.isWorker) {
 
   // start two workers and execute callback when both is listening
   const startCluster = function(cb) {
-    var workers = 8;
-    var online = 0;
+    const workers = 8;
+    let online = 0;
 
-    for (var i = 0, l = workers; i < l; i++) {
+    for (let i = 0, l = workers; i < l; i++) {
       cluster.fork().on('listening', common.mustCall(() => {
         online += 1;
         if (online === workers * servers) {
diff --git a/test/parallel/test-cluster-eaccess.js b/test/parallel/test-cluster-eaccess.js
index 2de4af8b1a..75b60248a2 100644
--- a/test/parallel/test-cluster-eaccess.js
+++ b/test/parallel/test-cluster-eaccess.js
@@ -37,7 +37,7 @@ if (cluster.isMaster) {
 
 } else {
   common.refreshTmpDir();
-  var cp = fork(common.fixturesDir + '/listen-on-socket-and-exit.js',
+  const cp = fork(common.fixturesDir + '/listen-on-socket-and-exit.js',
                 { stdio: 'inherit' });
 
   // message from the child indicates it's ready and listening
diff --git a/test/parallel/test-cluster-eaddrinuse.js b/test/parallel/test-cluster-eaddrinuse.js
index 8b94845635..a0936ef4b9 100644
--- a/test/parallel/test-cluster-eaddrinuse.js
+++ b/test/parallel/test-cluster-eaddrinuse.js
@@ -8,12 +8,12 @@ const assert = require('assert');
 const fork = require('child_process').fork;
 const net = require('net');
 
-var id = '' + process.argv[2];
+const id = '' + process.argv[2];
 
 if (id === 'undefined') {
   const server = net.createServer(common.fail);
   server.listen(common.PORT, function() {
-    var worker = fork(__filename, ['worker']);
+    const worker = fork(__filename, ['worker']);
     worker.on('message', function(msg) {
       if (msg !== 'stop-listening') return;
       server.close(function() {
diff --git a/test/parallel/test-cluster-fork-env.js b/test/parallel/test-cluster-fork-env.js
index 735ff73d2a..55ea684f3b 100644
--- a/test/parallel/test-cluster-fork-env.js
+++ b/test/parallel/test-cluster-fork-env.js
@@ -12,7 +12,7 @@ if (cluster.isWorker) {
   assert.strictEqual(result, true);
 } else if (cluster.isMaster) {
 
-  var checks = {
+  const checks = {
     using: false,
     overwrite: false
   };
@@ -22,7 +22,7 @@ if (cluster.isWorker) {
   process.env['cluster_test_overwrite'] = 'old';
 
   // Fork worker
-  var worker = cluster.fork({
+  const worker = cluster.fork({
     'cluster_test_prop': 'custom',
     'cluster_test_overwrite': 'new'
   });
diff --git a/test/parallel/test-cluster-master-error.js b/test/parallel/test-cluster-master-error.js
index 6f9c78a7ba..11ed09c64e 100644
--- a/test/parallel/test-cluster-master-error.js
+++ b/test/parallel/test-cluster-master-error.js
@@ -15,7 +15,7 @@ if (cluster.isWorker) {
 } else if (process.argv[2] === 'cluster') {
 
   // Send PID to testcase process
-  var forkNum = 0;
+  let forkNum = 0;
   cluster.on('fork', common.mustCall(function forkEvent(worker) {
 
     // Send PID
@@ -31,7 +31,7 @@ if (cluster.isWorker) {
   }));
 
   // Throw accidental error when all workers are listening
-  var listeningNum = 0;
+  let listeningNum = 0;
   cluster.on('listening', common.mustCall(function listeningEvent() {
 
     // When all workers are listening
@@ -56,8 +56,8 @@ if (cluster.isWorker) {
 
   const fork = require('child_process').fork;
 
-  var masterExited = false;
-  var workersExited = false;
+  let masterExited = false;
+  let workersExited = false;
 
   // List all workers
   const workers = [];
@@ -82,7 +82,7 @@ if (cluster.isWorker) {
 
     const pollWorkers = function() {
       // When master is dead all workers should be dead too
-      var alive = false;
+      let alive = false;
       workers.forEach((pid) => alive = common.isAlive(pid));
       if (alive) {
         setTimeout(pollWorkers, 50);
diff --git a/test/parallel/test-cluster-master-kill.js b/test/parallel/test-cluster-master-kill.js
index 44eedec47b..094e5ad256 100644
--- a/test/parallel/test-cluster-master-kill.js
+++ b/test/parallel/test-cluster-master-kill.js
@@ -34,13 +34,13 @@ if (cluster.isWorker) {
   const master = fork(process.argv[1], ['cluster']);
 
   // get pid info
-  var pid = null;
+  let pid = null;
   master.once('message', (data) => {
     pid = data.pid;
   });
 
   // When master is dead
-  var alive = true;
+  let alive = true;
   master.on('exit', common.mustCall((code) => {
 
     // make sure that the master died on purpose
diff --git a/test/parallel/test-cluster-message.js b/test/parallel/test-cluster-message.js
index a86b469e33..5b537bbe73 100644
--- a/test/parallel/test-cluster-message.js
+++ b/test/parallel/test-cluster-message.js
@@ -13,8 +13,8 @@ function forEach(obj, fn) {
 if (cluster.isWorker) {
   // Create a tcp server. This will be used as cluster-shared-server and as an
   // alternative IPC channel.
-  var server = net.Server();
-  var socket, message;
+  const server = net.Server();
+  let socket, message;
 
   function maybeReply() {
     if (!socket || !message) return;
@@ -42,7 +42,7 @@ if (cluster.isWorker) {
   server.listen(common.PORT, '127.0.0.1');
 } else if (cluster.isMaster) {
 
-  var checks = {
+  const checks = {
     global: {
       'receive': false,
       'correct': false
@@ -58,13 +58,13 @@ if (cluster.isWorker) {
   };
 
 
-  var client;
-  var check = function(type, result) {
+  let client;
+  const check = function(type, result) {
     checks[type].receive = true;
     checks[type].correct = result;
     console.error('check', checks);
 
-    var missing = false;
+    let missing = false;
     forEach(checks, function(type) {
       if (type.receive === false) missing = true;
     });
@@ -76,7 +76,7 @@ if (cluster.isWorker) {
   };
 
   // Spawn worker
-  var worker = cluster.fork();
+  const worker = cluster.fork();
 
   // When a IPC message is received from the worker
   worker.on('message', function(message) {
diff --git a/test/parallel/test-cluster-rr-domain-listen.js b/test/parallel/test-cluster-rr-domain-listen.js
index 0f39c47ed2..10b0530b50 100644
--- a/test/parallel/test-cluster-rr-domain-listen.js
+++ b/test/parallel/test-cluster-rr-domain-listen.js
@@ -7,14 +7,13 @@ const domain = require('domain');
 // cluster.schedulingPolicy = cluster.SCHED_RR;
 
 if (cluster.isWorker) {
-  var d = domain.create();
+  const d = domain.create();
   d.run(function() { });
 
   const http = require('http');
   http.Server(function() { }).listen(common.PORT, '127.0.0.1');
 
 } else if (cluster.isMaster) {
-  var worker;
 
   //Kill worker when listening
   cluster.on('listening', function() {
@@ -27,5 +26,5 @@ if (cluster.isWorker) {
   });
 
   //Create worker
-  worker = cluster.fork();
+  const worker = cluster.fork();
 }
diff --git a/test/parallel/test-cluster-send-deadlock.js b/test/parallel/test-cluster-send-deadlock.js
index 062ebf2b13..3fa64ac0b5 100644
--- a/test/parallel/test-cluster-send-deadlock.js
+++ b/test/parallel/test-cluster-send-deadlock.js
@@ -8,14 +8,14 @@ const cluster = require('cluster');
 const net = require('net');
 
 if (cluster.isMaster) {
-  var worker = cluster.fork();
+  const worker = cluster.fork();
   worker.on('exit', function(code, signal) {
     assert.strictEqual(code, 0, 'Worker exited with an error code');
     assert(!signal, 'Worker exited by a signal');
     server.close();
   });
 
-  var server = net.createServer(function(socket) {
+  const server = net.createServer(function(socket) {
     worker.send('handle', socket);
   });
 
@@ -25,9 +25,9 @@ if (cluster.isMaster) {
 } else {
   process.on('message', function(msg, handle) {
     if (msg === 'listen') {
-      var client1 = net.connect({ host: 'localhost', port: common.PORT });
-      var client2 = net.connect({ host: 'localhost', port: common.PORT });
-      var waiting = 2;
+      const client1 = net.connect({ host: 'localhost', port: common.PORT });
+      const client2 = net.connect({ host: 'localhost', port: common.PORT });
+      let waiting = 2;
       client1.on('close', onclose);
       client2.on('close', onclose);
       function onclose() {
diff --git a/test/parallel/test-cluster-setup-master-argv.js b/test/parallel/test-cluster-setup-master-argv.js
index 32c5a91b3c..72211918ef 100644
--- a/test/parallel/test-cluster-setup-master-argv.js
+++ b/test/parallel/test-cluster-setup-master-argv.js
@@ -6,8 +6,8 @@ const cluster = require('cluster');
 setTimeout(common.fail.bind(assert, 'setup not emitted'), 1000).unref();
 
 cluster.on('setup', common.mustCall(function() {
-  var clusterArgs = cluster.settings.args;
-  var realArgs = process.argv;
+  const clusterArgs = cluster.settings.args;
+  const realArgs = process.argv;
   assert.strictEqual(clusterArgs[clusterArgs.length - 1],
                      realArgs[realArgs.length - 1]);
 }));
diff --git a/test/parallel/test-cluster-setup-master-multiple.js b/test/parallel/test-cluster-setup-master-multiple.js
index a7611b3c23..25cff7cb33 100644
--- a/test/parallel/test-cluster-setup-master-multiple.js
+++ b/test/parallel/test-cluster-setup-master-multiple.js
@@ -13,7 +13,7 @@ function cheapClone(obj) {
   return JSON.parse(JSON.stringify(obj));
 }
 
-var configs = [];
+const configs = [];
 
 // Capture changes
 cluster.on('setup', function() {
@@ -21,7 +21,7 @@ cluster.on('setup', function() {
   configs.push(cheapClone(cluster.settings));
 });
 
-var execs = [
+const execs = [
   'node-next',
   'node-next-2',
   'node-next-3',
diff --git a/test/parallel/test-cluster-setup-master.js b/test/parallel/test-cluster-setup-master.js
index 4590309f93..5a92b988e6 100644
--- a/test/parallel/test-cluster-setup-master.js
+++ b/test/parallel/test-cluster-setup-master.js
@@ -10,14 +10,14 @@ if (cluster.isWorker) {
 
 } else if (cluster.isMaster) {
 
-  var checks = {
+  const checks = {
     args: false,
     setupEvent: false,
     settingsObject: false
   };
 
-  var totalWorkers = 2;
-  var onlineWorkers = 0;
+  const totalWorkers = 2;
+  let onlineWorkers = 0;
 
   // Setup master
   cluster.setupMaster({
@@ -28,7 +28,7 @@ if (cluster.isWorker) {
   cluster.once('setup', function() {
     checks.setupEvent = true;
 
-    var settings = cluster.settings;
+    const settings = cluster.settings;
     if (settings &&
         settings.args && settings.args[0] === 'custom argument' &&
         settings.silent === true &&
@@ -37,7 +37,7 @@ if (cluster.isWorker) {
     }
   });
 
-  var correctIn = 0;
+  let correctIn = 0;
 
   cluster.on('online', function lisenter(worker) {
 
@@ -66,7 +66,7 @@ if (cluster.isWorker) {
     assert.ok(checks.workers, 'Not all workers went online');
     assert.ok(checks.args, 'The arguments was noy send to the worker');
     assert.ok(checks.setupEvent, 'The setup event was never emitted');
-    var m = 'The settingsObject do not have correct properties';
+    const m = 'The settingsObject do not have correct properties';
     assert.ok(checks.settingsObject, m);
   });
 
diff --git a/test/parallel/test-cluster-shared-leak.js b/test/parallel/test-cluster-shared-leak.js
index ad13a869b2..d578ee6373 100644
--- a/test/parallel/test-cluster-shared-leak.js
+++ b/test/parallel/test-cluster-shared-leak.js
@@ -10,9 +10,9 @@ const cluster = require('cluster');
 cluster.schedulingPolicy = cluster.SCHED_NONE;
 
 if (cluster.isMaster) {
-  var conn, worker1, worker2;
+  let conn, worker2;
 
-  worker1 = cluster.fork();
+  const worker1 = cluster.fork();
   worker1.on('message', common.mustCall(function() {
     worker2 = cluster.fork();
     worker2.on('online', function() {
diff --git a/test/parallel/test-cluster-worker-death.js b/test/parallel/test-cluster-worker-death.js
index 6435232a81..7a33721a9a 100644
--- a/test/parallel/test-cluster-worker-death.js
+++ b/test/parallel/test-cluster-worker-death.js
@@ -6,7 +6,7 @@ const cluster = require('cluster');
 if (!cluster.isMaster) {
   process.exit(42);
 } else {
-  var worker = cluster.fork();
+  const worker = cluster.fork();
   worker.on('exit', common.mustCall(function(exitCode, signalCode) {
     assert.strictEqual(exitCode, 42);
     assert.strictEqual(signalCode, null);
diff --git a/test/parallel/test-cluster-worker-destroy.js b/test/parallel/test-cluster-worker-destroy.js
index 30da5954b2..6200025424 100644
--- a/test/parallel/test-cluster-worker-destroy.js
+++ b/test/parallel/test-cluster-worker-destroy.js
@@ -10,7 +10,7 @@
 const common = require('../common');
 const assert = require('assert');
 const cluster = require('cluster');
-var worker1, worker2;
+let worker1, worker2;
 
 if (cluster.isMaster) {
   worker1 = cluster.fork();
diff --git a/test/parallel/test-cluster-worker-events.js b/test/parallel/test-cluster-worker-events.js
index b77a32353b..6c2e278f5e 100644
--- a/test/parallel/test-cluster-worker-events.js
+++ b/test/parallel/test-cluster-worker-events.js
@@ -3,11 +3,11 @@ require('../common');
 const assert = require('assert');
 const cluster = require('cluster');
 
-var OK = 2;
+const OK = 2;
 
 if (cluster.isMaster) {
 
-  var worker = cluster.fork();
+  const worker = cluster.fork();
 
   worker.on('exit', function(code) {
     assert.strictEqual(code, OK);
@@ -25,8 +25,8 @@ if (cluster.isMaster) {
 
 assert(cluster.isWorker);
 
-var sawProcess;
-var sawWorker;
+let sawProcess;
+let sawWorker;
 
 process.on('message', function(m) {
   assert(!sawProcess);
@@ -40,7 +40,7 @@ cluster.worker.on('message', function(m) {
   check(m);
 });
 
-var messages = [];
+const messages = [];
 
 function check(m) {
   messages.push(m);
diff --git a/test/parallel/test-cluster-worker-forced-exit.js b/test/parallel/test-cluster-worker-forced-exit.js
index 1d553a9269..8281882547 100644
--- a/test/parallel/test-cluster-worker-forced-exit.js
+++ b/test/parallel/test-cluster-worker-forced-exit.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const cluster = require('cluster');
 
-var SENTINEL = 42;
+const SENTINEL = 42;
 
 // workers forcibly exit when control channel is disconnected, if
 // their .exitedAfterDisconnect flag isn't set
diff --git a/test/parallel/test-cluster-worker-isconnected.js b/test/parallel/test-cluster-worker-isconnected.js
index 3b17b53e8a..8b2ad86589 100644
--- a/test/parallel/test-cluster-worker-isconnected.js
+++ b/test/parallel/test-cluster-worker-isconnected.js
@@ -4,7 +4,7 @@ const cluster = require('cluster');
 const assert = require('assert');
 
 if (cluster.isMaster) {
-  var worker = cluster.fork();
+  const worker = cluster.fork();
 
   assert.ok(worker.isConnected(),
             'isConnected() should return true as soon as the worker has ' +
diff --git a/test/parallel/test-cluster-worker-isdead.js b/test/parallel/test-cluster-worker-isdead.js
index a80f8410b1..cd360ff248 100644
--- a/test/parallel/test-cluster-worker-isdead.js
+++ b/test/parallel/test-cluster-worker-isdead.js
@@ -4,7 +4,7 @@ const cluster = require('cluster');
 const assert = require('assert');
 
 if (cluster.isMaster) {
-  var worker = cluster.fork();
+  const worker = cluster.fork();
   assert.ok(!worker.isDead(),
             'isDead() should return false right after the worker has been ' +
             'created.');
diff --git a/test/parallel/test-cluster-worker-no-exit.js b/test/parallel/test-cluster-worker-no-exit.js
index 9921256c4a..9cfd8401a0 100644
--- a/test/parallel/test-cluster-worker-no-exit.js
+++ b/test/parallel/test-cluster-worker-no-exit.js
@@ -4,10 +4,10 @@ const assert = require('assert');
 const cluster = require('cluster');
 const net = require('net');
 
-var destroyed;
-var success;
-var worker;
-var server;
+let destroyed;
+let success;
+let worker;
+let server;
 
 // workers do not exit on disconnect, they exit under normal node rules: when
 // they have nothing keeping their loop alive, like an active connection
@@ -35,7 +35,7 @@ if (cluster.isMaster) {
     });
 
   }).listen(common.PORT, function() {
-    var port = this.address().port;
+    const port = this.address().port;
 
     worker = cluster.fork()
       .on('online', function() {
diff --git a/test/parallel/test-cluster-worker-wait-server-close.js b/test/parallel/test-cluster-worker-wait-server-close.js
index cb925822a0..d1a0d73678 100644
--- a/test/parallel/test-cluster-worker-wait-server-close.js
+++ b/test/parallel/test-cluster-worker-wait-server-close.js
@@ -5,10 +5,10 @@ const assert = require('assert');
 const cluster = require('cluster');
 const net = require('net');
 
-var serverClosed = false;
+let serverClosed = false;
 
 if (cluster.isWorker) {
-  var server = net.createServer(function(socket) {
+  const server = net.createServer(function(socket) {
     // Wait for any data, then close connection
     socket.write('.');
     socket.on('data', function discard() {});
@@ -20,7 +20,7 @@ if (cluster.isWorker) {
 
   // Although not typical, the worker process can exit before the disconnect
   // event fires. Use this to keep the process open until the event has fired.
-  var keepOpen = setInterval(function() {}, 9999);
+  const keepOpen = setInterval(function() {}, 9999);
 
   // Check worker events and properties
   process.once('disconnect', function() {
@@ -30,7 +30,7 @@ if (cluster.isWorker) {
   });
 } else if (cluster.isMaster) {
   // start worker
-  var worker = cluster.fork();
+  const worker = cluster.fork();
 
   // Disconnect worker when it is ready
   worker.once('listening', function() {
diff --git a/test/parallel/test-console-instance.js b/test/parallel/test-console-instance.js
index 4d2727d96b..4488e89373 100644
--- a/test/parallel/test-console-instance.js
+++ b/test/parallel/test-console-instance.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const Stream = require('stream');
 const Console = require('console').Console;
-var called = false;
+let called = false;
 
 const out = new Stream();
 const err = new Stream();
@@ -32,7 +32,7 @@ assert.throws(function() {
 
 out.write = err.write = function(d) {};
 
-var c = new Console(out, err);
+const c = new Console(out, err);
 
 out.write = err.write = function(d) {
   assert.strictEqual(d, 'test\n');
diff --git a/test/parallel/test-console-not-call-toString.js b/test/parallel/test-console-not-call-toString.js
index bf8a59072f..0c1c209374 100644
--- a/test/parallel/test-console-not-call-toString.js
+++ b/test/parallel/test-console-not-call-toString.js
@@ -2,8 +2,8 @@
 require('../common');
 const assert = require('assert');
 
-var func = function() {};
-var toStringCalled = false;
+const func = function() {};
+let toStringCalled = false;
 func.toString = function() {
   toStringCalled = true;
 };
diff --git a/test/parallel/test-crypto-binary-default.js b/test/parallel/test-crypto-binary-default.js
index 33a0783fe9..6ee3aeac36 100644
--- a/test/parallel/test-crypto-binary-default.js
+++ b/test/parallel/test-crypto-binary-default.js
@@ -53,7 +53,7 @@ const hmacHash = crypto.createHmac('sha1', 'Node')
 assert.strictEqual(hmacHash, '19fd6e1ba73d9ed2224dd5094a71babe85d9a892');
 
 // Test HMAC-SHA-* (rfc 4231 Test Cases)
-var rfc4231 = [
+const rfc4231 = [
   {
     key: Buffer.from('0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b', 'hex'),
     data: Buffer.from('4869205468657265', 'hex'), // 'Hi There'
@@ -200,8 +200,8 @@ var rfc4231 = [
 ];
 
 for (let i = 0, l = rfc4231.length; i < l; i++) {
-  for (var hash in rfc4231[i]['hmac']) {
-    var result = crypto.createHmac(hash, rfc4231[i]['key'])
+  for (const hash in rfc4231[i]['hmac']) {
+    let result = crypto.createHmac(hash, rfc4231[i]['key'])
                      .update(rfc4231[i]['data'])
                      .digest('hex');
     if (rfc4231[i]['truncate']) {
@@ -216,7 +216,7 @@ for (let i = 0, l = rfc4231.length; i < l; i++) {
 }
 
 // Test HMAC-MD5/SHA1 (rfc 2202 Test Cases)
-var rfc2202_md5 = [
+const rfc2202_md5 = [
   {
     key: Buffer.from('0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b', 'hex'),
     data: 'Hi There',
@@ -269,7 +269,7 @@ var rfc2202_md5 = [
     hmac: '6f630fad67cda0ee1fb1f562db3aa53e'
   }
 ];
-var rfc2202_sha1 = [
+const rfc2202_sha1 = [
   {
     key: Buffer.from('0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b', 'hex'),
     data: 'Hi There',
@@ -346,13 +346,13 @@ for (let i = 0, l = rfc2202_sha1.length; i < l; i++) {
 }
 
 // Test hashing
-var a1 = crypto.createHash('sha1').update('Test123').digest('hex');
-var a2 = crypto.createHash('sha256').update('Test123').digest('base64');
-var a3 = crypto.createHash('sha512').update('Test123').digest(); // binary
-var a4 = crypto.createHash('sha1').update('Test123').digest('buffer');
+const a1 = crypto.createHash('sha1').update('Test123').digest('hex');
+const a2 = crypto.createHash('sha256').update('Test123').digest('base64');
+const a3 = crypto.createHash('sha512').update('Test123').digest(); // binary
+const a4 = crypto.createHash('sha1').update('Test123').digest('buffer');
 
 if (!common.hasFipsCrypto) {
-  var a0 = crypto.createHash('md5').update('Test123').digest('latin1');
+  const a0 = crypto.createHash('md5').update('Test123').digest('latin1');
   assert.strictEqual(
     a0,
     'h\u00ea\u00cb\u0097\u00d8o\fF!\u00fa+\u000e\u0017\u00ca\u00bd\u008c',
@@ -382,14 +382,14 @@ assert.deepStrictEqual(
 );
 
 // Test multiple updates to same hash
-var h1 = crypto.createHash('sha1').update('Test123').digest('hex');
-var h2 = crypto.createHash('sha1').update('Test').update('123').digest('hex');
+const h1 = crypto.createHash('sha1').update('Test123').digest('hex');
+const h2 = crypto.createHash('sha1').update('Test').update('123').digest('hex');
 assert.strictEqual(h1, h2, 'multipled updates');
 
 // Test hashing for binary files
-var fn = path.join(common.fixturesDir, 'sample.png');
-var sha1Hash = crypto.createHash('sha1');
-var fileStream = fs.createReadStream(fn);
+const fn = path.join(common.fixturesDir, 'sample.png');
+const sha1Hash = crypto.createHash('sha1');
+const fileStream = fs.createReadStream(fn);
 fileStream.on('data', function(data) {
   sha1Hash.update(data);
 });
@@ -407,28 +407,28 @@ assert.throws(function() {
 }, /^Error: Digest method not supported$/);
 
 // Test signing and verifying
-var s1 = crypto.createSign('RSA-SHA1')
+const s1 = crypto.createSign('RSA-SHA1')
                .update('Test123')
                .sign(keyPem, 'base64');
-var s1Verified = crypto.createVerify('RSA-SHA1')
+const s1Verified = crypto.createVerify('RSA-SHA1')
                        .update('Test')
                        .update('123')
                        .verify(certPem, s1, 'base64');
 assert.strictEqual(s1Verified, true, 'sign and verify (base 64)');
 
-var s2 = crypto.createSign('RSA-SHA256')
+const s2 = crypto.createSign('RSA-SHA256')
                .update('Test123')
                .sign(keyPem); // binary
-var s2Verified = crypto.createVerify('RSA-SHA256')
+const s2Verified = crypto.createVerify('RSA-SHA256')
                        .update('Test')
                        .update('123')
                        .verify(certPem, s2); // binary
 assert.strictEqual(s2Verified, true, 'sign and verify (binary)');
 
-var s3 = crypto.createSign('RSA-SHA1')
+const s3 = crypto.createSign('RSA-SHA1')
                .update('Test123')
                .sign(keyPem, 'buffer');
-var s3Verified = crypto.createVerify('RSA-SHA1')
+const s3Verified = crypto.createVerify('RSA-SHA1')
                        .update('Test')
                        .update('123')
                        .verify(certPem, s3);
@@ -437,17 +437,17 @@ assert.strictEqual(s3Verified, true, 'sign and verify (buffer)');
 
 function testCipher1(key) {
   // Test encryption and decryption
-  var plaintext = 'Keep this a secret? No! Tell everyone about node.js!';
-  var cipher = crypto.createCipher('aes192', key);
+  const plaintext = 'Keep this a secret? No! Tell everyone about node.js!';
+  const cipher = crypto.createCipher('aes192', key);
 
   // encrypt plaintext which is in utf8 format
   // to a ciphertext which will be in hex
-  var ciph = cipher.update(plaintext, 'utf8', 'hex');
+  let ciph = cipher.update(plaintext, 'utf8', 'hex');
   // Only use binary or hex, not base64.
   ciph += cipher.final('hex');
 
-  var decipher = crypto.createDecipher('aes192', key);
-  var txt = decipher.update(ciph, 'hex', 'utf8');
+  const decipher = crypto.createDecipher('aes192', key);
+  let txt = decipher.update(ciph, 'hex', 'utf8');
   txt += decipher.final('utf8');
 
   assert.strictEqual(txt, plaintext, 'encryption and decryption');
@@ -457,19 +457,19 @@ function testCipher1(key) {
 function testCipher2(key) {
   // encryption and decryption with Base64
   // reported in https://github.com/joyent/node/issues/738
-  var plaintext =
+  const plaintext =
       '32|RmVZZkFUVmpRRkp0TmJaUm56ZU9qcnJkaXNNWVNpTTU*|iXmckfRWZBGWWELw' +
       'eCBsThSsfUHLeRe0KCsK8ooHgxie0zOINpXxfZi/oNG7uq9JWFVCk70gfzQH8ZUJ' +
       'jAfaFg**';
-  var cipher = crypto.createCipher('aes256', key);
+  const cipher = crypto.createCipher('aes256', key);
 
   // encrypt plaintext which is in utf8 format
   // to a ciphertext which will be in Base64
-  var ciph = cipher.update(plaintext, 'utf8', 'base64');
+  let ciph = cipher.update(plaintext, 'utf8', 'base64');
   ciph += cipher.final('base64');
 
-  var decipher = crypto.createDecipher('aes256', key);
-  var txt = decipher.update(ciph, 'base64', 'utf8');
+  const decipher = crypto.createDecipher('aes256', key);
+  let txt = decipher.update(ciph, 'base64', 'utf8');
   txt += decipher.final('utf8');
 
   assert.strictEqual(txt, plaintext, 'encryption and decryption with Base64');
@@ -478,16 +478,16 @@ function testCipher2(key) {
 
 function testCipher3(key, iv) {
   // Test encyrption and decryption with explicit key and iv
-  var plaintext =
+  const plaintext =
       '32|RmVZZkFUVmpRRkp0TmJaUm56ZU9qcnJkaXNNWVNpTTU*|iXmckfRWZBGWWELw' +
       'eCBsThSsfUHLeRe0KCsK8ooHgxie0zOINpXxfZi/oNG7uq9JWFVCk70gfzQH8ZUJ' +
       'jAfaFg**';
-  var cipher = crypto.createCipheriv('des-ede3-cbc', key, iv);
-  var ciph = cipher.update(plaintext, 'utf8', 'hex');
+  const cipher = crypto.createCipheriv('des-ede3-cbc', key, iv);
+  let ciph = cipher.update(plaintext, 'utf8', 'hex');
   ciph += cipher.final('hex');
 
-  var decipher = crypto.createDecipheriv('des-ede3-cbc', key, iv);
-  var txt = decipher.update(ciph, 'hex', 'utf8');
+  const decipher = crypto.createDecipheriv('des-ede3-cbc', key, iv);
+  let txt = decipher.update(ciph, 'hex', 'utf8');
   txt += decipher.final('utf8');
 
   assert.strictEqual(txt, plaintext,
@@ -497,16 +497,16 @@ function testCipher3(key, iv) {
 
 function testCipher4(key, iv) {
   // Test encyrption and decryption with explicit key and iv
-  var plaintext =
+  const plaintext =
       '32|RmVZZkFUVmpRRkp0TmJaUm56ZU9qcnJkaXNNWVNpTTU*|iXmckfRWZBGWWELw' +
       'eCBsThSsfUHLeRe0KCsK8ooHgxie0zOINpXxfZi/oNG7uq9JWFVCk70gfzQH8ZUJ' +
       'jAfaFg**';
-  var cipher = crypto.createCipheriv('des-ede3-cbc', key, iv);
-  var ciph = cipher.update(plaintext, 'utf8', 'buffer');
+  const cipher = crypto.createCipheriv('des-ede3-cbc', key, iv);
+  let ciph = cipher.update(plaintext, 'utf8', 'buffer');
   ciph = Buffer.concat([ciph, cipher.final('buffer')]);
 
-  var decipher = crypto.createDecipheriv('des-ede3-cbc', key, iv);
-  var txt = decipher.update(ciph, 'buffer', 'utf8');
+  const decipher = crypto.createDecipheriv('des-ede3-cbc', key, iv);
+  let txt = decipher.update(ciph, 'buffer', 'utf8');
   txt += decipher.final('utf8');
 
   assert.strictEqual(txt, plaintext,
@@ -537,20 +537,20 @@ assert.throws(function() {
 
 // Test Diffie-Hellman with two parties sharing a secret,
 // using various encodings as we go along
-var dh1 = crypto.createDiffieHellman(common.hasFipsCrypto ? 1024 : 256);
-var p1 = dh1.getPrime('buffer');
-var dh2 = crypto.createDiffieHellman(p1, 'base64');
-var key1 = dh1.generateKeys();
-var key2 = dh2.generateKeys('hex');
-var secret1 = dh1.computeSecret(key2, 'hex', 'base64');
-var secret2 = dh2.computeSecret(key1, 'latin1', 'buffer');
+const dh1 = crypto.createDiffieHellman(common.hasFipsCrypto ? 1024 : 256);
+const p1 = dh1.getPrime('buffer');
+const dh2 = crypto.createDiffieHellman(p1, 'base64');
+const key1 = dh1.generateKeys();
+const key2 = dh2.generateKeys('hex');
+const secret1 = dh1.computeSecret(key2, 'hex', 'base64');
+const secret2 = dh2.computeSecret(key1, 'latin1', 'buffer');
 
 assert.strictEqual(secret1, secret2.toString('base64'));
 
 // Create "another dh1" using generated keys from dh1,
 // and compute secret again
-var dh3 = crypto.createDiffieHellman(p1, 'buffer');
-var privkey1 = dh1.getPrivateKey();
+const dh3 = crypto.createDiffieHellman(p1, 'buffer');
+const privkey1 = dh1.getPrivateKey();
 dh3.setPublicKey(key1);
 dh3.setPrivateKey(privkey1);
 
@@ -559,26 +559,26 @@ assert.strictEqual(dh1.getGenerator(), dh3.getGenerator());
 assert.strictEqual(dh1.getPublicKey(), dh3.getPublicKey());
 assert.strictEqual(dh1.getPrivateKey(), dh3.getPrivateKey());
 
-var secret3 = dh3.computeSecret(key2, 'hex', 'base64');
+const secret3 = dh3.computeSecret(key2, 'hex', 'base64');
 
 assert.strictEqual(secret1, secret3);
 
 // https://github.com/joyent/node/issues/2338
-var p = 'FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74' +
-        '020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F1437' +
-        '4FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED' +
-        'EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF';
-var d = crypto.createDiffieHellman(p, 'hex');
+const p = 'FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74' +
+          '020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F1437' +
+          '4FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED' +
+          'EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF';
+const d = crypto.createDiffieHellman(p, 'hex');
 assert.strictEqual(d.verifyError, DH_NOT_SUITABLE_GENERATOR);
 
 // Test RSA key signing/verification
-var rsaSign = crypto.createSign('RSA-SHA1');
-var rsaVerify = crypto.createVerify('RSA-SHA1');
+const rsaSign = crypto.createSign('RSA-SHA1');
+const rsaVerify = crypto.createVerify('RSA-SHA1');
 assert.ok(rsaSign instanceof crypto.Sign);
 assert.ok(rsaVerify instanceof crypto.Verify);
 
 rsaSign.update(rsaPubPem);
-var rsaSignature = rsaSign.sign(rsaKeyPem, 'hex');
+const rsaSignature = rsaSign.sign(rsaKeyPem, 'hex');
 assert.strictEqual(
   rsaSignature,
   '5c50e3145c4e2497aadb0eabc83b342d0b0021ece0d4c4a064b7c' +
diff --git a/test/parallel/test-crypto-certificate.js b/test/parallel/test-crypto-certificate.js
index 24c3163e3b..ebd83b019b 100644
--- a/test/parallel/test-crypto-certificate.js
+++ b/test/parallel/test-crypto-certificate.js
@@ -13,11 +13,11 @@ crypto.DEFAULT_ENCODING = 'buffer';
 const fs = require('fs');
 
 // Test Certificates
-var spkacValid = fs.readFileSync(common.fixturesDir + '/spkac.valid');
-var spkacFail = fs.readFileSync(common.fixturesDir + '/spkac.fail');
-var spkacPem = fs.readFileSync(common.fixturesDir + '/spkac.pem');
+const spkacValid = fs.readFileSync(common.fixturesDir + '/spkac.valid');
+const spkacFail = fs.readFileSync(common.fixturesDir + '/spkac.fail');
+const spkacPem = fs.readFileSync(common.fixturesDir + '/spkac.pem');
 
-var certificate = new crypto.Certificate();
+const certificate = new crypto.Certificate();
 
 assert.strictEqual(certificate.verifySpkac(spkacValid), true);
 assert.strictEqual(certificate.verifySpkac(spkacFail), false);
diff --git a/test/parallel/test-crypto-dh-odd-key.js b/test/parallel/test-crypto-dh-odd-key.js
index 92fab2b8c2..f6bfcf72c0 100644
--- a/test/parallel/test-crypto-dh-odd-key.js
+++ b/test/parallel/test-crypto-dh-odd-key.js
@@ -9,9 +9,9 @@ if (!common.hasCrypto) {
 const crypto = require('crypto');
 
 function test() {
-  var odd = Buffer.alloc(39, 'A');
+  const odd = Buffer.alloc(39, 'A');
 
-  var c = crypto.createDiffieHellman(32);
+  const c = crypto.createDiffieHellman(32);
   c.setPrivateKey(odd);
   c.generateKeys();
 }
diff --git a/test/parallel/test-crypto-dh.js b/test/parallel/test-crypto-dh.js
index 6a067bef1b..2a06fc6e7b 100644
--- a/test/parallel/test-crypto-dh.js
+++ b/test/parallel/test-crypto-dh.js
@@ -11,13 +11,13 @@ const DH_NOT_SUITABLE_GENERATOR = crypto.constants.DH_NOT_SUITABLE_GENERATOR;
 
 // Test Diffie-Hellman with two parties sharing a secret,
 // using various encodings as we go along
-var dh1 = crypto.createDiffieHellman(common.hasFipsCrypto ? 1024 : 256);
-var p1 = dh1.getPrime('buffer');
-var dh2 = crypto.createDiffieHellman(p1, 'buffer');
-var key1 = dh1.generateKeys();
-var key2 = dh2.generateKeys('hex');
-var secret1 = dh1.computeSecret(key2, 'hex', 'base64');
-var secret2 = dh2.computeSecret(key1, 'latin1', 'buffer');
+const dh1 = crypto.createDiffieHellman(common.hasFipsCrypto ? 1024 : 256);
+const p1 = dh1.getPrime('buffer');
+const dh2 = crypto.createDiffieHellman(p1, 'buffer');
+let key1 = dh1.generateKeys();
+let key2 = dh2.generateKeys('hex');
+let secret1 = dh1.computeSecret(key2, 'hex', 'base64');
+let secret2 = dh2.computeSecret(key1, 'latin1', 'buffer');
 
 assert.equal(secret1, secret2.toString('base64'));
 assert.equal(dh1.verifyError, 0);
@@ -41,8 +41,8 @@ assert.throws(function() {
 
 // Create "another dh1" using generated keys from dh1,
 // and compute secret again
-var dh3 = crypto.createDiffieHellman(p1, 'buffer');
-var privkey1 = dh1.getPrivateKey();
+const dh3 = crypto.createDiffieHellman(p1, 'buffer');
+const privkey1 = dh1.getPrivateKey();
 dh3.setPublicKey(key1);
 dh3.setPrivateKey(privkey1);
 
@@ -52,7 +52,7 @@ assert.deepStrictEqual(dh1.getPublicKey(), dh3.getPublicKey());
 assert.deepStrictEqual(dh1.getPrivateKey(), dh3.getPrivateKey());
 assert.equal(dh3.verifyError, 0);
 
-var secret3 = dh3.computeSecret(key2, 'hex', 'base64');
+const secret3 = dh3.computeSecret(key2, 'hex', 'base64');
 
 assert.equal(secret1, secret3);
 
@@ -72,12 +72,12 @@ assert.throws(function() {
 }
 
 // Create a shared using a DH group.
-var alice = crypto.createDiffieHellmanGroup('modp5');
-var bob = crypto.createDiffieHellmanGroup('modp5');
+const alice = crypto.createDiffieHellmanGroup('modp5');
+const bob = crypto.createDiffieHellmanGroup('modp5');
 alice.generateKeys();
 bob.generateKeys();
-var aSecret = alice.computeSecret(bob.getPublicKey()).toString('hex');
-var bSecret = bob.computeSecret(alice.getPublicKey()).toString('hex');
+const aSecret = alice.computeSecret(bob.getPublicKey()).toString('hex');
+const bSecret = bob.computeSecret(alice.getPublicKey()).toString('hex');
 assert.equal(aSecret, bSecret);
 assert.equal(alice.verifyError, DH_NOT_SUITABLE_GENERATOR);
 assert.equal(bob.verifyError, DH_NOT_SUITABLE_GENERATOR);
@@ -85,8 +85,8 @@ assert.equal(bob.verifyError, DH_NOT_SUITABLE_GENERATOR);
 /* Ensure specific generator (buffer) works as expected.
  * The values below (modp2/modp2buf) are for a 1024 bits long prime from
  * RFC 2412 E.2, see https://tools.ietf.org/html/rfc2412. */
-var modp2 = crypto.createDiffieHellmanGroup('modp2');
-var modp2buf = Buffer.from([
+const modp2 = crypto.createDiffieHellmanGroup('modp2');
+const modp2buf = Buffer.from([
   0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xc9, 0x0f,
   0xda, 0xa2, 0x21, 0x68, 0xc2, 0x34, 0xc4, 0xc6, 0x62, 0x8b,
   0x80, 0xdc, 0x1c, 0xd1, 0x29, 0x02, 0x4e, 0x08, 0x8a, 0x67,
@@ -101,51 +101,52 @@ var modp2buf = Buffer.from([
   0x1f, 0xe6, 0x49, 0x28, 0x66, 0x51, 0xec, 0xe6, 0x53, 0x81,
   0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff
 ]);
-var exmodp2 = crypto.createDiffieHellman(modp2buf, Buffer.from([2]));
+const exmodp2 = crypto.createDiffieHellman(modp2buf, Buffer.from([2]));
 modp2.generateKeys();
 exmodp2.generateKeys();
-var modp2Secret = modp2.computeSecret(exmodp2.getPublicKey()).toString('hex');
-var exmodp2Secret = exmodp2.computeSecret(modp2.getPublicKey()).toString('hex');
+let modp2Secret = modp2.computeSecret(exmodp2.getPublicKey()).toString('hex');
+const exmodp2Secret = exmodp2.computeSecret(modp2.getPublicKey())
+                             .toString('hex');
 assert.equal(modp2Secret, exmodp2Secret);
 assert.equal(modp2.verifyError, DH_NOT_SUITABLE_GENERATOR);
 assert.equal(exmodp2.verifyError, DH_NOT_SUITABLE_GENERATOR);
 
 
 // Ensure specific generator (string with encoding) works as expected.
-var exmodp2_2 = crypto.createDiffieHellman(modp2buf, '02', 'hex');
+const exmodp2_2 = crypto.createDiffieHellman(modp2buf, '02', 'hex');
 exmodp2_2.generateKeys();
 modp2Secret = modp2.computeSecret(exmodp2_2.getPublicKey()).toString('hex');
-var exmodp2_2Secret = exmodp2_2.computeSecret(modp2.getPublicKey())
-                               .toString('hex');
+const exmodp2_2Secret = exmodp2_2.computeSecret(modp2.getPublicKey())
+                                 .toString('hex');
 assert.equal(modp2Secret, exmodp2_2Secret);
 assert.equal(exmodp2_2.verifyError, DH_NOT_SUITABLE_GENERATOR);
 
 
 // Ensure specific generator (string without encoding) works as expected.
-var exmodp2_3 = crypto.createDiffieHellman(modp2buf, '\x02');
+const exmodp2_3 = crypto.createDiffieHellman(modp2buf, '\x02');
 exmodp2_3.generateKeys();
 modp2Secret = modp2.computeSecret(exmodp2_3.getPublicKey()).toString('hex');
-var exmodp2_3Secret = exmodp2_3.computeSecret(modp2.getPublicKey())
+const exmodp2_3Secret = exmodp2_3.computeSecret(modp2.getPublicKey())
                                .toString('hex');
 assert.equal(modp2Secret, exmodp2_3Secret);
 assert.equal(exmodp2_3.verifyError, DH_NOT_SUITABLE_GENERATOR);
 
 
 // Ensure specific generator (numeric) works as expected.
-var exmodp2_4 = crypto.createDiffieHellman(modp2buf, 2);
+const exmodp2_4 = crypto.createDiffieHellman(modp2buf, 2);
 exmodp2_4.generateKeys();
 modp2Secret = modp2.computeSecret(exmodp2_4.getPublicKey()).toString('hex');
-var exmodp2_4Secret = exmodp2_4.computeSecret(modp2.getPublicKey())
+const exmodp2_4Secret = exmodp2_4.computeSecret(modp2.getPublicKey())
                                .toString('hex');
 assert.equal(modp2Secret, exmodp2_4Secret);
 assert.equal(exmodp2_4.verifyError, DH_NOT_SUITABLE_GENERATOR);
 
 
-var p = 'FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74' +
-        '020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F1437' +
-        '4FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED' +
-        'EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF';
-var bad_dh = crypto.createDiffieHellman(p, 'hex');
+const p = 'FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74' +
+          '020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F1437' +
+          '4FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED' +
+          'EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF';
+const bad_dh = crypto.createDiffieHellman(p, 'hex');
 assert.equal(bad_dh.verifyError, DH_NOT_SUITABLE_GENERATOR);
 
 
diff --git a/test/parallel/test-crypto-domain.js b/test/parallel/test-crypto-domain.js
index fcbbcb8e05..6586f7d48a 100644
--- a/test/parallel/test-crypto-domain.js
+++ b/test/parallel/test-crypto-domain.js
@@ -10,12 +10,12 @@ if (!common.hasCrypto) {
 const crypto = require('crypto');
 
 function test(fn) {
-  var ex = new Error('BAM');
-  var d = domain.create();
+  const ex = new Error('BAM');
+  const d = domain.create();
   d.on('error', common.mustCall(function(err) {
     assert.strictEqual(err, ex);
   }));
-  var cb = common.mustCall(function() {
+  const cb = common.mustCall(function() {
     throw ex;
   });
   d.run(cb);
diff --git a/test/parallel/test-crypto-domains.js b/test/parallel/test-crypto-domains.js
index 738ed15323..2749382fa5 100644
--- a/test/parallel/test-crypto-domains.js
+++ b/test/parallel/test-crypto-domains.js
@@ -2,8 +2,8 @@
 const common = require('../common');
 const domain = require('domain');
 const assert = require('assert');
-var d = domain.create();
-var expect = ['pbkdf2', 'randomBytes', 'pseudoRandomBytes'];
+const d = domain.create();
+const expect = ['pbkdf2', 'randomBytes', 'pseudoRandomBytes'];
 
 if (!common.hasCrypto) {
   common.skip('missing crypto');
diff --git a/test/parallel/test-crypto-fips.js b/test/parallel/test-crypto-fips.js
index f27c596247..6fd3352740 100644
--- a/test/parallel/test-crypto-fips.js
+++ b/test/parallel/test-crypto-fips.js
@@ -15,14 +15,14 @@ const FIPS_ERROR_STRING = 'Error: Cannot set FIPS mode';
 const OPTION_ERROR_STRING = 'bad option';
 const CNF_FIPS_ON = path.join(common.fixturesDir, 'openssl_fips_enabled.cnf');
 const CNF_FIPS_OFF = path.join(common.fixturesDir, 'openssl_fips_disabled.cnf');
-var num_children_ok = 0;
+let num_children_ok = 0;
 
 function compiledWithFips() {
   return process.config.variables.openssl_fips ? true : false;
 }
 
 function addToEnv(newVar, value) {
-  var envCopy = {};
+  const envCopy = {};
   for (const e in process.env) {
     envCopy[e] = process.env[e];
   }
diff --git a/test/parallel/test-crypto-from-binary.js b/test/parallel/test-crypto-from-binary.js
index ec39606506..485735e83c 100644
--- a/test/parallel/test-crypto-from-binary.js
+++ b/test/parallel/test-crypto-from-binary.js
@@ -12,10 +12,10 @@ if (!common.hasCrypto) {
 }
 const crypto = require('crypto');
 
-var EXTERN_APEX = 0xFBEE9;
+const EXTERN_APEX = 0xFBEE9;
 
 // manually controlled string for checking binary output
-var ucs2_control = 'a\u0000';
+let ucs2_control = 'a\u0000';
 
 // grow the strings to proper length
 while (ucs2_control.length <= EXTERN_APEX) {
@@ -24,7 +24,7 @@ while (ucs2_control.length <= EXTERN_APEX) {
 
 
 // check resultant buffer and output string
-var b = Buffer.from(ucs2_control + ucs2_control, 'ucs2');
+const b = Buffer.from(ucs2_control + ucs2_control, 'ucs2');
 
 //
 // Test updating from birant data
diff --git a/test/parallel/test-crypto-hash.js b/test/parallel/test-crypto-hash.js
index 10135d2950..0d55d4236a 100644
--- a/test/parallel/test-crypto-hash.js
+++ b/test/parallel/test-crypto-hash.js
@@ -11,34 +11,34 @@ if (!common.hasCrypto) {
 const crypto = require('crypto');
 
 // Test hashing
-var a1 = crypto.createHash('sha1').update('Test123').digest('hex');
-var a2 = crypto.createHash('sha256').update('Test123').digest('base64');
-var a3 = crypto.createHash('sha512').update('Test123').digest(); // buffer
-var a4 = crypto.createHash('sha1').update('Test123').digest('buffer');
+const a1 = crypto.createHash('sha1').update('Test123').digest('hex');
+const a2 = crypto.createHash('sha256').update('Test123').digest('base64');
+const a3 = crypto.createHash('sha512').update('Test123').digest(); // buffer
+const a4 = crypto.createHash('sha1').update('Test123').digest('buffer');
 
 // stream interface
-var a5 = crypto.createHash('sha512');
+let a5 = crypto.createHash('sha512');
 a5.end('Test123');
 a5 = a5.read();
 
-var a6 = crypto.createHash('sha512');
+let a6 = crypto.createHash('sha512');
 a6.write('Te');
 a6.write('st');
 a6.write('123');
 a6.end();
 a6 = a6.read();
 
-var a7 = crypto.createHash('sha512');
+let a7 = crypto.createHash('sha512');
 a7.end();
 a7 = a7.read();
 
-var a8 = crypto.createHash('sha512');
+let a8 = crypto.createHash('sha512');
 a8.write('');
 a8.end();
 a8 = a8.read();
 
 if (!common.hasFipsCrypto) {
-  var a0 = crypto.createHash('md5').update('Test123').digest('latin1');
+  const a0 = crypto.createHash('md5').update('Test123').digest('latin1');
   assert.strictEqual(
     a0,
     'h\u00ea\u00cb\u0097\u00d8o\fF!\u00fa+\u000e\u0017\u00ca\u00bd\u008c',
@@ -71,14 +71,14 @@ assert.notStrictEqual(a7, undefined, 'no data should return data');
 assert.notStrictEqual(a8, undefined, 'empty string should generate data');
 
 // Test multiple updates to same hash
-var h1 = crypto.createHash('sha1').update('Test123').digest('hex');
-var h2 = crypto.createHash('sha1').update('Test').update('123').digest('hex');
+const h1 = crypto.createHash('sha1').update('Test123').digest('hex');
+const h2 = crypto.createHash('sha1').update('Test').update('123').digest('hex');
 assert.strictEqual(h1, h2, 'multipled updates');
 
 // Test hashing for binary files
-var fn = path.join(common.fixturesDir, 'sample.png');
-var sha1Hash = crypto.createHash('sha1');
-var fileStream = fs.createReadStream(fn);
+const fn = path.join(common.fixturesDir, 'sample.png');
+const sha1Hash = crypto.createHash('sha1');
+const fileStream = fs.createReadStream(fn);
 fileStream.on('data', function(data) {
   sha1Hash.update(data);
 });
@@ -94,7 +94,7 @@ assert.throws(function() {
 }, /Digest method not supported/);
 
 // Default UTF-8 encoding
-var hutf8 = crypto.createHash('sha512').update('УТФ-8 text').digest('hex');
+const hutf8 = crypto.createHash('sha512').update('УТФ-8 text').digest('hex');
 assert.strictEqual(
     hutf8,
     '4b21bbd1a68e690a730ddcb5a8bc94ead9879ffe82580767ad7ec6fa8ba2dea6' +
@@ -104,7 +104,7 @@ assert.notStrictEqual(
     hutf8,
     crypto.createHash('sha512').update('УТФ-8 text', 'latin1').digest('hex'));
 
-var h3 = crypto.createHash('sha256');
+const h3 = crypto.createHash('sha256');
 h3.digest();
 assert.throws(function() {
   h3.digest();
diff --git a/test/parallel/test-crypto-hmac.js b/test/parallel/test-crypto-hmac.js
index 5b7021521f..f267e82038 100644
--- a/test/parallel/test-crypto-hmac.js
+++ b/test/parallel/test-crypto-hmac.js
@@ -9,14 +9,14 @@ if (!common.hasCrypto) {
 const crypto = require('crypto');
 
 // Test HMAC
-var h1 = crypto.createHmac('sha1', 'Node')
+const h1 = crypto.createHmac('sha1', 'Node')
                .update('some data')
                .update('to hmac')
                .digest('hex');
 assert.strictEqual(h1, '19fd6e1ba73d9ed2224dd5094a71babe85d9a892', 'test HMAC');
 
 // Test HMAC (Wikipedia Test Cases)
-var wikipedia = [
+const wikipedia = [
   {
     key: 'key', data: 'The quick brown fox jumps over the lazy dog',
     hmac: {  // HMACs lifted from Wikipedia.
@@ -75,7 +75,7 @@ for (let i = 0, l = wikipedia.length; i < l; i++) {
 
 
 // Test HMAC-SHA-* (rfc 4231 Test Cases)
-var rfc4231 = [
+const rfc4231 = [
   {
     key: Buffer.from('0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b', 'hex'),
     data: Buffer.from('4869205468657265', 'hex'), // 'Hi There'
@@ -241,7 +241,7 @@ for (let i = 0, l = rfc4231.length; i < l; i++) {
 }
 
 // Test HMAC-MD5/SHA1 (rfc 2202 Test Cases)
-var rfc2202_md5 = [
+const rfc2202_md5 = [
   {
     key: Buffer.from('0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b', 'hex'),
     data: 'Hi There',
@@ -294,7 +294,7 @@ var rfc2202_md5 = [
     hmac: '6f630fad67cda0ee1fb1f562db3aa53e'
   }
 ];
-var rfc2202_sha1 = [
+const rfc2202_sha1 = [
   {
     key: Buffer.from('0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b', 'hex'),
     data: 'Hi There',
diff --git a/test/parallel/test-crypto-rsa-dsa.js b/test/parallel/test-crypto-rsa-dsa.js
index d894e0dcaf..bb529f2454 100644
--- a/test/parallel/test-crypto-rsa-dsa.js
+++ b/test/parallel/test-crypto-rsa-dsa.js
@@ -158,7 +158,7 @@ assert.strictEqual(rsaVerify.verify(rsaPubPem, rsaSignature, 'hex'), true);
 rsaSign = crypto.createSign('RSA-SHA1');
 rsaSign.update(rsaPubPem);
 assert.doesNotThrow(() => {
-  var signOptions = { key: rsaKeyPemEncrypted, passphrase: 'password' };
+  const signOptions = { key: rsaKeyPemEncrypted, passphrase: 'password' };
   rsaSignature = rsaSign.sign(signOptions, 'hex');
 });
 assert.strictEqual(rsaSignature, expectedSignature);
@@ -170,7 +170,7 @@ assert.strictEqual(rsaVerify.verify(rsaPubPem, rsaSignature, 'hex'), true);
 rsaSign = crypto.createSign('RSA-SHA1');
 rsaSign.update(rsaPubPem);
 assert.throws(() => {
-  var signOptions = { key: rsaKeyPemEncrypted, passphrase: 'wrong' };
+  const signOptions = { key: rsaKeyPemEncrypted, passphrase: 'wrong' };
   rsaSign.sign(signOptions, 'hex');
 }, decryptError);
 
diff --git a/test/parallel/test-crypto-sign-verify.js b/test/parallel/test-crypto-sign-verify.js
index 05f29a7104..ae78cd9d8b 100644
--- a/test/parallel/test-crypto-sign-verify.js
+++ b/test/parallel/test-crypto-sign-verify.js
@@ -10,8 +10,8 @@ if (!common.hasCrypto) {
 const crypto = require('crypto');
 
 // Test certificates
-var certPem = fs.readFileSync(common.fixturesDir + '/test_cert.pem', 'ascii');
-var keyPem = fs.readFileSync(common.fixturesDir + '/test_key.pem', 'ascii');
+const certPem = fs.readFileSync(common.fixturesDir + '/test_cert.pem', 'ascii');
+const keyPem = fs.readFileSync(common.fixturesDir + '/test_key.pem', 'ascii');
 
 // Test signing and verifying
 {
diff --git a/test/parallel/test-crypto-stream.js b/test/parallel/test-crypto-stream.js
index 06b483b326..f558851b79 100644
--- a/test/parallel/test-crypto-stream.js
+++ b/test/parallel/test-crypto-stream.js
@@ -28,7 +28,7 @@ Stream2buffer.prototype._write = function(data, encodeing, done) {
 
 if (!common.hasFipsCrypto) {
   // Create an md5 hash of "Hallo world"
-  var hasher1 = crypto.createHash('md5');
+  const hasher1 = crypto.createHash('md5');
   hasher1.pipe(new Stream2buffer(common.mustCall(function end(err, hash) {
     assert.strictEqual(err, null);
     assert.strictEqual(
diff --git a/test/parallel/test-crypto-verify-failure.js b/test/parallel/test-crypto-verify-failure.js
index f840c017b4..c171814dd3 100644
--- a/test/parallel/test-crypto-verify-failure.js
+++ b/test/parallel/test-crypto-verify-failure.js
@@ -12,14 +12,14 @@ crypto.DEFAULT_ENCODING = 'buffer';
 
 const fs = require('fs');
 
-var certPem = fs.readFileSync(common.fixturesDir + '/test_cert.pem', 'ascii');
+const certPem = fs.readFileSync(common.fixturesDir + '/test_cert.pem', 'ascii');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var server = tls.Server(options, function(socket) {
+const server = tls.Server(options, function(socket) {
   setImmediate(function() {
     console.log('sending');
     verify();
diff --git a/test/parallel/test-cwd-enoent-repl.js b/test/parallel/test-cwd-enoent-repl.js
index da7729b286..942fc07b64 100644
--- a/test/parallel/test-cwd-enoent-repl.js
+++ b/test/parallel/test-cwd-enoent-repl.js
@@ -10,13 +10,13 @@ if (common.isSunOS || common.isWindows || common.isAix) {
   return;
 }
 
-var dirname = common.tmpDir + '/cwd-does-not-exist-' + process.pid;
+const dirname = common.tmpDir + '/cwd-does-not-exist-' + process.pid;
 common.refreshTmpDir();
 fs.mkdirSync(dirname);
 process.chdir(dirname);
 fs.rmdirSync(dirname);
 
-var proc = spawn(process.execPath, ['--interactive']);
+const proc = spawn(process.execPath, ['--interactive']);
 proc.stdout.pipe(process.stdout);
 proc.stderr.pipe(process.stderr);
 proc.stdin.write('require("path");\n');
diff --git a/test/parallel/test-cwd-enoent.js b/test/parallel/test-cwd-enoent.js
index bb26a6e879..736121a7c8 100644
--- a/test/parallel/test-cwd-enoent.js
+++ b/test/parallel/test-cwd-enoent.js
@@ -10,13 +10,13 @@ if (common.isSunOS || common.isWindows || common.isAix) {
   return;
 }
 
-var dirname = common.tmpDir + '/cwd-does-not-exist-' + process.pid;
+const dirname = common.tmpDir + '/cwd-does-not-exist-' + process.pid;
 common.refreshTmpDir();
 fs.mkdirSync(dirname);
 process.chdir(dirname);
 fs.rmdirSync(dirname);
 
-var proc = spawn(process.execPath, ['-e', '0']);
+const proc = spawn(process.execPath, ['-e', '0']);
 proc.stdout.pipe(process.stdout);
 proc.stderr.pipe(process.stderr);
 
diff --git a/test/parallel/test-debug-brk.js b/test/parallel/test-debug-brk.js
index 89d9c89214..769e3a6c5b 100644
--- a/test/parallel/test-debug-brk.js
+++ b/test/parallel/test-debug-brk.js
@@ -7,13 +7,13 @@ let run = () => {};
 function test(extraArgs, stdoutPattern) {
   const next = run;
   run = () => {
-    var procStdout = '';
-    var procStderr = '';
-    var agentStdout = '';
-    var debuggerListening = false;
-    var outputMatched = false;
-    var needToSpawnAgent = true;
-    var needToExit = true;
+    let procStdout = '';
+    let procStderr = '';
+    let agentStdout = '';
+    let debuggerListening = false;
+    let outputMatched = false;
+    let needToSpawnAgent = true;
+    let needToExit = true;
 
     const procArgs = [`--debug-brk=${common.PORT}`].concat(extraArgs);
     const proc = spawn(process.execPath, procArgs);
diff --git a/test/parallel/test-debug-port-from-cmdline.js b/test/parallel/test-debug-port-from-cmdline.js
index f0ff853b16..53f35877a2 100644
--- a/test/parallel/test-debug-port-from-cmdline.js
+++ b/test/parallel/test-debug-port-from-cmdline.js
@@ -3,15 +3,15 @@ const common = require('../common');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-var debugPort = common.PORT;
-var args = ['--interactive', '--debug-port=' + debugPort];
-var childOptions = { stdio: ['pipe', 'pipe', 'pipe', 'ipc'] };
-var child = spawn(process.execPath, args, childOptions);
+const debugPort = common.PORT;
+const args = ['--interactive', '--debug-port=' + debugPort];
+const childOptions = { stdio: ['pipe', 'pipe', 'pipe', 'ipc'] };
+const child = spawn(process.execPath, args, childOptions);
 
 child.stdin.write("process.send({ msg: 'childready' });\n");
 
 child.stderr.on('data', function(data) {
-  var lines = data.toString().replace(/\r/g, '').trim().split('\n');
+  const lines = data.toString().replace(/\r/g, '').trim().split('\n');
   lines.forEach(processStderrLine);
 });
 
@@ -26,7 +26,7 @@ process.on('exit', function() {
   assertOutputLines();
 });
 
-var outputLines = [];
+const outputLines = [];
 function processStderrLine(line) {
   console.log('> ' + line);
   outputLines.push(line);
@@ -37,12 +37,12 @@ function processStderrLine(line) {
 }
 
 function assertOutputLines() {
-  var expectedLines = [
+  const expectedLines = [
     'Starting debugger agent.',
     'Debugger listening on 127.0.0.1:' + debugPort,
   ];
 
   assert.strictEqual(outputLines.length, expectedLines.length);
-  for (var i = 0; i < expectedLines.length; i++)
+  for (let i = 0; i < expectedLines.length; i++)
     assert(expectedLines[i].includes(outputLines[i]));
 }
diff --git a/test/parallel/test-debug-signal-cluster.js b/test/parallel/test-debug-signal-cluster.js
index 24b362a687..89b3ca456d 100644
--- a/test/parallel/test-debug-signal-cluster.js
+++ b/test/parallel/test-debug-signal-cluster.js
@@ -12,7 +12,7 @@ const args = [`--debug-port=${port}`, serverPath];
 const options = { stdio: ['inherit', 'inherit', 'pipe', 'ipc'] };
 const child = spawn(process.execPath, args, options);
 
-var expectedContent = [
+let expectedContent = [
   'Starting debugger agent.',
   'Debugger listening on 127.0.0.1:' + (port + 0),
   'Starting debugger agent.',
@@ -22,10 +22,10 @@ var expectedContent = [
 ].join(os.EOL);
 expectedContent += os.EOL; // the last line also contains an EOL character
 
-var debuggerAgentsOutput = '';
-var debuggerAgentsStarted = false;
+let debuggerAgentsOutput = '';
+let debuggerAgentsStarted = false;
 
-var pids;
+let pids;
 
 child.stderr.on('data', function(data) {
   const childStderrOutputString = data.toString();
diff --git a/test/parallel/test-debug-uncaught-exception-async.js b/test/parallel/test-debug-uncaught-exception-async.js
index 29947cfd92..8d0d7957c2 100644
--- a/test/parallel/test-debug-uncaught-exception-async.js
+++ b/test/parallel/test-debug-uncaught-exception-async.js
@@ -7,7 +7,7 @@ const spawn = require('child_process').spawn;
 const emitUncaught = path.join(common.fixturesDir, 'debug-uncaught-async.js');
 const result = spawn(process.execPath, [emitUncaught], {encoding: 'utf8'});
 
-var stderr = '';
+let stderr = '';
 result.stderr.on('data', (data) => {
   stderr += data;
 });
diff --git a/test/parallel/test-debug-usage.js b/test/parallel/test-debug-usage.js
index 5406a82cc8..67ee08ce72 100644
--- a/test/parallel/test-debug-usage.js
+++ b/test/parallel/test-debug-usage.js
@@ -10,7 +10,7 @@ const expectedUsageMessage = `Usage: node debug script.js
        node debug <host>:<port>
        node debug -p <pid>
 `;
-var actualUsageMessage = '';
+let actualUsageMessage = '';
 child.stderr.on('data', function(data) {
   actualUsageMessage += data.toString();
 });
diff --git a/test/parallel/test-debugger-pid.js b/test/parallel/test-debugger-pid.js
index 2b81da700f..e8ba516bfc 100644
--- a/test/parallel/test-debugger-pid.js
+++ b/test/parallel/test-debugger-pid.js
@@ -3,15 +3,15 @@ const common = require('../common');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-var buffer = '';
+let buffer = '';
 
 // connect to debug agent
-var interfacer = spawn(process.execPath, ['debug', '-p', '655555']);
+const interfacer = spawn(process.execPath, ['debug', '-p', '655555']);
 
 console.error(process.execPath, 'debug', '-p', '655555');
 interfacer.stdout.setEncoding('utf-8');
 interfacer.stderr.setEncoding('utf-8');
-var onData = function(data) {
+const onData = function(data) {
   data = (buffer + data).split('\n');
   buffer = data.pop();
   data.forEach(function(line) {
@@ -21,9 +21,9 @@ var onData = function(data) {
 interfacer.stdout.on('data', onData);
 interfacer.stderr.on('data', onData);
 
-var lineCount = 0;
+let lineCount = 0;
 interfacer.on('line', function(line) {
-  var expected;
+  let expected;
   const pid = interfacer.pid;
   if (common.isWindows) {
     switch (++lineCount) {
diff --git a/test/parallel/test-debugger-repeat-last.js b/test/parallel/test-debugger-repeat-last.js
index 7a43bbb6c7..cacedbbd7c 100644
--- a/test/parallel/test-debugger-repeat-last.js
+++ b/test/parallel/test-debugger-repeat-last.js
@@ -17,11 +17,11 @@ const args = [
 const proc = spawn(process.execPath, args, { stdio: 'pipe' });
 proc.stdout.setEncoding('utf8');
 
-var stdout = '';
+let stdout = '';
 
-var sentCommand = false;
-var sentEmpty = false;
-var sentExit = false;
+let sentCommand = false;
+let sentEmpty = false;
+let sentExit = false;
 
 proc.stdout.on('data', (data) => {
   stdout += data;
diff --git a/test/parallel/test-dgram-address.js b/test/parallel/test-dgram-address.js
index 03fcdf3446..09c5ba31f6 100644
--- a/test/parallel/test-dgram-address.js
+++ b/test/parallel/test-dgram-address.js
@@ -4,11 +4,11 @@ const assert = require('assert');
 const dgram = require('dgram');
 
 // IPv4 Test
-var socket_ipv4 = dgram.createSocket('udp4');
-var family_ipv4 = 'IPv4';
+const socket_ipv4 = dgram.createSocket('udp4');
+const family_ipv4 = 'IPv4';
 
 socket_ipv4.on('listening', function() {
-  var address_ipv4 = socket_ipv4.address();
+  const address_ipv4 = socket_ipv4.address();
   assert.strictEqual(address_ipv4.address, common.localhostIPv4);
   assert.strictEqual(typeof address_ipv4.port, 'number');
   assert.ok(isFinite(address_ipv4.port));
@@ -25,12 +25,12 @@ socket_ipv4.on('error', function(e) {
 socket_ipv4.bind(0, common.localhostIPv4);
 
 // IPv6 Test
-var localhost_ipv6 = '::1';
-var socket_ipv6 = dgram.createSocket('udp6');
-var family_ipv6 = 'IPv6';
+const localhost_ipv6 = '::1';
+const socket_ipv6 = dgram.createSocket('udp6');
+const family_ipv6 = 'IPv6';
 
 socket_ipv6.on('listening', function() {
-  var address_ipv6 = socket_ipv6.address();
+  const address_ipv6 = socket_ipv6.address();
   assert.strictEqual(address_ipv6.address, localhost_ipv6);
   assert.strictEqual(typeof address_ipv6.port, 'number');
   assert.ok(isFinite(address_ipv6.port));
diff --git a/test/parallel/test-dgram-bind.js b/test/parallel/test-dgram-bind.js
index 6df102b32d..882f2e87bd 100644
--- a/test/parallel/test-dgram-bind.js
+++ b/test/parallel/test-dgram-bind.js
@@ -3,12 +3,12 @@ require('../common');
 const assert = require('assert');
 const dgram = require('dgram');
 
-var socket = dgram.createSocket('udp4');
+const socket = dgram.createSocket('udp4');
 
 socket.on('listening', function() {
   socket.close();
 });
 
-var result = socket.bind(); // should not throw
+const result = socket.bind(); // should not throw
 
 assert.strictEqual(result, socket); // should have returned itself
diff --git a/test/parallel/test-dgram-bytes-length.js b/test/parallel/test-dgram-bytes-length.js
index da686d850e..07a8bb0e43 100644
--- a/test/parallel/test-dgram-bytes-length.js
+++ b/test/parallel/test-dgram-bytes-length.js
@@ -3,8 +3,8 @@ require('../common');
 const assert = require('assert');
 const dgram = require('dgram');
 
-var message = Buffer.from('Some bytes');
-var client = dgram.createSocket('udp4');
+const message = Buffer.from('Some bytes');
+const client = dgram.createSocket('udp4');
 client.send(
   message,
   0,
diff --git a/test/parallel/test-dgram-close-is-not-callback.js b/test/parallel/test-dgram-close-is-not-callback.js
index beccc2420d..d0f23d5808 100644
--- a/test/parallel/test-dgram-close-is-not-callback.js
+++ b/test/parallel/test-dgram-close-is-not-callback.js
@@ -2,9 +2,9 @@
 const common = require('../common');
 const dgram = require('dgram');
 
-var buf = Buffer.alloc(1024, 42);
+const buf = Buffer.alloc(1024, 42);
 
-var socket = dgram.createSocket('udp4');
+const socket = dgram.createSocket('udp4');
 
 socket.send(buf, 0, buf.length, common.PORT, 'localhost');
 
diff --git a/test/parallel/test-dgram-close.js b/test/parallel/test-dgram-close.js
index 41d28553ca..43047db697 100644
--- a/test/parallel/test-dgram-close.js
+++ b/test/parallel/test-dgram-close.js
@@ -6,10 +6,10 @@ const common = require('../common');
 const assert = require('assert');
 const dgram = require('dgram');
 
-var buf = Buffer.alloc(1024, 42);
+const buf = Buffer.alloc(1024, 42);
 
-var socket = dgram.createSocket('udp4');
-var handle = socket._handle;
+let socket = dgram.createSocket('udp4');
+const handle = socket._handle;
 
 socket.send(buf, 0, buf.length, common.PORT, 'localhost');
 assert.strictEqual(socket.close(common.mustCall(function() {})), socket);
diff --git a/test/parallel/test-dgram-exclusive-implicit-bind.js b/test/parallel/test-dgram-exclusive-implicit-bind.js
index 97fd6bed5d..927c9f815f 100644
--- a/test/parallel/test-dgram-exclusive-implicit-bind.js
+++ b/test/parallel/test-dgram-exclusive-implicit-bind.js
@@ -40,11 +40,11 @@ const dgram = require('dgram');
 // with ENOTSUP.
 
 if (cluster.isMaster) {
-  var messages = 0;
+  let messages = 0;
   const ports = {};
   const pids = [];
 
-  var target = dgram.createSocket('udp4');
+  const target = dgram.createSocket('udp4');
 
   const done = common.mustCall(function() {
     cluster.disconnect();
@@ -84,7 +84,6 @@ if (cluster.isMaster) {
 }
 
 const source = dgram.createSocket('udp4');
-var interval;
 
 source.on('close', function() {
   clearInterval(interval);
@@ -100,6 +99,6 @@ if (process.env.BOUND === 'y') {
 }
 
 const buf = Buffer.from(process.pid.toString());
-interval = setInterval(() => {
+const interval = setInterval(() => {
   source.send(buf, common.PORT, '127.0.0.1');
 }, 1).unref();
diff --git a/test/parallel/test-dgram-implicit-bind.js b/test/parallel/test-dgram-implicit-bind.js
index 25c61903e8..bfc86803b4 100644
--- a/test/parallel/test-dgram-implicit-bind.js
+++ b/test/parallel/test-dgram-implicit-bind.js
@@ -2,9 +2,9 @@
 const common = require('../common');
 const dgram = require('dgram');
 
-var source = dgram.createSocket('udp4');
-var target = dgram.createSocket('udp4');
-var messages = 0;
+const source = dgram.createSocket('udp4');
+const target = dgram.createSocket('udp4');
+let messages = 0;
 
 target.on('message', common.mustCall(function(buf) {
   if (buf.toString() === 'abc') ++messages;
diff --git a/test/parallel/test-dgram-listen-after-bind.js b/test/parallel/test-dgram-listen-after-bind.js
index 2060a47380..e60687649c 100644
--- a/test/parallel/test-dgram-listen-after-bind.js
+++ b/test/parallel/test-dgram-listen-after-bind.js
@@ -3,12 +3,12 @@ require('../common');
 const assert = require('assert');
 const dgram = require('dgram');
 
-var socket = dgram.createSocket('udp4');
+const socket = dgram.createSocket('udp4');
 
 socket.bind();
 
-var fired = false;
-var timer = setTimeout(function() {
+let fired = false;
+const timer = setTimeout(function() {
   socket.close();
 }, 100);
 
diff --git a/test/parallel/test-dgram-oob-buffer.js b/test/parallel/test-dgram-oob-buffer.js
index 0bf302e7e3..247a35588c 100644
--- a/test/parallel/test-dgram-oob-buffer.js
+++ b/test/parallel/test-dgram-oob-buffer.js
@@ -6,8 +6,8 @@
 const common = require('../common');
 const dgram = require('dgram');
 
-var socket = dgram.createSocket('udp4');
-var buf = Buffer.from([1, 2, 3, 4]);
+const socket = dgram.createSocket('udp4');
+const buf = Buffer.from([1, 2, 3, 4]);
 
 function ok() {}
 socket.send(buf, 0, 0, common.PORT, '127.0.0.1', ok); // useful? no
diff --git a/test/parallel/test-dgram-regress-4496.js b/test/parallel/test-dgram-regress-4496.js
index 2b509d7907..b4ec8195d7 100644
--- a/test/parallel/test-dgram-regress-4496.js
+++ b/test/parallel/test-dgram-regress-4496.js
@@ -6,7 +6,7 @@ const assert = require('assert');
 const dgram = require('dgram');
 
 // Should throw but not crash.
-var socket = dgram.createSocket('udp4');
+const socket = dgram.createSocket('udp4');
 assert.throws(function() { socket.send(true, 0, 1, 1, 'host'); }, TypeError);
 assert.throws(function() { socket.sendto(5, 0, 1, 1, 'host'); }, TypeError);
 socket.close();
diff --git a/test/parallel/test-dgram-send-bad-arguments.js b/test/parallel/test-dgram-send-bad-arguments.js
index 8505c8bb67..c7ec6b51ee 100644
--- a/test/parallel/test-dgram-send-bad-arguments.js
+++ b/test/parallel/test-dgram-send-bad-arguments.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const dgram = require('dgram');
 
-var buf = Buffer.from('test');
-var host = '127.0.0.1';
-var sock = dgram.createSocket('udp4');
+const buf = Buffer.from('test');
+const host = '127.0.0.1';
+const sock = dgram.createSocket('udp4');
 
 assert.throws(function() {
   sock.send();
diff --git a/test/parallel/test-dgram-send-empty-array.js b/test/parallel/test-dgram-send-empty-array.js
index 442803e6db..1bfcacd9a4 100644
--- a/test/parallel/test-dgram-send-empty-array.js
+++ b/test/parallel/test-dgram-send-empty-array.js
@@ -12,7 +12,7 @@ const dgram = require('dgram');
 
 const client = dgram.createSocket('udp4');
 
-var interval;
+let interval;
 
 client.on('message', common.mustCall(function onMessage(buf, info) {
   const expected = Buffer.alloc(0);
diff --git a/test/parallel/test-dgram-send-empty-buffer.js b/test/parallel/test-dgram-send-empty-buffer.js
index 70e7e46e0a..d612dbc7e5 100644
--- a/test/parallel/test-dgram-send-empty-buffer.js
+++ b/test/parallel/test-dgram-send-empty-buffer.js
@@ -21,7 +21,7 @@ client.bind(0, common.mustCall(function() {
   }));
 
   const buf = Buffer.alloc(0);
-  var interval = setInterval(function() {
+  const interval = setInterval(function() {
     client.send(buf, 0, 0, port, '127.0.0.1', common.mustCall(function() {}));
   }, 10);
 }));
diff --git a/test/parallel/test-dgram-setTTL.js b/test/parallel/test-dgram-setTTL.js
index 9393e53c7f..3985297217 100644
--- a/test/parallel/test-dgram-setTTL.js
+++ b/test/parallel/test-dgram-setTTL.js
@@ -6,7 +6,7 @@ const socket = dgram.createSocket('udp4');
 
 socket.bind(0);
 socket.on('listening', function() {
-  var result = socket.setTTL(16);
+  const result = socket.setTTL(16);
   assert.strictEqual(result, 16);
 
   assert.throws(function() {
diff --git a/test/parallel/test-dgram-unref.js b/test/parallel/test-dgram-unref.js
index 1b95487a30..ee463d7ab8 100644
--- a/test/parallel/test-dgram-unref.js
+++ b/test/parallel/test-dgram-unref.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const dgram = require('dgram');
 
-var s = dgram.createSocket('udp4');
+const s = dgram.createSocket('udp4');
 s.bind();
 s.unref();
 
diff --git a/test/parallel/test-dh-padding.js b/test/parallel/test-dh-padding.js
index 35202f6a51..1d9d47ec0e 100644
--- a/test/parallel/test-dh-padding.js
+++ b/test/parallel/test-dh-padding.js
@@ -45,7 +45,7 @@ const crypto = require('crypto');
  * }
  */
 
-var apub =
+const apub =
 '5484455905d3eff34c70980e871f27f05448e66f5a6efbb97cbcba4e927196c2bd9ea272cded91\
 10a4977afa8d9b16c9139a444ed2d954a794650e5d7cb525204f385e1af81530518563822ecd0f9\
 524a958d02b3c269e79d6d69850f0968ad567a4404fbb0b19efc8bc73e267b6136b88cafb33299f\
@@ -53,7 +53,7 @@ f7c7cace3ffab1a88c2c9ee841f88b4c3679b4efc465f5c93cca11d487be57373e4c5926f634c4e\
 efee6721d01db91cd66321615b2522f96368dbc818875d422140d0edf30bdb97d9721feddcb9ff6\
 453741a4f687ee46fc54bf1198801f1210ac789879a5ee123f79e2d2ce1209df2445d32166bc9e4\
 8f89e944ec9c3b2e16c8066cd8eebd4e33eb941';
-var bpub =
+const bpub =
 '3fca64510e36bc7da8a3a901c7b74c2eabfa25deaf7cbe1d0c50235866136ad677317279e1fb0\
 06e9c0a07f63e14a3363c8e016fbbde2b2c7e79fed1cc3e08e95f7459f547a8cd0523ee9dc744d\
 e5a956d92b937db4448917e1f6829437f05e408ee7aea70c0362b37370c7c75d14449d8b2d2133\
@@ -61,7 +61,7 @@ e5a956d92b937db4448917e1f6829437f05e408ee7aea70c0362b37370c7c75d14449d8b2d2133\
 788a1fdc6cdf17f5fffaf024ce8711a2ebde0b52e9f1cb56224483826d6e5ac6ecfaae07b75d20\
 6e8ac97f5be1a5b68f20382f2a7dac189cf169325c4cf845b26a0cd616c31fec905c5d9035e5f7\
 8e9880c812374ac0f3ca3d365f06e4be526b5affd4b79';
-var apriv =
+const apriv =
 '62411e34704637d99c6c958a7db32ac22fcafafbe1c33d2cfdb76e12ded41f38fc16b792b9041\
 2e4c82755a3815ba52f780f0ee296ad46e348fc4d1dcd6b64f4eea1b231b2b7d95c5b1c2e26d34\
 83520558b9860a6eb668f01422a54e6604aa7702b4e67511397ef3ecb912bff1a83899c5a5bfb2\
@@ -69,7 +69,7 @@ var apriv =
 08c3b3e6fc60668f7be81cce6784482af228dd7f489005253a165e292802cfd0399924f6c56827\
 7012f68255207722355634290acc7fddeefbba75650a85ece95b6a12de67eac016ba78960108dd\
 5dbadfaa43cc9fed515a1f307b7d90ae0623bc7b8cefb';
-var secret =
+const secret =
 '00c37b1e06a436d6717816a40e6d72907a6f255638b93032267dcb9a5f0b4a9aa0236f3dce63b\
 1c418c60978a00acd1617dfeecf1661d8a3fafb4d0d8824386750f4853313400e7e4afd22847e4\
 fa56bc9713872021265111906673b38db83d10cbfa1dea3b6b4c97c8655f4ae82125281af7f234\
@@ -79,7 +79,7 @@ dc8fe984ddaf532fc1531ce43155fa0ab32532bf1ece5356b8a3447b5267798a904f16f3f4e635\
 8612314311231f905f91c63a1aea52e0b60cead8b57df';
 
 /* FIPS-friendly 2048 bit prime */
-var p = crypto.createDiffieHellman(
+const p = crypto.createDiffieHellman(
   crypto.getDiffieHellman('modp14').getPrime());
 
 p.setPublicKey(apub, 'hex');
diff --git a/test/parallel/test-dns-lookup-cb-error.js b/test/parallel/test-dns-lookup-cb-error.js
index 212f037868..d866a8a13b 100644
--- a/test/parallel/test-dns-lookup-cb-error.js
+++ b/test/parallel/test-dns-lookup-cb-error.js
@@ -11,7 +11,7 @@ cares.getaddrinfo = function() {
 };
 
 assert.doesNotThrow(() => {
-  var tickValue = 0;
+  let tickValue = 0;
 
   dns.lookup('example.com', common.mustCall((error, result, addressType) => {
     assert(error);
diff --git a/test/parallel/test-domain-abort-on-uncaught.js b/test/parallel/test-domain-abort-on-uncaught.js
index d9183af60b..f754541f37 100644
--- a/test/parallel/test-domain-abort-on-uncaught.js
+++ b/test/parallel/test-domain-abort-on-uncaught.js
@@ -9,7 +9,7 @@ const assert = require('assert');
 const domain = require('domain');
 const child_process = require('child_process');
 
-var errorHandlerCalled = false;
+let errorHandlerCalled = false;
 
 const tests = [
   function nextTick() {
@@ -232,7 +232,7 @@ if (process.argv[2] === 'child') {
 } else {
 
   tests.forEach(function(test, testIndex) {
-    var testCmd = '';
+    let testCmd = '';
     if (!common.isWindows) {
       // Do not create core files, as it can take a lot of disk space on
       // continuous testing and developers' machines
@@ -245,7 +245,7 @@ if (process.argv[2] === 'child') {
     testCmd += ' ' + 'child';
     testCmd += ' ' + testIndex;
 
-    var child = child_process.exec(testCmd);
+    const child = child_process.exec(testCmd);
 
     child.on('exit', function onExit(code, signal) {
       assert.strictEqual(code, 0, 'Test at index ' + testIndex +
diff --git a/test/parallel/test-domain-enter-exit.js b/test/parallel/test-domain-enter-exit.js
index 5f12a0bd4c..4a11c3a206 100644
--- a/test/parallel/test-domain-enter-exit.js
+++ b/test/parallel/test-domain-enter-exit.js
@@ -11,11 +11,11 @@ function names(array) {
   }).join(', ');
 }
 
-var a = domain.create();
+const a = domain.create();
 a.name = 'a';
-var b = domain.create();
+const b = domain.create();
 b.name = 'b';
-var c = domain.create();
+const c = domain.create();
 c.name = 'c';
 
 a.enter(); // push
diff --git a/test/parallel/test-domain-exit-dispose-again.js b/test/parallel/test-domain-exit-dispose-again.js
index 2a4ffd655c..542950f8af 100644
--- a/test/parallel/test-domain-exit-dispose-again.js
+++ b/test/parallel/test-domain-exit-dispose-again.js
@@ -8,20 +8,20 @@
 require('../common');
 const assert = require('assert');
 const domain = require('domain');
-var disposalFailed = false;
+let disposalFailed = false;
 
 // Repeatedly schedule a timer with a delay different than the timers attached
 // to a domain that will eventually be disposed to make sure that they are
 // called, regardless of what happens with those timers attached to domains
 // that will eventually be disposed.
-var a = 0;
+let a = 0;
 log();
 function log() {
   console.log(a++, process.domain);
   if (a < 10) setTimeout(log, 20);
 }
 
-var secondTimerRan = false;
+let secondTimerRan = false;
 
 // Use the same timeout duration for both "firstTimer" and "secondTimer"
 // callbacks so that they are called during the same invocation of the
diff --git a/test/parallel/test-domain-exit-dispose.js b/test/parallel/test-domain-exit-dispose.js
index c53b840bf5..e1797cb660 100644
--- a/test/parallel/test-domain-exit-dispose.js
+++ b/test/parallel/test-domain-exit-dispose.js
@@ -5,7 +5,7 @@ const assert = require('assert');
 const domain = require('domain');
 
 // no matter what happens, we should increment a 10 times.
-var a = 0;
+let a = 0;
 log();
 function log() {
   console.log(a++, process.domain);
@@ -15,7 +15,7 @@ function log() {
 // in 50ms we'll throw an error.
 setTimeout(err, 50);
 function err() {
-  var d = domain.create();
+  const d = domain.create();
   d.on('error', handle);
   d.run(err2);
 
diff --git a/test/parallel/test-domain-http-server.js b/test/parallel/test-domain-http-server.js
index ee0388b3cd..d8bb5c8436 100644
--- a/test/parallel/test-domain-http-server.js
+++ b/test/parallel/test-domain-http-server.js
@@ -4,14 +4,14 @@ const domain = require('domain');
 const http = require('http');
 const assert = require('assert');
 
-var objects = { foo: 'bar', baz: {}, num: 42, arr: [1, 2, 3] };
+const objects = { foo: 'bar', baz: {}, num: 42, arr: [1, 2, 3] };
 objects.baz.asdf = objects;
 
-var serverCaught = 0;
-var clientCaught = 0;
+let serverCaught = 0;
+let clientCaught = 0;
 
-var server = http.createServer(function(req, res) {
-  var dom = domain.create();
+const server = http.createServer(function(req, res) {
+  const dom = domain.create();
   req.resume();
   dom.add(req);
   dom.add(res);
@@ -27,7 +27,7 @@ var server = http.createServer(function(req, res) {
   dom.run(function() {
     // Now, an action that has the potential to fail!
     // if you request 'baz', then it'll throw a JSON circular ref error.
-    var data = JSON.stringify(objects[req.url.replace(/[^a-z]/g, '')]);
+    const data = JSON.stringify(objects[req.url.replace(/[^a-z]/g, '')]);
 
     // this line will throw if you pick an unknown key
     assert.notStrictEqual(data, undefined, 'Data should not be undefined');
@@ -43,8 +43,8 @@ function next() {
   const port = this.address().port;
   console.log('listening on localhost:%d', port);
 
-  var requests = 0;
-  var responses = 0;
+  let requests = 0;
+  let responses = 0;
 
   makeReq('/');
   makeReq('/foo');
@@ -55,14 +55,14 @@ function next() {
   function makeReq(p) {
     requests++;
 
-    var dom = domain.create();
+    const dom = domain.create();
     dom.on('error', function(er) {
       clientCaught++;
       console.log('client error', er);
       req.socket.destroy();
     });
 
-    var req = http.get({ host: 'localhost', port: port, path: p });
+    const req = http.get({ host: 'localhost', port: port, path: p });
     dom.add(req);
     req.on('response', function(res) {
       responses++;
@@ -74,7 +74,7 @@ function next() {
       }
 
       dom.add(res);
-      var d = '';
+      let d = '';
       res.on('data', function(c) {
         d += c;
       });
diff --git a/test/parallel/test-domain-implicit-fs.js b/test/parallel/test-domain-implicit-fs.js
index 7773bc46d7..265d291f61 100644
--- a/test/parallel/test-domain-implicit-fs.js
+++ b/test/parallel/test-domain-implicit-fs.js
@@ -5,7 +5,7 @@ const common = require('../common');
 const assert = require('assert');
 const domain = require('domain');
 
-var d = new domain.Domain();
+const d = new domain.Domain();
 
 d.on('error', common.mustCall(function(er) {
   console.error('caught', er);
diff --git a/test/parallel/test-domain-multi.js b/test/parallel/test-domain-multi.js
index 2e541ab5e4..b800572621 100644
--- a/test/parallel/test-domain-multi.js
+++ b/test/parallel/test-domain-multi.js
@@ -5,12 +5,12 @@ require('../common');
 const assert = require('assert');
 const domain = require('domain');
 
-var caughtA = false;
-var caughtB = false;
-var caughtC = false;
+let caughtA = false;
+let caughtB = false;
+let caughtC = false;
 
 
-var a = domain.create();
+const a = domain.create();
 a.enter(); // this will be our "root" domain
 a.on('error', function(er) {
   caughtA = true;
@@ -20,9 +20,9 @@ a.on('error', function(er) {
 
 
 const http = require('http');
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   // child domain of a.
-  var b = domain.create();
+  const b = domain.create();
   a.add(b);
 
   // treat these EE objects as if they are a part of the b domain
@@ -50,8 +50,8 @@ var server = http.createServer(function(req, res) {
   }));
 
 }).listen(0, function() {
-  var c = domain.create();
-  var req = http.get({ host: 'localhost', port: this.address().port });
+  const c = domain.create();
+  const req = http.get({ host: 'localhost', port: this.address().port });
 
   // add the request to the C domain
   c.add(req);
diff --git a/test/parallel/test-domain-nested-throw.js b/test/parallel/test-domain-nested-throw.js
index 008eaaa5f7..39507446a3 100644
--- a/test/parallel/test-domain-nested-throw.js
+++ b/test/parallel/test-domain-nested-throw.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 
 const domain = require('domain');
 
-var dispose;
+let dispose;
 switch (process.argv[2]) {
   case 'true':
     dispose = true;
@@ -18,10 +18,10 @@ switch (process.argv[2]) {
 }
 
 function parent() {
-  var node = process.execPath;
+  const node = process.execPath;
   const spawn = require('child_process').spawn;
-  var opt = { stdio: 'inherit' };
-  var child = spawn(node, [__filename, 'true'], opt);
+  const opt = { stdio: 'inherit' };
+  let child = spawn(node, [__filename, 'true'], opt);
   child.on('exit', function(c) {
     assert(!c);
     child = spawn(node, [__filename, 'false'], opt);
@@ -32,11 +32,11 @@ function parent() {
   });
 }
 
-var gotDomain1Error = false;
-var gotDomain2Error = false;
+let gotDomain1Error = false;
+let gotDomain2Error = false;
 
-var threw1 = false;
-var threw2 = false;
+let threw1 = false;
+let threw2 = false;
 
 function throw1() {
   threw1 = true;
@@ -49,7 +49,7 @@ function throw2() {
 }
 
 function inner(throw1, throw2) {
-  var domain1 = domain.createDomain();
+  const domain1 = domain.createDomain();
 
   domain1.on('error', function(err) {
     if (gotDomain1Error) {
@@ -67,7 +67,7 @@ function inner(throw1, throw2) {
 }
 
 function outer() {
-  var domain2 = domain.createDomain();
+  const domain2 = domain.createDomain();
 
   domain2.on('error', function(err) {
     if (gotDomain2Error) {
diff --git a/test/parallel/test-domain-safe-exit.js b/test/parallel/test-domain-safe-exit.js
index c00aa306f1..bbc3b2fe22 100644
--- a/test/parallel/test-domain-safe-exit.js
+++ b/test/parallel/test-domain-safe-exit.js
@@ -5,8 +5,8 @@ require('../common');
 const assert = require('assert');
 const domain = require('domain');
 
-var a = domain.create();
-var b = domain.create();
+const a = domain.create();
+const b = domain.create();
 
 a.enter(); // push
 b.enter(); // push
diff --git a/test/parallel/test-domain-stack.js b/test/parallel/test-domain-stack.js
index 5772a56e39..43152a4a3b 100644
--- a/test/parallel/test-domain-stack.js
+++ b/test/parallel/test-domain-stack.js
@@ -4,7 +4,7 @@
 require('../common');
 const domain = require('domain');
 
-var a = domain.create();
+const a = domain.create();
 a.name = 'a';
 
 a.on('error', function() {
@@ -14,11 +14,11 @@ a.on('error', function() {
   }
 });
 
-var foo = a.bind(function() {
+const foo = a.bind(function() {
   throw new Error('error from foo');
 });
 
-for (var i = 0; i < 1000; i++) {
+for (let i = 0; i < 1000; i++) {
   process.nextTick(foo);
 }
 
diff --git a/test/parallel/test-domain-throw-error-then-throw-from-uncaught-exception-handler.js b/test/parallel/test-domain-throw-error-then-throw-from-uncaught-exception-handler.js
index 6cbc9ba3e0..c106e40e27 100644
--- a/test/parallel/test-domain-throw-error-then-throw-from-uncaught-exception-handler.js
+++ b/test/parallel/test-domain-throw-error-then-throw-from-uncaught-exception-handler.js
@@ -82,7 +82,7 @@ function runTestWithAbortOnUncaughtException() {
 }
 
 function createTestCmdLine(options) {
-  var testCmd = '';
+  let testCmd = '';
 
   if (!common.isWindows) {
     // Do not create core files, as it can take a lot of disk space on
diff --git a/test/parallel/test-domain-timers.js b/test/parallel/test-domain-timers.js
index 6ddec404cd..79bd300545 100644
--- a/test/parallel/test-domain-timers.js
+++ b/test/parallel/test-domain-timers.js
@@ -3,9 +3,7 @@ const common = require('../common');
 const domain = require('domain');
 const assert = require('assert');
 
-var timeout;
-
-var timeoutd = domain.create();
+const timeoutd = domain.create();
 
 timeoutd.on('error', common.mustCall(function(e) {
   assert.strictEqual(e.message, 'Timeout UNREFd',
@@ -19,7 +17,7 @@ timeoutd.run(function() {
   }, 0).unref();
 });
 
-var immediated = domain.create();
+const immediated = domain.create();
 
 immediated.on('error', common.mustCall(function(e) {
   assert.strictEqual(e.message, 'Immediate Error',
@@ -32,4 +30,4 @@ immediated.run(function() {
   });
 });
 
-timeout = setTimeout(function() {}, 10 * 1000);
+const timeout = setTimeout(function() {}, 10 * 1000);
diff --git a/test/parallel/test-domain-top-level-error-handler-throw.js b/test/parallel/test-domain-top-level-error-handler-throw.js
index 8062d10736..53a73eac42 100644
--- a/test/parallel/test-domain-top-level-error-handler-throw.js
+++ b/test/parallel/test-domain-top-level-error-handler-throw.js
@@ -14,7 +14,7 @@ const internalExMessage = 'You should NOT see me';
 
 if (process.argv[2] === 'child') {
   const domain = require('domain');
-  var d = domain.create();
+  const d = domain.create();
 
   d.on('error', function() {
     throw new Error(domainErrHandlerExMessage);
@@ -29,8 +29,8 @@ if (process.argv[2] === 'child') {
   const fork = require('child_process').fork;
   const assert = require('assert');
 
-  var child = fork(process.argv[1], ['child'], {silent: true});
-  var stderrOutput = '';
+  const child = fork(process.argv[1], ['child'], {silent: true});
+  let stderrOutput = '';
   if (child) {
     child.stderr.on('data', function onStderrData(data) {
       stderrOutput += data.toString();
@@ -45,8 +45,8 @@ if (process.argv[2] === 'child') {
     });
 
     child.on('exit', function onChildExited(exitCode, signal) {
-      var expectedExitCode = 7;
-      var expectedSignal = null;
+      const expectedExitCode = 7;
+      const expectedSignal = null;
 
       assert.strictEqual(exitCode, expectedExitCode);
       assert.strictEqual(signal, expectedSignal);
diff --git a/test/parallel/test-domain-with-abort-on-uncaught-exception.js b/test/parallel/test-domain-with-abort-on-uncaught-exception.js
index e11935be68..0da8f1368f 100644
--- a/test/parallel/test-domain-with-abort-on-uncaught-exception.js
+++ b/test/parallel/test-domain-with-abort-on-uncaught-exception.js
@@ -30,7 +30,7 @@ const domainErrHandlerExMessage = 'exception from domain error handler';
 
 if (process.argv[2] === 'child') {
   const domain = require('domain');
-  var d = domain.create();
+  const d = domain.create();
 
   process.on('uncaughtException', function onUncaughtException() {
     // The process' uncaughtException event must not be emitted when
@@ -88,18 +88,18 @@ if (process.argv[2] === 'child') {
       cmdLineOption = undefined;
     }
 
-    var throwInDomainErrHandlerOpt;
+    let throwInDomainErrHandlerOpt;
     if (options.throwInDomainErrHandler)
       throwInDomainErrHandlerOpt = 'throwInDomainErrHandler';
 
-    var cmdToExec = '';
+    let cmdToExec = '';
     if (!common.isWindows) {
       // Do not create core files, as it can take a lot of disk space on
       // continuous testing and developers' machines
       cmdToExec += 'ulimit -c 0 && ';
     }
 
-    var useTryCatchOpt;
+    let useTryCatchOpt;
     if (options.useTryCatch)
       useTryCatchOpt = 'useTryCatch';
 
@@ -112,7 +112,7 @@ if (process.argv[2] === 'child') {
       useTryCatchOpt
     ].join(' ');
 
-    var child = exec(cmdToExec);
+    const child = exec(cmdToExec);
 
     if (child) {
       child.on('exit', function onChildExited(exitCode, signal) {
diff --git a/test/parallel/test-domain.js b/test/parallel/test-domain.js
index 73ab3b9616..11c14f3c01 100644
--- a/test/parallel/test-domain.js
+++ b/test/parallel/test-domain.js
@@ -6,24 +6,24 @@ const assert = require('assert');
 const domain = require('domain');
 const events = require('events');
 const fs = require('fs');
-var caught = 0;
-var expectCaught = 0;
+let caught = 0;
+let expectCaught = 0;
 
-var d = new domain.Domain();
-var e = new events.EventEmitter();
+const d = new domain.Domain();
+const e = new events.EventEmitter();
 
 d.on('error', function(er) {
   console.error('caught', er && (er.message || er));
 
-  var er_message = er.message;
-  var er_path = er.path;
+  let er_message = er.message;
+  let er_path = er.path;
 
   // On windows, error messages can contain full path names. If this is the
   // case, remove the directory part.
   if (typeof er_path === 'string') {
-    var slash = er_path.lastIndexOf('\\');
+    const slash = er_path.lastIndexOf('\\');
     if (slash !== -1) {
-      var dir = er_path.slice(0, slash + 1);
+      const dir = er_path.slice(0, slash + 1);
       er_path = er_path.replace(dir, '');
       er_message = er_message.replace(dir, '');
     }
@@ -129,7 +129,7 @@ expectCaught++;
 // set up while in the scope of the d domain.
 d.run(function() {
   process.nextTick(function() {
-    var i = setInterval(function() {
+    const i = setInterval(function() {
       clearInterval(i);
       setTimeout(function() {
         fs.stat('this file does not exist', function(er, stat) {
@@ -166,7 +166,7 @@ function fn() {
   throw new Error('This function should never be called!');
 }
 
-var bound = d.intercept(fn);
+let bound = d.intercept(fn);
 bound(new Error('bound'));
 expectCaught++;
 
@@ -210,7 +210,7 @@ expectCaught++;
 
 
 // implicit addition by being created within a domain-bound context.
-var implicit;
+let implicit;
 
 d.run(function() {
   implicit = new events.EventEmitter();
@@ -223,7 +223,7 @@ setTimeout(function() {
 expectCaught++;
 
 
-var result = d.run(function() {
+let result = d.run(function() {
   return 'return value';
 });
 assert.strictEqual(result, 'return value');
@@ -236,12 +236,12 @@ result = d.run(function(a, b) {
 assert.strictEqual(result, 'return value');
 
 
-var fst = fs.createReadStream('stream for nonexistent file');
+const fst = fs.createReadStream('stream for nonexistent file');
 d.add(fst);
 expectCaught++;
 
 [42, null, , false, function() {}, 'string'].forEach(function(something) {
-  var d = new domain.Domain();
+  const d = new domain.Domain();
   d.run(function() {
     process.nextTick(function() {
       throw something;
diff --git a/test/parallel/test-dsa-fips-invalid-key.js b/test/parallel/test-dsa-fips-invalid-key.js
index d0716a3e21..6055a9b4c4 100644
--- a/test/parallel/test-dsa-fips-invalid-key.js
+++ b/test/parallel/test-dsa-fips-invalid-key.js
@@ -10,11 +10,11 @@ if (!common.hasFipsCrypto) {
 const crypto = require('crypto');
 const fs = require('fs');
 
-var input = 'hello';
+const input = 'hello';
 
-var dsapri = fs.readFileSync(common.fixturesDir +
+const dsapri = fs.readFileSync(common.fixturesDir +
                              '/keys/dsa_private_1025.pem');
-var sign = crypto.createSign('DSS1');
+const sign = crypto.createSign('DSS1');
 sign.update(input);
 
 assert.throws(function() {
diff --git a/test/parallel/test-error-reporting.js b/test/parallel/test-error-reporting.js
index 058f99126a..fa7332a67a 100644
--- a/test/parallel/test-error-reporting.js
+++ b/test/parallel/test-error-reporting.js
@@ -5,8 +5,8 @@ const exec = require('child_process').exec;
 const path = require('path');
 
 function errExec(script, callback) {
-  var cmd = '"' + process.argv[0] + '" "' +
-            path.join(common.fixturesDir, script) + '"';
+  const cmd = '"' + process.argv[0] + '" "' +
+              path.join(common.fixturesDir, script) + '"';
   return exec(cmd, function(err, stdout, stderr) {
     // There was some error
     assert.ok(err);
diff --git a/test/parallel/test-event-emitter-check-listener-leaks.js b/test/parallel/test-event-emitter-check-listener-leaks.js
index 18d2d065b8..5b9787f1b8 100644
--- a/test/parallel/test-event-emitter-check-listener-leaks.js
+++ b/test/parallel/test-event-emitter-check-listener-leaks.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const events = require('events');
 
-var e = new events.EventEmitter();
+let e = new events.EventEmitter();
 
 // default
 for (let i = 0; i < 10; i++) {
diff --git a/test/parallel/test-event-emitter-errors.js b/test/parallel/test-event-emitter-errors.js
index e2c9fa2a0c..2b4a93ae98 100644
--- a/test/parallel/test-event-emitter-errors.js
+++ b/test/parallel/test-event-emitter-errors.js
@@ -3,7 +3,7 @@ require('../common');
 const EventEmitter = require('events');
 const assert = require('assert');
 
-var EE = new EventEmitter();
+const EE = new EventEmitter();
 
 assert.throws(function() {
   EE.emit('error', 'Accepts a string');
diff --git a/test/parallel/test-event-emitter-get-max-listeners.js b/test/parallel/test-event-emitter-get-max-listeners.js
index 13f1dcb97c..9acc71fd79 100644
--- a/test/parallel/test-event-emitter-get-max-listeners.js
+++ b/test/parallel/test-event-emitter-get-max-listeners.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const EventEmitter = require('events');
 
-var emitter = new EventEmitter();
+const emitter = new EventEmitter();
 
 assert.strictEqual(emitter.getMaxListeners(), EventEmitter.defaultMaxListeners);
 
@@ -14,6 +14,6 @@ emitter.setMaxListeners(3);
 assert.strictEqual(emitter.getMaxListeners(), 3);
 
 // https://github.com/nodejs/node/issues/523 - second call should not throw.
-var recv = {};
+const recv = {};
 EventEmitter.prototype.on.call(recv, 'event', function() {});
 EventEmitter.prototype.on.call(recv, 'event', function() {});
diff --git a/test/parallel/test-event-emitter-listeners-side-effects.js b/test/parallel/test-event-emitter-listeners-side-effects.js
index 60d64c6614..022750da40 100644
--- a/test/parallel/test-event-emitter-listeners-side-effects.js
+++ b/test/parallel/test-event-emitter-listeners-side-effects.js
@@ -5,8 +5,8 @@ const assert = require('assert');
 
 const EventEmitter = require('events').EventEmitter;
 
-var e = new EventEmitter();
-var fl;  // foo listeners
+const e = new EventEmitter();
+let fl;  // foo listeners
 
 fl = e.listeners('foo');
 assert(Array.isArray(fl));
diff --git a/test/parallel/test-event-emitter-method-names.js b/test/parallel/test-event-emitter-method-names.js
index 27c01b2a07..d0464b80a4 100644
--- a/test/parallel/test-event-emitter-method-names.js
+++ b/test/parallel/test-event-emitter-method-names.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const events = require('events');
 
-var E = events.EventEmitter.prototype;
+const E = events.EventEmitter.prototype;
 assert.strictEqual(E.constructor.name, 'EventEmitter');
 assert.strictEqual(E.on, E.addListener);  // Same method.
 Object.getOwnPropertyNames(E).forEach(function(name) {
diff --git a/test/parallel/test-event-emitter-no-error-provided-to-error-event.js b/test/parallel/test-event-emitter-no-error-provided-to-error-event.js
index 99bdeea79d..3e7242915e 100644
--- a/test/parallel/test-event-emitter-no-error-provided-to-error-event.js
+++ b/test/parallel/test-event-emitter-no-error-provided-to-error-event.js
@@ -3,9 +3,9 @@ const common = require('../common');
 const assert = require('assert');
 const events = require('events');
 const domain = require('domain');
-var e = new events.EventEmitter();
+const e = new events.EventEmitter();
 
-var d = domain.create();
+const d = domain.create();
 d.add(e);
 d.on('error', common.mustCall(function(er) {
   assert(er instanceof Error, 'error created');
diff --git a/test/parallel/test-event-emitter-num-args.js b/test/parallel/test-event-emitter-num-args.js
index a5660b259e..d1027befa3 100644
--- a/test/parallel/test-event-emitter-num-args.js
+++ b/test/parallel/test-event-emitter-num-args.js
@@ -7,7 +7,7 @@ const e = new events.EventEmitter();
 const num_args_emited = [];
 
 e.on('numArgs', function() {
-  var numArgs = arguments.length;
+  const numArgs = arguments.length;
   console.log('numArgs: ' + numArgs);
   num_args_emited.push(numArgs);
 });
diff --git a/test/parallel/test-event-emitter-once.js b/test/parallel/test-event-emitter-once.js
index cca21a454b..724b2ffd16 100644
--- a/test/parallel/test-event-emitter-once.js
+++ b/test/parallel/test-event-emitter-once.js
@@ -12,7 +12,7 @@ e.emit('hello', 'a', 'b');
 e.emit('hello', 'a', 'b');
 e.emit('hello', 'a', 'b');
 
-var remove = function() {
+const remove = function() {
   common.fail('once->foo should not be emitted');
 };
 
diff --git a/test/parallel/test-event-emitter-prepend.js b/test/parallel/test-event-emitter-prepend.js
index aa755a723c..5a3b66b0cb 100644
--- a/test/parallel/test-event-emitter-prepend.js
+++ b/test/parallel/test-event-emitter-prepend.js
@@ -5,7 +5,7 @@ const EventEmitter = require('events');
 const assert = require('assert');
 
 const myEE = new EventEmitter();
-var m = 0;
+let m = 0;
 // This one comes last.
 myEE.on('foo', common.mustCall(() => assert.equal(m, 2)));
 
diff --git a/test/parallel/test-event-emitter-remove-all-listeners.js b/test/parallel/test-event-emitter-remove-all-listeners.js
index 076c1ea1d7..38f9b7101e 100644
--- a/test/parallel/test-event-emitter-remove-all-listeners.js
+++ b/test/parallel/test-event-emitter-remove-all-listeners.js
@@ -66,7 +66,7 @@ function listener() {}
 
 {
   const ee = new events.EventEmitter();
-  var expectLength = 2;
+  let expectLength = 2;
   ee.on('removeListener', function(name, listener) {
     assert.strictEqual(expectLength--, this.listeners('baz').length);
   });
diff --git a/test/parallel/test-event-emitter-set-max-listeners-side-effects.js b/test/parallel/test-event-emitter-set-max-listeners-side-effects.js
index 64f7420fb2..5235d05638 100644
--- a/test/parallel/test-event-emitter-set-max-listeners-side-effects.js
+++ b/test/parallel/test-event-emitter-set-max-listeners-side-effects.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const events = require('events');
 
-var e = new events.EventEmitter();
+const e = new events.EventEmitter();
 
 assert(!(e._events instanceof Object));
 assert.deepStrictEqual(Object.keys(e._events), []);
diff --git a/test/parallel/test-event-emitter-subclass.js b/test/parallel/test-event-emitter-subclass.js
index 27532dae46..d35802b413 100644
--- a/test/parallel/test-event-emitter-subclass.js
+++ b/test/parallel/test-event-emitter-subclass.js
@@ -13,7 +13,7 @@ function MyEE(cb) {
   EventEmitter.call(this);
 }
 
-var myee = new MyEE(common.mustCall(function() {}));
+const myee = new MyEE(common.mustCall(function() {}));
 
 
 util.inherits(ErrorEE, EventEmitter);
@@ -38,8 +38,8 @@ function MyEE2() {
 
 MyEE2.prototype = new EventEmitter();
 
-var ee1 = new MyEE2();
-var ee2 = new MyEE2();
+const ee1 = new MyEE2();
+const ee2 = new MyEE2();
 
 ee1.on('x', function() {});
 
diff --git a/test/parallel/test-exception-handler.js b/test/parallel/test-exception-handler.js
index 1e2b2b8901..506fbaed0e 100644
--- a/test/parallel/test-exception-handler.js
+++ b/test/parallel/test-exception-handler.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const assert = require('assert');
 
-var MESSAGE = 'catch me if you can';
+const MESSAGE = 'catch me if you can';
 
 process.on('uncaughtException', common.mustCall(function(e) {
   console.log('uncaught exception! 1');
diff --git a/test/parallel/test-file-write-stream2.js b/test/parallel/test-file-write-stream2.js
index 77e8119d25..504a572dde 100644
--- a/test/parallel/test-file-write-stream2.js
+++ b/test/parallel/test-file-write-stream2.js
@@ -7,14 +7,13 @@ const fs = require('fs');
 
 
 const filepath = path.join(common.tmpDir, 'write.txt');
-var file;
 
 const EXPECTED = '012345678910';
 
 const cb_expected = 'write open drain write drain close error ';
-var cb_occurred = '';
+let cb_occurred = '';
 
-var countDrains = 0;
+let countDrains = 0;
 
 
 process.on('exit', function() {
@@ -41,7 +40,7 @@ function removeTestFile() {
 common.refreshTmpDir();
 
 // drain at 0, return false at 10.
-file = fs.createWriteStream(filepath, {
+const file = fs.createWriteStream(filepath, {
   highWaterMark: 11
 });
 
@@ -80,7 +79,7 @@ file.on('error', function(err) {
 });
 
 
-for (var i = 0; i < 11; i++) {
+for (let i = 0; i < 11; i++) {
   const ret = file.write(i + '');
   console.error('%d %j', i, ret);
 
diff --git a/test/parallel/test-force-repl-with-eval.js b/test/parallel/test-force-repl-with-eval.js
index e5044ba59d..8c75818508 100644
--- a/test/parallel/test-force-repl-with-eval.js
+++ b/test/parallel/test-force-repl-with-eval.js
@@ -5,11 +5,11 @@ const spawn = require('child_process').spawn;
 
 // spawn a node child process in "interactive" mode (force the repl) and eval
 const cp = spawn(process.execPath, ['-i', '-e', 'console.log("42")']);
-var gotToEnd = false;
+let gotToEnd = false;
 
 cp.stdout.setEncoding('utf8');
 
-var output = '';
+let output = '';
 cp.stdout.on('data', function(b) {
   output += b;
   if (output === '> 42\n') {
diff --git a/test/parallel/test-force-repl.js b/test/parallel/test-force-repl.js
index 37f2f60334..5507d7401b 100644
--- a/test/parallel/test-force-repl.js
+++ b/test/parallel/test-force-repl.js
@@ -5,7 +5,7 @@ const spawn = require('child_process').spawn;
 
 // spawn a node child process in "interactive" mode (force the repl)
 const cp = spawn(process.execPath, ['-i']);
-var timeoutId = setTimeout(function() {
+const timeoutId = setTimeout(function() {
   common.fail('timeout!');
 }, common.platformTimeout(5000)); // give node + the repl 5 seconds to start
 
diff --git a/test/parallel/test-freelist.js b/test/parallel/test-freelist.js
index 65758bd711..8ec7134dad 100644
--- a/test/parallel/test-freelist.js
+++ b/test/parallel/test-freelist.js
@@ -12,7 +12,7 @@ assert.strictEqual(typeof freelist.FreeList, 'function');
 const flist1 = new freelist.FreeList('flist1', 3, String);
 
 // Allocating when empty, should not change the list size
-var result = flist1.alloc('test');
+const result = flist1.alloc('test');
 assert.strictEqual(typeof result, 'string');
 assert.strictEqual(result, 'test');
 assert.strictEqual(flist1.list.length, 0);
diff --git a/test/parallel/test-fs-access.js b/test/parallel/test-fs-access.js
index a74ad9dcce..4157f92f8b 100644
--- a/test/parallel/test-fs-access.js
+++ b/test/parallel/test-fs-access.js
@@ -107,7 +107,7 @@ assert.doesNotThrow(() => {
 });
 
 assert.doesNotThrow(() => {
-  var mode = fs.F_OK | fs.R_OK | fs.W_OK;
+  const mode = fs.F_OK | fs.R_OK | fs.W_OK;
 
   fs.accessSync(readWriteFile, mode);
 });
diff --git a/test/parallel/test-fs-chmod.js b/test/parallel/test-fs-chmod.js
index 2e5d839ae9..5121fd89f9 100644
--- a/test/parallel/test-fs-chmod.js
+++ b/test/parallel/test-fs-chmod.js
@@ -99,7 +99,7 @@ fs.open(file2, 'a', common.mustCall((err, fd) => {
 
 // lchmod
 if (fs.lchmod) {
-  var link = path.join(common.tmpDir, 'symbolic-link');
+  const link = path.join(common.tmpDir, 'symbolic-link');
 
   common.refreshTmpDir();
   fs.symlinkSync(file2, link);
diff --git a/test/parallel/test-fs-exists.js b/test/parallel/test-fs-exists.js
index 5f9e9f50bf..14e1446f5a 100644
--- a/test/parallel/test-fs-exists.js
+++ b/test/parallel/test-fs-exists.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const assert = require('assert');
 const fs = require('fs');
-var f = __filename;
+const f = __filename;
 
 fs.exists(f, common.mustCall(function(y) {
   assert.strictEqual(y, true);
diff --git a/test/parallel/test-fs-long-path.js b/test/parallel/test-fs-long-path.js
index dd091ae1b1..8d22159e24 100644
--- a/test/parallel/test-fs-long-path.js
+++ b/test/parallel/test-fs-long-path.js
@@ -10,9 +10,9 @@ if (!common.isWindows) {
 }
 
 // make a path that will be at least 260 chars long.
-var fileNameLen = Math.max(260 - common.tmpDir.length - 1, 1);
-var fileName = path.join(common.tmpDir, new Array(fileNameLen + 1).join('x'));
-var fullPath = path.resolve(fileName);
+const fileNameLen = Math.max(260 - common.tmpDir.length - 1, 1);
+const fileName = path.join(common.tmpDir, new Array(fileNameLen + 1).join('x'));
+const fullPath = path.resolve(fileName);
 
 common.refreshTmpDir();
 
diff --git a/test/parallel/test-fs-open-flags.js b/test/parallel/test-fs-open-flags.js
index 9ee896f3cf..e15d36c249 100644
--- a/test/parallel/test-fs-open-flags.js
+++ b/test/parallel/test-fs-open-flags.js
@@ -5,13 +5,13 @@ const assert = require('assert');
 
 const fs = require('fs');
 
-var O_APPEND = fs.constants.O_APPEND || 0;
-var O_CREAT = fs.constants.O_CREAT || 0;
-var O_EXCL = fs.constants.O_EXCL || 0;
-var O_RDONLY = fs.constants.O_RDONLY || 0;
-var O_RDWR = fs.constants.O_RDWR || 0;
-var O_TRUNC = fs.constants.O_TRUNC || 0;
-var O_WRONLY = fs.constants.O_WRONLY || 0;
+const O_APPEND = fs.constants.O_APPEND || 0;
+const O_CREAT = fs.constants.O_CREAT || 0;
+const O_EXCL = fs.constants.O_EXCL || 0;
+const O_RDONLY = fs.constants.O_RDONLY || 0;
+const O_RDWR = fs.constants.O_RDWR || 0;
+const O_TRUNC = fs.constants.O_TRUNC || 0;
+const O_WRONLY = fs.constants.O_WRONLY || 0;
 
 const { stringToFlags } = require('internal/fs');
 
diff --git a/test/parallel/test-fs-read-stream-fd-leak.js b/test/parallel/test-fs-read-stream-fd-leak.js
index 4584727009..0a7a42a3ba 100644
--- a/test/parallel/test-fs-read-stream-fd-leak.js
+++ b/test/parallel/test-fs-read-stream-fd-leak.js
@@ -5,7 +5,7 @@ const assert = require('assert');
 const fs = require('fs');
 const path = require('path');
 
-var openCount = 0;
+let openCount = 0;
 const _fsopen = fs.open;
 const _fsclose = fs.close;
 
@@ -26,8 +26,8 @@ fs.close = function() {
 function testLeak(endFn, callback) {
   console.log('testing for leaks from fs.createReadStream().%s()...', endFn);
 
-  var i = 0;
-  var check = 0;
+  let i = 0;
+  let check = 0;
 
   const checkFunction = function() {
     if (openCount !== 0 && check < totalCheck) {
diff --git a/test/parallel/test-fs-read-stream-resume.js b/test/parallel/test-fs-read-stream-resume.js
index 3ff89644e5..6c8b2c2421 100644
--- a/test/parallel/test-fs-read-stream-resume.js
+++ b/test/parallel/test-fs-read-stream-resume.js
@@ -9,7 +9,7 @@ const file = path.join(common.fixturesDir, 'x.txt');
 let data = '';
 let first = true;
 
-var stream = fs.createReadStream(file);
+const stream = fs.createReadStream(file);
 stream.setEncoding('utf8');
 stream.on('data', function(chunk) {
   data += chunk;
diff --git a/test/parallel/test-fs-read-stream.js b/test/parallel/test-fs-read-stream.js
index c8da0275c5..d4fad6e6f7 100644
--- a/test/parallel/test-fs-read-stream.js
+++ b/test/parallel/test-fs-read-stream.js
@@ -7,13 +7,13 @@ const fs = require('fs');
 const fn = path.join(common.fixturesDir, 'elipses.txt');
 const rangeFile = path.join(common.fixturesDir, 'x.txt');
 
-var callbacks = { open: 0, end: 0, close: 0 };
+const callbacks = { open: 0, end: 0, close: 0 };
 
-var paused = false;
-var bytesRead = 0;
+let paused = false;
+let bytesRead = 0;
 
-var file = fs.ReadStream(fn);
-var fileSize = fs.statSync(fn).size;
+const file = fs.ReadStream(fn);
+const fileSize = fs.statSync(fn).size;
 
 assert.strictEqual(file.bytesRead, 0);
 
@@ -64,13 +64,13 @@ file.on('close', function() {
   //assert.equal(fs.readFileSync(fn), fileContent);
 });
 
-var file3 = fs.createReadStream(fn, {encoding: 'utf8'});
+const file3 = fs.createReadStream(fn, {encoding: 'utf8'});
 file3.length = 0;
 file3.on('data', function(data) {
   assert.strictEqual('string', typeof data);
   file3.length += data.length;
 
-  for (var i = 0; i < data.length; i++) {
+  for (let i = 0; i < data.length; i++) {
     // http://www.fileformat.info/info/unicode/char/2026/index.htm
     assert.strictEqual('\u2026', data[i]);
   }
@@ -89,8 +89,8 @@ process.on('exit', function() {
   console.error('ok');
 });
 
-var file4 = fs.createReadStream(rangeFile, {bufferSize: 1, start: 1, end: 2});
-var contentRead = '';
+const file4 = fs.createReadStream(rangeFile, {bufferSize: 1, start: 1, end: 2});
+let contentRead = '';
 file4.on('data', function(data) {
   contentRead += data.toString('utf-8');
 });
@@ -98,7 +98,7 @@ file4.on('end', function(data) {
   assert.strictEqual(contentRead, 'yz');
 });
 
-var file5 = fs.createReadStream(rangeFile, {bufferSize: 1, start: 1});
+const file5 = fs.createReadStream(rangeFile, {bufferSize: 1, start: 1});
 file5.data = '';
 file5.on('data', function(data) {
   file5.data += data.toString('utf-8');
@@ -108,7 +108,7 @@ file5.on('end', function() {
 });
 
 // https://github.com/joyent/node/issues/2320
-var file6 = fs.createReadStream(rangeFile, {bufferSize: 1.23, start: 1});
+const file6 = fs.createReadStream(rangeFile, {bufferSize: 1.23, start: 1});
 file6.data = '';
 file6.on('data', function(data) {
   file6.data += data.toString('utf-8');
@@ -121,7 +121,7 @@ assert.throws(function() {
   fs.createReadStream(rangeFile, {start: 10, end: 2});
 }, /"start" option must be <= "end" option/);
 
-var stream = fs.createReadStream(rangeFile, { start: 0, end: 0 });
+const stream = fs.createReadStream(rangeFile, { start: 0, end: 0 });
 stream.data = '';
 
 stream.on('data', function(chunk) {
@@ -133,11 +133,11 @@ stream.on('end', function() {
 });
 
 // pause and then resume immediately.
-var pauseRes = fs.createReadStream(rangeFile);
+const pauseRes = fs.createReadStream(rangeFile);
 pauseRes.pause();
 pauseRes.resume();
 
-var file7 = fs.createReadStream(rangeFile, {autoClose: false });
+let file7 = fs.createReadStream(rangeFile, {autoClose: false });
 file7.on('data', function() {});
 file7.on('end', function() {
   process.nextTick(function() {
@@ -160,12 +160,12 @@ function file7Next() {
 }
 
 // Just to make sure autoClose won't close the stream because of error.
-var file8 = fs.createReadStream(null, {fd: 13337, autoClose: false });
+const file8 = fs.createReadStream(null, {fd: 13337, autoClose: false });
 file8.on('data', function() {});
 file8.on('error', common.mustCall(function() {}));
 
 // Make sure stream is destroyed when file does not exist.
-var file9 = fs.createReadStream('/path/to/file/that/does/not/exist');
+const file9 = fs.createReadStream('/path/to/file/that/does/not/exist');
 file9.on('data', function() {});
 file9.on('error', common.mustCall(function() {}));
 
diff --git a/test/parallel/test-fs-readfile-fd.js b/test/parallel/test-fs-readfile-fd.js
index 190a10fc53..85fc6ebef5 100644
--- a/test/parallel/test-fs-readfile-fd.js
+++ b/test/parallel/test-fs-readfile-fd.js
@@ -39,7 +39,7 @@ function tempFd(callback) {
 }
 
 function tempFdSync(callback) {
-  var fd = fs.openSync(fn, 'r');
+  const fd = fs.openSync(fn, 'r');
   callback(fd);
   fs.closeSync(fd);
 }
diff --git a/test/parallel/test-fs-readfile-tostring-fail.js b/test/parallel/test-fs-readfile-tostring-fail.js
index f3d728f57f..ee962652fc 100644
--- a/test/parallel/test-fs-readfile-tostring-fail.js
+++ b/test/parallel/test-fs-readfile-tostring-fail.js
@@ -23,7 +23,7 @@ const stream = fs.createWriteStream(file, {
 const size = kStringMaxLength / 200;
 const a = Buffer.alloc(size, 'a');
 
-for (var i = 0; i < 201; i++) {
+for (let i = 0; i < 201; i++) {
   stream.write(a);
 }
 
diff --git a/test/parallel/test-fs-realpath-buffer-encoding.js b/test/parallel/test-fs-realpath-buffer-encoding.js
index fd7b0d5c3d..0ce6ec576d 100644
--- a/test/parallel/test-fs-realpath-buffer-encoding.js
+++ b/test/parallel/test-fs-realpath-buffer-encoding.js
@@ -8,14 +8,15 @@ const buffer_dir = Buffer.from(string_dir);
 
 const encodings = ['ascii', 'utf8', 'utf16le', 'ucs2',
                    'base64', 'binary', 'hex'];
-var expected = {};
+const expected = {};
 encodings.forEach((encoding) => {
   expected[encoding] = buffer_dir.toString(encoding);
 });
 
 
 // test sync version
-for (var encoding in expected) {
+let encoding;
+for (encoding in expected) {
   const expected_value = expected[encoding];
   let result;
 
diff --git a/test/parallel/test-fs-realpath-on-substed-drive.js b/test/parallel/test-fs-realpath-on-substed-drive.js
index ae2895553e..f8d9e86de5 100644
--- a/test/parallel/test-fs-realpath-on-substed-drive.js
+++ b/test/parallel/test-fs-realpath-on-substed-drive.js
@@ -14,7 +14,8 @@ let result;
 // create a subst drive
 const driveLetters = 'ABCDEFGHIJKLMNOPQRSTUWXYZ';
 let drive;
-for (var i = 0; i < driveLetters.length; ++i) {
+let i;
+for (i = 0; i < driveLetters.length; ++i) {
   drive = `${driveLetters[i]}:`;
   result = spawnSync('subst', [drive, common.fixturesDir]);
   if (result.status === 0)
diff --git a/test/parallel/test-fs-realpath.js b/test/parallel/test-fs-realpath.js
index 986e2089b4..a444bf4984 100644
--- a/test/parallel/test-fs-realpath.js
+++ b/test/parallel/test-fs-realpath.js
@@ -4,13 +4,14 @@ const assert = require('assert');
 const fs = require('fs');
 const path = require('path');
 const exec = require('child_process').exec;
-var async_completed = 0, async_expected = 0, unlink = [];
-var skipSymlinks = false;
+let async_completed = 0, async_expected = 0;
+const unlink = [];
+let skipSymlinks = false;
 
 common.refreshTmpDir();
 
-var root = '/';
-var assertEqualPath = assert.strictEqual;
+let root = '/';
+let assertEqualPath = assert.strictEqual;
 if (common.isWindows) {
   // something like "C:\\"
   root = process.cwd().substr(0, 3);
@@ -54,7 +55,7 @@ fs.mkdirSync(path.join(targetsAbsDir, 'nested-index', 'two'));
 function asynctest(testBlock, args, callback, assertBlock) {
   async_expected++;
   testBlock.apply(testBlock, args.concat(function(err) {
-    var ignoreError = false;
+    let ignoreError = false;
     if (assertBlock) {
       try {
         ignoreError = assertBlock.apply(assertBlock, arguments);
@@ -216,7 +217,7 @@ function test_cyclic_link_overprotection(callback) {
   const expected = fs.realpathSync(cycles);
   const folder = cycles + '/folder';
   const link = folder + '/cycles';
-  var testPath = cycles;
+  let testPath = cycles;
   testPath += '/folder/cycles'.repeat(10);
   try { fs.unlinkSync(link); } catch (ex) {}
   fs.symlinkSync(cycles, link, 'dir');
@@ -474,7 +475,7 @@ const tests = [
   test_up_multiple
 ];
 const numtests = tests.length;
-var testsRun = 0;
+let testsRun = 0;
 function runNextTest(err) {
   assert.ifError(err);
   const test = tests.shift();
diff --git a/test/parallel/test-fs-sir-writes-alot.js b/test/parallel/test-fs-sir-writes-alot.js
index 85feac59dd..7bf97ff1d5 100644
--- a/test/parallel/test-fs-sir-writes-alot.js
+++ b/test/parallel/test-fs-sir-writes-alot.js
@@ -4,33 +4,35 @@ const fs = require('fs');
 const assert = require('assert');
 const join = require('path').join;
 
-var filename = join(common.tmpDir, 'out.txt');
+const filename = join(common.tmpDir, 'out.txt');
 
 common.refreshTmpDir();
 
-var fd = fs.openSync(filename, 'w');
+const fd = fs.openSync(filename, 'w');
 
-var line = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaa\n';
+const line = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaa\n';
 
-var N = 10240, complete = 0;
-for (var i = 0; i < N; i++) {
+const N = 10240;
+let complete = 0;
+
+for (let i = 0; i < N; i++) {
   // Create a new buffer for each write. Before the write is actually
   // executed by the thread pool, the buffer will be collected.
-  var buffer = Buffer.from(line);
+  const buffer = Buffer.from(line);
   fs.write(fd, buffer, 0, buffer.length, null, function(er, written) {
     complete++;
     if (complete === N) {
       fs.closeSync(fd);
-      var s = fs.createReadStream(filename);
+      const s = fs.createReadStream(filename);
       s.on('data', testBuffer);
     }
   });
 }
 
-var bytesChecked = 0;
+let bytesChecked = 0;
 
 function testBuffer(b) {
-  for (var i = 0; i < b.length; i++) {
+  for (let i = 0; i < b.length; i++) {
     bytesChecked++;
     if (b[i] !== 'a'.charCodeAt(0) && b[i] !== '\n'.charCodeAt(0)) {
       throw new Error('invalid char ' + i + ',' + b[i]);
diff --git a/test/parallel/test-fs-stat.js b/test/parallel/test-fs-stat.js
index 48e59b8840..32ba47c2fe 100644
--- a/test/parallel/test-fs-stat.js
+++ b/test/parallel/test-fs-stat.js
@@ -37,7 +37,7 @@ fs.open('.', 'r', undefined, common.mustCall(function(err, fd) {
 
 // fstatSync
 fs.open('.', 'r', undefined, common.mustCall(function(err, fd) {
-  var stats;
+  let stats;
   try {
     stats = fs.fstatSync(fd);
   } catch (err) {
diff --git a/test/parallel/test-fs-symlink-dir-junction-relative.js b/test/parallel/test-fs-symlink-dir-junction-relative.js
index 166894c97e..3a2ba1a06c 100644
--- a/test/parallel/test-fs-symlink-dir-junction-relative.js
+++ b/test/parallel/test-fs-symlink-dir-junction-relative.js
@@ -6,10 +6,10 @@ const assert = require('assert');
 const path = require('path');
 const fs = require('fs');
 
-var linkPath1 = path.join(common.tmpDir, 'junction1');
-var linkPath2 = path.join(common.tmpDir, 'junction2');
-var linkTarget = path.join(common.fixturesDir);
-var linkData = path.join(common.fixturesDir);
+const linkPath1 = path.join(common.tmpDir, 'junction1');
+const linkPath2 = path.join(common.tmpDir, 'junction2');
+const linkTarget = path.join(common.fixturesDir);
+const linkData = path.join(common.fixturesDir);
 
 common.refreshTmpDir();
 
@@ -24,11 +24,11 @@ fs.symlinkSync(linkData, linkPath2, 'junction');
 verifyLink(linkPath2);
 
 function verifyLink(linkPath) {
-  var stats = fs.lstatSync(linkPath);
+  const stats = fs.lstatSync(linkPath);
   assert.ok(stats.isSymbolicLink());
 
-  var data1 = fs.readFileSync(linkPath + '/x.txt', 'ascii');
-  var data2 = fs.readFileSync(linkTarget + '/x.txt', 'ascii');
+  const data1 = fs.readFileSync(linkPath + '/x.txt', 'ascii');
+  const data2 = fs.readFileSync(linkTarget + '/x.txt', 'ascii');
   assert.strictEqual(data1, data2);
 
   // Clean up.
diff --git a/test/parallel/test-fs-symlink.js b/test/parallel/test-fs-symlink.js
index 69b78dad3d..c8c6fc933d 100644
--- a/test/parallel/test-fs-symlink.js
+++ b/test/parallel/test-fs-symlink.js
@@ -4,8 +4,8 @@ const assert = require('assert');
 const path = require('path');
 const fs = require('fs');
 
-var linkTime;
-var fileTime;
+let linkTime;
+let fileTime;
 
 if (!common.canCreateSymLink()) {
   common.skip('insufficient privileges');
diff --git a/test/parallel/test-fs-sync-fd-leak.js b/test/parallel/test-fs-sync-fd-leak.js
index 67150f637d..353519593e 100644
--- a/test/parallel/test-fs-sync-fd-leak.js
+++ b/test/parallel/test-fs-sync-fd-leak.js
@@ -22,6 +22,7 @@ fs.fstatSync = function() {
   throw new Error('BAM');
 };
 
+let close_called = 0;
 ensureThrows(function() {
   fs.readFileSync('dummy');
 });
@@ -32,9 +33,8 @@ ensureThrows(function() {
   fs.appendFileSync('dummy', 'xxx');
 });
 
-var close_called = 0;
 function ensureThrows(cb) {
-  var got_exception = false;
+  let got_exception = false;
 
   close_called = 0;
   try {
diff --git a/test/parallel/test-fs-truncate-GH-6233.js b/test/parallel/test-fs-truncate-GH-6233.js
index ee6ca1ca13..95eaf12cbb 100644
--- a/test/parallel/test-fs-truncate-GH-6233.js
+++ b/test/parallel/test-fs-truncate-GH-6233.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const fs = require('fs');
 
-var filename = common.tmpDir + '/truncate-file.txt';
+const filename = common.tmpDir + '/truncate-file.txt';
 
 common.refreshTmpDir();
 
diff --git a/test/parallel/test-fs-watch-recursive.js b/test/parallel/test-fs-watch-recursive.js
index 97c65fccf4..5fb13623df 100644
--- a/test/parallel/test-fs-watch-recursive.js
+++ b/test/parallel/test-fs-watch-recursive.js
@@ -22,7 +22,7 @@ const filepathOne = path.join(testsubdir, filenameOne);
 
 const watcher = fs.watch(testDir, {recursive: true});
 
-var watcherClosed = false;
+let watcherClosed = false;
 watcher.on('change', function(event, filename) {
   assert.ok('change' === event || 'rename' === event);
 
@@ -37,8 +37,9 @@ watcher.on('change', function(event, filename) {
   watcherClosed = true;
 });
 
+let interval;
 if (common.isOSX) {
-  var interval = setInterval(function() {
+  interval = setInterval(function() {
     fs.writeFileSync(filepathOne, 'world');
   }, 10);
 } else {
diff --git a/test/parallel/test-fs-watchfile.js b/test/parallel/test-fs-watchfile.js
index d30261859c..fcd28be0c1 100644
--- a/test/parallel/test-fs-watchfile.js
+++ b/test/parallel/test-fs-watchfile.js
@@ -40,7 +40,7 @@ common.refreshTmpDir();
 
 // If the file initially didn't exist, and gets created at a later point of
 // time, the callback should be invoked again with proper values in stat object
-var fileExists = false;
+let fileExists = false;
 
 fs.watchFile(enoentFile, {interval: 0}, common.mustCall(function(curr, prev) {
   if (!fileExists) {
diff --git a/test/parallel/test-fs-write-buffer.js b/test/parallel/test-fs-write-buffer.js
index 927ebb1541..89570314cc 100644
--- a/test/parallel/test-fs-write-buffer.js
+++ b/test/parallel/test-fs-write-buffer.js
@@ -19,7 +19,7 @@ common.refreshTmpDir();
       assert.strictEqual(expected.length, written);
       fs.closeSync(fd);
 
-      var found = fs.readFileSync(filename, 'utf8');
+      const found = fs.readFileSync(filename, 'utf8');
       assert.strictEqual(expected.toString(), found);
     });
 
diff --git a/test/parallel/test-fs-write-file-buffer.js b/test/parallel/test-fs-write-file-buffer.js
index e2c7b1d01e..409ddf1d51 100644
--- a/test/parallel/test-fs-write-file-buffer.js
+++ b/test/parallel/test-fs-write-file-buffer.js
@@ -4,7 +4,7 @@ const join = require('path').join;
 const util = require('util');
 const fs = require('fs');
 
-var data = [
+let data = [
   '/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAUDBAQEAwUEBAQFBQUGBwwIBwcH',
   'Bw8LCwkMEQ8SEhEPERETFhwXExQaFRERGCEYGh0dHx8fExciJCIeJBweHx7/',
   '2wBDAQUFBQcGBw4ICA4eFBEUHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4e',
@@ -27,7 +27,7 @@ data = data.join('\n');
 
 common.refreshTmpDir();
 
-var buf = Buffer.from(data, 'base64');
+const buf = Buffer.from(data, 'base64');
 fs.writeFileSync(join(common.tmpDir, 'test.jpg'), buf);
 
 util.log('Done!');
diff --git a/test/parallel/test-fs-write-stream-change-open.js b/test/parallel/test-fs-write-stream-change-open.js
index a9d31a3074..f68cc0a6d3 100644
--- a/test/parallel/test-fs-write-stream-change-open.js
+++ b/test/parallel/test-fs-write-stream-change-open.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const path = require('path');
 const fs = require('fs');
 
-var file = path.join(common.tmpDir, 'write.txt');
+const file = path.join(common.tmpDir, 'write.txt');
 
 common.refreshTmpDir();
 
diff --git a/test/parallel/test-fs-write-stream-err.js b/test/parallel/test-fs-write-stream-err.js
index 22137329bf..d50fd07103 100644
--- a/test/parallel/test-fs-write-stream-err.js
+++ b/test/parallel/test-fs-write-stream-err.js
@@ -5,13 +5,13 @@ const fs = require('fs');
 
 common.refreshTmpDir();
 
-var stream = fs.createWriteStream(common.tmpDir + '/out', {
+const stream = fs.createWriteStream(common.tmpDir + '/out', {
   highWaterMark: 10
 });
-var err = new Error('BAM');
+const err = new Error('BAM');
 
-var write = fs.write;
-var writeCalls = 0;
+const write = fs.write;
+let writeCalls = 0;
 fs.write = function() {
   switch (writeCalls++) {
     case 0:
@@ -21,7 +21,7 @@ fs.write = function() {
     case 1:
       // then it breaks
       console.error('second write');
-      var cb = arguments[arguments.length - 1];
+      const cb = arguments[arguments.length - 1];
       return process.nextTick(function() {
         cb(err);
       });
diff --git a/test/parallel/test-fs-write-stream.js b/test/parallel/test-fs-write-stream.js
index 71aaaa75dd..a3ba0377e2 100644
--- a/test/parallel/test-fs-write-stream.js
+++ b/test/parallel/test-fs-write-stream.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const path = require('path');
 const fs = require('fs');
 
-var file = path.join(common.tmpDir, 'write.txt');
+const file = path.join(common.tmpDir, 'write.txt');
 
 common.refreshTmpDir();
 
diff --git a/test/parallel/test-fs-write-string-coerce.js b/test/parallel/test-fs-write-string-coerce.js
index 4179ba0332..2694312b86 100644
--- a/test/parallel/test-fs-write-string-coerce.js
+++ b/test/parallel/test-fs-write-string-coerce.js
@@ -7,9 +7,9 @@ const fs = require('fs');
 
 common.refreshTmpDir();
 
-var fn = path.join(common.tmpDir, 'write-string-coerce.txt');
-var data = true;
-var expected = data + '';
+const fn = path.join(common.tmpDir, 'write-string-coerce.txt');
+const data = true;
+const expected = data + '';
 
 fs.open(fn, 'w', 0o644, common.mustCall(function(err, fd) {
   assert.ifError(err);
diff --git a/test/parallel/test-global-console-exists.js b/test/parallel/test-global-console-exists.js
index 32e640cc52..d4a7c21222 100644
--- a/test/parallel/test-global-console-exists.js
+++ b/test/parallel/test-global-console-exists.js
@@ -9,7 +9,7 @@ const assert = require('assert');
 const EventEmitter = require('events');
 const leak_warning = /EventEmitter memory leak detected\. 2 hello listeners/;
 
-var write_calls = 0;
+let write_calls = 0;
 
 process.on('warning', (warning) => {
   // This will be called after the default internal
diff --git a/test/parallel/test-http-1.0-keep-alive.js b/test/parallel/test-http-1.0-keep-alive.js
index 301c054fc9..dea5f65c54 100644
--- a/test/parallel/test-http-1.0-keep-alive.js
+++ b/test/parallel/test-http-1.0-keep-alive.js
@@ -83,12 +83,12 @@ check([{
 }]);
 
 function check(tests) {
-  var test = tests[0];
-  var server;
+  const test = tests[0];
+  let server;
   if (test) {
     server = http.createServer(serverHandler).listen(0, '127.0.0.1', client);
   }
-  var current = 0;
+  let current = 0;
 
   function next() {
     check(tests.slice(1));
@@ -96,7 +96,7 @@ function check(tests) {
 
   function serverHandler(req, res) {
     if (current + 1 === test.responses.length) this.close();
-    var ctx = test.responses[current];
+    const ctx = test.responses[current];
     console.error('<  SERVER SENDING RESPONSE', ctx);
     res.writeHead(200, ctx.headers);
     ctx.chunks.slice(0, -1).forEach(function(chunk) { res.write(chunk); });
@@ -106,10 +106,10 @@ function check(tests) {
   function client() {
     if (current === test.requests.length) return next();
     const port = server.address().port;
-    var conn = net.createConnection(port, '127.0.0.1', connected);
+    const conn = net.createConnection(port, '127.0.0.1', connected);
 
     function connected() {
-      var ctx = test.requests[current];
+      const ctx = test.requests[current];
       console.error(' > CLIENT SENDING REQUEST', ctx);
       conn.setEncoding('utf8');
       conn.write(ctx.data);
diff --git a/test/parallel/test-http-1.0.js b/test/parallel/test-http-1.0.js
index 926497d002..d98da182a9 100644
--- a/test/parallel/test-http-1.0.js
+++ b/test/parallel/test-http-1.0.js
@@ -4,17 +4,17 @@ const assert = require('assert');
 const net = require('net');
 const http = require('http');
 
-var body = 'hello world\n';
+const body = 'hello world\n';
 
 function test(handler, request_generator, response_validator) {
-  var server = http.createServer(handler);
+  const server = http.createServer(handler);
 
-  var client_got_eof = false;
-  var server_response = '';
+  let client_got_eof = false;
+  let server_response = '';
 
   server.listen(0);
   server.on('listening', function() {
-    var c = net.createConnection(this.address().port);
+    const c = net.createConnection(this.address().port);
 
     c.setEncoding('utf8');
 
diff --git a/test/parallel/test-http-abort-before-end.js b/test/parallel/test-http-abort-before-end.js
index 5e0baf3cdf..046f439cb3 100644
--- a/test/parallel/test-http-abort-before-end.js
+++ b/test/parallel/test-http-abort-before-end.js
@@ -3,10 +3,10 @@ const common = require('../common');
 const http = require('http');
 const assert = require('assert');
 
-var server = http.createServer(common.fail);
+const server = http.createServer(common.fail);
 
 server.listen(0, function() {
-  var req = http.request({
+  const req = http.request({
     method: 'GET',
     host: '127.0.0.1',
     port: this.address().port
diff --git a/test/parallel/test-http-abort-client.js b/test/parallel/test-http-abort-client.js
index 9ee2974c46..6c00a1e6c6 100644
--- a/test/parallel/test-http-abort-client.js
+++ b/test/parallel/test-http-abort-client.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const http = require('http');
 
-var server = http.Server(function(req, res) {
+const server = http.Server(function(req, res) {
   console.log('Server accepted request.');
   res.writeHead(200);
   res.write('Part of my res.');
diff --git a/test/parallel/test-http-abort-queued.js b/test/parallel/test-http-abort-queued.js
index 0a993a2658..8e8339aa82 100644
--- a/test/parallel/test-http-abort-queued.js
+++ b/test/parallel/test-http-abort-queued.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var complete;
+let complete;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   // We should not see the queued /thatotherone request within the server
   // as it should be aborted before it is sent.
   assert.equal(req.url, '/');
@@ -22,10 +22,10 @@ var server = http.createServer(function(req, res) {
 server.listen(0, function() {
   console.log('listen', server.address().port);
 
-  var agent = new http.Agent({maxSockets: 1});
+  const agent = new http.Agent({maxSockets: 1});
   assert.equal(Object.keys(agent.sockets).length, 0);
 
-  var options = {
+  const options = {
     hostname: 'localhost',
     port: server.address().port,
     method: 'GET',
@@ -33,12 +33,12 @@ server.listen(0, function() {
     agent: agent
   };
 
-  var req1 = http.request(options);
+  const req1 = http.request(options);
   req1.on('response', function(res1) {
     assert.equal(Object.keys(agent.sockets).length, 1);
     assert.equal(Object.keys(agent.requests).length, 0);
 
-    var req2 = http.request({
+    const req2 = http.request({
       method: 'GET',
       host: 'localhost',
       port: server.address().port,
diff --git a/test/parallel/test-http-after-connect.js b/test/parallel/test-http-after-connect.js
index 0580f428ed..a93d0b83aa 100644
--- a/test/parallel/test-http-after-connect.js
+++ b/test/parallel/test-http-after-connect.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var clientResponses = 0;
+let clientResponses = 0;
 
 const server = http.createServer(common.mustCall(function(req, res) {
   console.error('Server got GET request');
@@ -46,7 +46,7 @@ function doRequest(i) {
     path: '/request' + i
   }, common.mustCall(function(res) {
     console.error('Client got GET response');
-    var data = '';
+    let data = '';
     res.setEncoding('utf8');
     res.on('data', function(chunk) {
       data += chunk;
diff --git a/test/parallel/test-http-agent-destroyed-socket.js b/test/parallel/test-http-agent-destroyed-socket.js
index 3a6cc53658..d983d44793 100644
--- a/test/parallel/test-http-agent-destroyed-socket.js
+++ b/test/parallel/test-http-agent-destroyed-socket.js
@@ -3,26 +3,26 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.end('Hello World\n');
 }).listen(0, function() {
-  var agent = new http.Agent({maxSockets: 1});
+  const agent = new http.Agent({maxSockets: 1});
 
   agent.on('free', function(socket, host, port) {
     console.log('freeing socket. destroyed? ', socket.destroyed);
   });
 
-  var requestOptions = {
+  const requestOptions = {
     agent: agent,
     host: 'localhost',
     port: this.address().port,
     path: '/'
   };
 
-  var request1 = http.get(requestOptions, function(response) {
+  const request1 = http.get(requestOptions, function(response) {
     // assert request2 is queued in the agent
-    var key = agent.getName(requestOptions);
+    const key = agent.getName(requestOptions);
     assert.strictEqual(agent.requests[key].length, 1);
     console.log('got response1');
     request1.socket.on('close', function() {
@@ -58,14 +58,14 @@ var server = http.createServer(function(req, res) {
     });
   });
 
-  var request2 = http.get(requestOptions, function(response) {
+  const request2 = http.get(requestOptions, function(response) {
     assert(!request2.socket.destroyed);
     assert(request1.socket.destroyed);
     // assert not reusing the same socket, since it was destroyed.
     assert.notStrictEqual(request1.socket, request2.socket);
     console.log('got response2');
-    var gotClose = false;
-    var gotResponseEnd = false;
+    let gotClose = false;
+    let gotResponseEnd = false;
     request2.socket.on('close', function() {
       console.log('request2 socket closed');
       gotClose = true;
diff --git a/test/parallel/test-http-agent-error-on-idle.js b/test/parallel/test-http-agent-error-on-idle.js
index 14a058a6f8..b2bf6334dc 100644
--- a/test/parallel/test-http-agent-error-on-idle.js
+++ b/test/parallel/test-http-agent-error-on-idle.js
@@ -2,37 +2,37 @@
 require('../common');
 const assert = require('assert');
 const http = require('http');
-var Agent = http.Agent;
+const Agent = http.Agent;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.end('hello world');
 });
 
 server.listen(0, function() {
-  var agent = new Agent({
+  const agent = new Agent({
     keepAlive: true,
   });
 
-  var requestParams = {
+  const requestParams = {
     host: 'localhost',
     port: this.address().port,
     agent: agent,
     path: '/'
   };
 
-  var socketKey = agent.getName(requestParams);
+  const socketKey = agent.getName(requestParams);
 
   get(function(res) {
     assert.equal(res.statusCode, 200);
     res.resume();
     res.on('end', function() {
       process.nextTick(function() {
-        var freeSockets = agent.freeSockets[socketKey];
+        const freeSockets = agent.freeSockets[socketKey];
         assert.equal(freeSockets.length, 1,
                      'expect a free socket on ' + socketKey);
 
         //generate a random error on the free socket
-        var freeSocket = freeSockets[0];
+        const freeSocket = freeSockets[0];
         freeSocket.emit('error', new Error('ECONNRESET: test'));
 
         get(done);
diff --git a/test/parallel/test-http-agent-false.js b/test/parallel/test-http-agent-false.js
index 65da4b836a..5c9907bb70 100644
--- a/test/parallel/test-http-agent-false.js
+++ b/test/parallel/test-http-agent-false.js
@@ -6,7 +6,7 @@ const http = require('http');
 // sending `agent: false` when `port: null` is also passed in (i.e. the result
 // of a `url.parse()` call with the default port used, 80 or 443), should not
 // result in an assertion error...
-var opts = {
+const opts = {
   host: '127.0.0.1',
   port: null,
   path: '/',
@@ -14,7 +14,7 @@ var opts = {
   agent: false
 };
 
-var good = false;
+let good = false;
 process.on('exit', function() {
   assert(good, 'expected either an "error" or "response" event');
 });
@@ -22,7 +22,7 @@ process.on('exit', function() {
 // we just want an "error" (no local HTTP server on port 80) or "response"
 // to happen (user happens ot have HTTP server running on port 80).
 // As long as the process doesn't crash from a C++ assertion then we're good.
-var req = http.request(opts);
+const req = http.request(opts);
 req.on('response', function(res) {
   good = true;
 });
diff --git a/test/parallel/test-http-agent-maxsockets-regress-4050.js b/test/parallel/test-http-agent-maxsockets-regress-4050.js
index 0ba3d1304f..fd3a8c33f3 100644
--- a/test/parallel/test-http-agent-maxsockets-regress-4050.js
+++ b/test/parallel/test-http-agent-maxsockets-regress-4050.js
@@ -26,9 +26,9 @@ function get(path, callback) {
 }
 
 server.listen(0, function() {
-  var finished = 0;
+  let finished = 0;
   const num_requests = 6;
-  for (var i = 0; i < num_requests; i++) {
+  for (let i = 0; i < num_requests; i++) {
     const request = get('/1', function() {
     });
     request.on('response', function() {
diff --git a/test/parallel/test-http-agent-maxsockets.js b/test/parallel/test-http-agent-maxsockets.js
index b71cbd95c4..6814aca3e7 100644
--- a/test/parallel/test-http-agent-maxsockets.js
+++ b/test/parallel/test-http-agent-maxsockets.js
@@ -3,14 +3,14 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var agent = new http.Agent({
+const agent = new http.Agent({
   keepAlive: true,
   keepAliveMsecs: 1000,
   maxSockets: 2,
   maxFreeSockets: 2
 });
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.end('hello world');
 });
 
@@ -23,12 +23,12 @@ function get(path, callback) {
   }, callback);
 }
 
-var count = 0;
+let count = 0;
 function done() {
   if (++count !== 2) {
     return;
   }
-  var freepool = agent.freeSockets[Object.keys(agent.freeSockets)[0]];
+  const freepool = agent.freeSockets[Object.keys(agent.freeSockets)[0]];
   assert.equal(freepool.length, 2,
                'expect keep 2 free sockets, but got ' + freepool.length);
   agent.destroy();
diff --git a/test/parallel/test-http-agent-no-protocol.js b/test/parallel/test-http-agent-no-protocol.js
index e2c748b011..a11489b6c5 100644
--- a/test/parallel/test-http-agent-no-protocol.js
+++ b/test/parallel/test-http-agent-no-protocol.js
@@ -3,10 +3,10 @@ const common = require('../common');
 const http = require('http');
 const url = require('url');
 
-var server = http.createServer(common.mustCall(function(req, res) {
+const server = http.createServer(common.mustCall(function(req, res) {
   res.end();
 })).listen(0, '127.0.0.1', common.mustCall(function() {
-  var opts = url.parse(`http://127.0.0.1:${this.address().port}/`);
+  const opts = url.parse(`http://127.0.0.1:${this.address().port}/`);
 
   // remove the `protocol` field… the `http` module should fall back
   // to "http:", as defined by the global, default `http.Agent` instance.
diff --git a/test/parallel/test-http-agent-null.js b/test/parallel/test-http-agent-null.js
index 426ec3bb76..9071b88b98 100644
--- a/test/parallel/test-http-agent-null.js
+++ b/test/parallel/test-http-agent-null.js
@@ -2,10 +2,10 @@
 const common = require('../common');
 const http = require('http');
 
-var server = http.createServer(common.mustCall(function(req, res) {
+const server = http.createServer(common.mustCall(function(req, res) {
   res.end();
 })).listen(0, common.mustCall(function() {
-  var options = {
+  const options = {
     agent: null,
     port: this.address().port
   };
diff --git a/test/parallel/test-http-agent.js b/test/parallel/test-http-agent.js
index 443af96215..85975979cb 100644
--- a/test/parallel/test-http-agent.js
+++ b/test/parallel/test-http-agent.js
@@ -3,20 +3,20 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var server = http.Server(function(req, res) {
+const server = http.Server(function(req, res) {
   res.writeHead(200);
   res.end('hello world\n');
 });
 
-var responses = 0;
-var N = 4;
-var M = 4;
+let responses = 0;
+const N = 4;
+const M = 4;
 
 server.listen(0, function() {
   const port = this.address().port;
-  for (var i = 0; i < N; i++) {
+  for (let i = 0; i < N; i++) {
     setTimeout(function() {
-      for (var j = 0; j < M; j++) {
+      for (let j = 0; j < M; j++) {
         http.get({ port: port, path: '/' }, function(res) {
           console.log('%d %d', responses, res.statusCode);
           if (++responses === N * M) {
diff --git a/test/parallel/test-http-automatic-headers.js b/test/parallel/test-http-automatic-headers.js
index b15330342c..5fc8d780af 100644
--- a/test/parallel/test-http-automatic-headers.js
+++ b/test/parallel/test-http-automatic-headers.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.setHeader('X-Date', 'foo');
   res.setHeader('X-Connection', 'bar');
   res.setHeader('X-Content-Length', 'baz');
@@ -12,7 +12,7 @@ var server = http.createServer(function(req, res) {
 server.listen(0);
 
 server.on('listening', function() {
-  var agent = new http.Agent({ port: this.address().port, maxSockets: 1 });
+  const agent = new http.Agent({ port: this.address().port, maxSockets: 1 });
   http.get({
     port: this.address().port,
     path: '/hello',
diff --git a/test/parallel/test-http-blank-header.js b/test/parallel/test-http-blank-header.js
index 863cbed19d..2a6bf48207 100644
--- a/test/parallel/test-http-blank-header.js
+++ b/test/parallel/test-http-blank-header.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var server = http.createServer(common.mustCall(function(req, res) {
+const server = http.createServer(common.mustCall(function(req, res) {
   assert.equal('GET', req.method);
   assert.equal('/blah', req.url);
   assert.deepStrictEqual({
@@ -16,7 +16,7 @@ var server = http.createServer(common.mustCall(function(req, res) {
 
 
 server.listen(0, function() {
-  var c = net.createConnection(this.address().port);
+  const c = net.createConnection(this.address().port);
 
   c.on('connect', function() {
     c.write('GET /blah HTTP/1.1\r\n' +
diff --git a/test/parallel/test-http-buffer-sanity.js b/test/parallel/test-http-buffer-sanity.js
index d71f3bdb78..c240cd13b1 100644
--- a/test/parallel/test-http-buffer-sanity.js
+++ b/test/parallel/test-http-buffer-sanity.js
@@ -3,26 +3,26 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var bufferSize = 5 * 1024 * 1024;
-var measuredSize = 0;
+const bufferSize = 5 * 1024 * 1024;
+let measuredSize = 0;
 
-var buffer = Buffer.allocUnsafe(bufferSize);
-for (var i = 0; i < buffer.length; i++) {
+const buffer = Buffer.allocUnsafe(bufferSize);
+for (let i = 0; i < buffer.length; i++) {
   buffer[i] = i % 256;
 }
 
 
-var web = http.Server(function(req, res) {
+const web = http.Server(function(req, res) {
   web.close();
 
   console.log(req.headers);
 
-  var i = 0;
+  let i = 0;
 
   req.on('data', function(d) {
     process.stdout.write(',');
     measuredSize += d.length;
-    for (var j = 0; j < d.length; j++) {
+    for (let j = 0; j < d.length; j++) {
       assert.equal(buffer[i], d[j]);
       i++;
     }
@@ -45,7 +45,7 @@ var web = http.Server(function(req, res) {
 web.listen(0, common.mustCall(function() {
   console.log('Making request');
 
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'GET',
     path: '/',
diff --git a/test/parallel/test-http-byteswritten.js b/test/parallel/test-http-byteswritten.js
index 1ef5f2398d..3b295ebdcc 100644
--- a/test/parallel/test-http-byteswritten.js
+++ b/test/parallel/test-http-byteswritten.js
@@ -3,9 +3,9 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var body = 'hello world\n';
+const body = 'hello world\n';
 
-var httpServer = http.createServer(common.mustCall(function(req, res) {
+const httpServer = http.createServer(common.mustCall(function(req, res) {
   httpServer.close();
 
   res.on('finish', common.mustCall(function() {
@@ -16,9 +16,9 @@ var httpServer = http.createServer(common.mustCall(function(req, res) {
 
   // Write 1.5mb to cause some requests to buffer
   // Also, mix up the encodings a bit.
-  var chunk = new Array(1024 + 1).join('7');
-  var bchunk = Buffer.from(chunk);
-  for (var i = 0; i < 1024; i++) {
+  const chunk = new Array(1024 + 1).join('7');
+  const bchunk = Buffer.from(chunk);
+  for (let i = 0; i < 1024; i++) {
     res.write(chunk);
     res.write(bchunk);
     res.write(chunk, 'hex');
diff --git a/test/parallel/test-http-chunked.js b/test/parallel/test-http-chunked.js
index d8891db6c7..5165232211 100644
--- a/test/parallel/test-http-chunked.js
+++ b/test/parallel/test-http-chunked.js
@@ -3,23 +3,23 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var UTF8_STRING = '南越国是前203年至前111年存在于岭南地区的一个国家，' +
-                  '国都位于番禺，疆域包括今天中国的广东、广西两省区的大部份地区，福建省、湖南、' +
-                  '贵州、云南的一小部份地区和越南的北部。南越国是秦朝灭亡后，' +
-                  '由南海郡尉赵佗于前203年起兵兼并桂林郡和象郡后建立。前196年和前179年，' +
-                  '南越国曾先后两次名义上臣属于西汉，成为西汉的“外臣”。前112年，' +
-                  '南越国末代君主赵建德与西汉发生战争，被汉武帝于前111年所灭。' +
-                  '南越国共存在93年，历经五代君主。南越国是岭南地区的第一个有记载的政权国家，' +
-                  '采用封建制和郡县制并存的制度，它的建立保证了秦末乱世岭南地区社会秩序的稳定，' +
-                  '有效的改善了岭南地区落后的政治、经济现状。';
+const UTF8_STRING = '南越国是前203年至前111年存在于岭南地区的一个国家，' +
+                    '国都位于番禺，疆域包括今天中国的广东、广西两省区的大部份地区，福建省、湖南、' +
+                    '贵州、云南的一小部份地区和越南的北部。南越国是秦朝灭亡后，' +
+                    '由南海郡尉赵佗于前203年起兵兼并桂林郡和象郡后建立。前196年和前179年，' +
+                    '南越国曾先后两次名义上臣属于西汉，成为西汉的“外臣”。前112年，' +
+                    '南越国末代君主赵建德与西汉发生战争，被汉武帝于前111年所灭。' +
+                    '南越国共存在93年，历经五代君主。南越国是岭南地区的第一个有记载的政权国家，' +
+                    '采用封建制和郡县制并存的制度，它的建立保证了秦末乱世岭南地区社会秩序的稳定，' +
+                    '有效的改善了岭南地区落后的政治、经济现状。';
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200, {'Content-Type': 'text/plain; charset=utf8'});
   res.end(UTF8_STRING, 'utf8');
 });
 server.listen(0, function() {
-  var data = '';
-  var get = http.get({
+  let data = '';
+  const get = http.get({
     path: '/',
     host: 'localhost',
     port: this.address().port
diff --git a/test/parallel/test-http-client-abort-event.js b/test/parallel/test-http-client-abort-event.js
index 00c989ca91..b62186e724 100644
--- a/test/parallel/test-http-client-abort-event.js
+++ b/test/parallel/test-http-client-abort-event.js
@@ -1,12 +1,12 @@
 'use strict';
 const common = require('../common');
 const http = require('http');
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.end();
 });
 
 server.listen(0, common.mustCall(function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port
   }, common.fail);
 
diff --git a/test/parallel/test-http-client-abort.js b/test/parallel/test-http-client-abort.js
index 4f46128f0e..738c620815 100644
--- a/test/parallel/test-http-client-abort.js
+++ b/test/parallel/test-http-client-abort.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var clientAborts = 0;
+let clientAborts = 0;
 
 const server = http.Server(function(req, res) {
   console.log('Got connection');
@@ -23,17 +23,17 @@ const server = http.Server(function(req, res) {
   });
 });
 
-var responses = 0;
+let responses = 0;
 const N = 8;
 const requests = [];
 
 server.listen(0, function() {
   console.log('Server listening.');
 
-  for (var i = 0; i < N; i++) {
+  for (let i = 0; i < N; i++) {
     console.log('Making client ' + i);
-    var options = { port: this.address().port, path: '/?id=' + i };
-    var req = http.get(options, function(res) {
+    const options = { port: this.address().port, path: '/?id=' + i };
+    const req = http.get(options, function(res) {
       console.log('Client response code ' + res.statusCode);
 
       res.resume();
diff --git a/test/parallel/test-http-client-abort2.js b/test/parallel/test-http-client-abort2.js
index 346b781ef0..7c7be9b530 100644
--- a/test/parallel/test-http-client-abort2.js
+++ b/test/parallel/test-http-client-abort2.js
@@ -2,12 +2,12 @@
 require('../common');
 const http = require('http');
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.end('Hello');
 });
 
 server.listen(0, function() {
-  var req = http.get({port: this.address().port}, function(res) {
+  const req = http.get({port: this.address().port}, function(res) {
     res.on('data', function(data) {
       req.abort();
       server.close();
diff --git a/test/parallel/test-http-client-agent.js b/test/parallel/test-http-client-agent.js
index 8093100fce..3c2b9c8972 100644
--- a/test/parallel/test-http-client-agent.js
+++ b/test/parallel/test-http-client-agent.js
@@ -3,11 +3,11 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var name;
-var max = 3;
-var count = 0;
+let name;
+const max = 3;
+let count = 0;
 
-var server = http.Server(function(req, res) {
+const server = http.Server(function(req, res) {
   if (req.url === '/0') {
     setTimeout(function() {
       res.writeHead(200);
@@ -20,17 +20,17 @@ var server = http.Server(function(req, res) {
 });
 server.listen(0, function() {
   name = http.globalAgent.getName({ port: this.address().port });
-  for (var i = 0; i < max; ++i) {
+  for (let i = 0; i < max; ++i) {
     request(i);
   }
 });
 
 function request(i) {
-  var req = http.get({
+  const req = http.get({
     port: server.address().port,
     path: '/' + i
   }, function(res) {
-    var socket = req.socket;
+    const socket = req.socket;
     socket.on('close', function() {
       ++count;
       if (count < max) {
diff --git a/test/parallel/test-http-client-default-headers-exist.js b/test/parallel/test-http-client-default-headers-exist.js
index 3a1f528193..2271944773 100644
--- a/test/parallel/test-http-client-default-headers-exist.js
+++ b/test/parallel/test-http-client-default-headers-exist.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var expectedHeaders = {
+const expectedHeaders = {
   'DELETE': ['host', 'connection'],
   'GET': ['host', 'connection'],
   'HEAD': ['host', 'connection'],
@@ -12,18 +12,18 @@ var expectedHeaders = {
   'PUT': ['host', 'connection', 'content-length']
 };
 
-var expectedMethods = Object.keys(expectedHeaders);
+const expectedMethods = Object.keys(expectedHeaders);
 
-var requestCount = 0;
+let requestCount = 0;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   requestCount++;
   res.end();
 
   assert(expectedHeaders.hasOwnProperty(req.method),
          req.method + ' was an unexpected method');
 
-  var requestHeaders = Object.keys(req.headers);
+  const requestHeaders = Object.keys(req.headers);
   requestHeaders.forEach(function(header) {
     assert.notStrictEqual(
       expectedHeaders[req.method].indexOf(header.toLowerCase()),
diff --git a/test/parallel/test-http-client-get-url.js b/test/parallel/test-http-client-get-url.js
index d9a5e93256..9311110bdc 100644
--- a/test/parallel/test-http-client-get-url.js
+++ b/test/parallel/test-http-client-get-url.js
@@ -5,7 +5,7 @@ const http = require('http');
 const url = require('url');
 const URL = url.URL;
 
-var server = http.createServer(common.mustCall(function(req, res) {
+const server = http.createServer(common.mustCall(function(req, res) {
   assert.equal('GET', req.method);
   assert.equal('/foo?bar', req.url);
   res.writeHead(200, {'Content-Type': 'text/plain'});
diff --git a/test/parallel/test-http-client-parse-error.js b/test/parallel/test-http-client-parse-error.js
index c7469c1562..0d8f577a78 100644
--- a/test/parallel/test-http-client-parse-error.js
+++ b/test/parallel/test-http-client-parse-error.js
@@ -5,8 +5,8 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var connects = 0;
-var parseErrors = 0;
+let connects = 0;
+let parseErrors = 0;
 
 // Create a TCP server
 net.createServer(function(c) {
@@ -18,7 +18,7 @@ net.createServer(function(c) {
     this.close();
   }
 }).listen(0, '127.0.0.1', function() {
-  for (var i = 0; i < 2; i++) {
+  for (let i = 0; i < 2; i++) {
     http.request({
       host: '127.0.0.1',
       port: this.address().port,
diff --git a/test/parallel/test-http-client-pipe-end.js b/test/parallel/test-http-client-pipe-end.js
index 04f308e76d..d4ccba8d55 100644
--- a/test/parallel/test-http-client-pipe-end.js
+++ b/test/parallel/test-http-client-pipe-end.js
@@ -4,7 +4,7 @@
 const common = require('../common');
 const http = require('http');
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   req.resume();
   req.once('end', function() {
     res.writeHead(200);
@@ -16,7 +16,7 @@ var server = http.createServer(function(req, res) {
 common.refreshTmpDir();
 
 server.listen(common.PIPE, function() {
-  var req = http.request({
+  const req = http.request({
     socketPath: common.PIPE,
     headers: {'Content-Length': '1'},
     method: 'POST',
diff --git a/test/parallel/test-http-client-race-2.js b/test/parallel/test-http-client-race-2.js
index 80a8252528..d9e89b85ec 100644
--- a/test/parallel/test-http-client-race-2.js
+++ b/test/parallel/test-http-client-race-2.js
@@ -9,14 +9,14 @@ const url = require('url');
 // condition involving the parsers FreeList used internally by http.Client.
 //
 
-var body1_s = '1111111111111111';
-var body2_s = '22222';
-var body3_s = '3333333333333333333';
+const body1_s = '1111111111111111';
+const body2_s = '22222';
+const body3_s = '3333333333333333333';
 
-var server = http.createServer(function(req, res) {
-  var pathname = url.parse(req.url).pathname;
+const server = http.createServer(function(req, res) {
+  const pathname = url.parse(req.url).pathname;
 
-  var body;
+  let body;
   switch (pathname) {
     case '/1': body = body1_s; break;
     case '/2': body = body2_s; break;
@@ -29,15 +29,15 @@ var server = http.createServer(function(req, res) {
 });
 server.listen(0);
 
-var body1 = '';
-var body2 = '';
-var body3 = '';
+let body1 = '';
+let body2 = '';
+let body3 = '';
 
 server.on('listening', function() {
   //
   // Client #1 is assigned Parser #1
   //
-  var req1 = http.get({ port: this.address().port, path: '/1' });
+  const req1 = http.get({ port: this.address().port, path: '/1' });
   req1.on('response', function(res1) {
     res1.setEncoding('utf8');
 
@@ -59,7 +59,7 @@ server.on('listening', function() {
         // At this point, the bug would manifest itself and crash because the
         // internal state of the parser was no longer valid for use by Client #1
         //
-        var req2 = http.get({ port: server.address().port, path: '/2' });
+        const req2 = http.get({ port: server.address().port, path: '/2' });
         req2.on('response', function(res2) {
           res2.setEncoding('utf8');
           res2.on('data', function(chunk) { body2 += chunk; });
@@ -69,7 +69,7 @@ server.on('listening', function() {
             // Just to be really sure we've covered all our bases, execute a
             // request using client2.
             //
-            var req3 = http.get({ port: server.address().port, path: '/3' });
+            const req3 = http.get({ port: server.address().port, path: '/3' });
             req3.on('response', function(res3) {
               res3.setEncoding('utf8');
               res3.on('data', function(chunk) { body3 += chunk; });
diff --git a/test/parallel/test-http-client-race.js b/test/parallel/test-http-client-race.js
index af8e18686b..14e3300f20 100644
--- a/test/parallel/test-http-client-race.js
+++ b/test/parallel/test-http-client-race.js
@@ -4,22 +4,22 @@ const assert = require('assert');
 const http = require('http');
 const url = require('url');
 
-var body1_s = '1111111111111111';
-var body2_s = '22222';
+const body1_s = '1111111111111111';
+const body2_s = '22222';
 
-var server = http.createServer(function(req, res) {
-  var body = url.parse(req.url).pathname === '/1' ? body1_s : body2_s;
+const server = http.createServer(function(req, res) {
+  const body = url.parse(req.url).pathname === '/1' ? body1_s : body2_s;
   res.writeHead(200,
                 {'Content-Type': 'text/plain', 'Content-Length': body.length});
   res.end(body);
 });
 server.listen(0);
 
-var body1 = '';
-var body2 = '';
+let body1 = '';
+let body2 = '';
 
 server.on('listening', function() {
-  var req1 = http.request({ port: this.address().port, path: '/1' });
+  const req1 = http.request({ port: this.address().port, path: '/1' });
   req1.end();
   req1.on('response', function(res1) {
     res1.setEncoding('utf8');
@@ -29,7 +29,7 @@ server.on('listening', function() {
     });
 
     res1.on('end', function() {
-      var req2 = http.request({ port: server.address().port, path: '/2' });
+      const req2 = http.request({ port: server.address().port, path: '/2' });
       req2.end();
       req2.on('response', function(res2) {
         res2.setEncoding('utf8');
diff --git a/test/parallel/test-http-client-read-in-error.js b/test/parallel/test-http-client-read-in-error.js
index 96085ae51f..69b24f7377 100644
--- a/test/parallel/test-http-client-read-in-error.js
+++ b/test/parallel/test-http-client-read-in-error.js
@@ -10,8 +10,8 @@ function Agent() {
 util.inherits(Agent, http.Agent);
 
 Agent.prototype.createConnection = function() {
-  var self = this;
-  var socket = new net.Socket();
+  const self = this;
+  const socket = new net.Socket();
 
   socket.on('error', function() {
     socket.push('HTTP/1.1 200\r\n\r\n');
@@ -35,7 +35,7 @@ Agent.prototype.breakSocket = function breakSocket(socket) {
   socket.emit('error', new Error('Intentional error'));
 };
 
-var agent = new Agent();
+const agent = new Agent();
 
 http.request({
   agent: agent
diff --git a/test/parallel/test-http-client-readable.js b/test/parallel/test-http-client-readable.js
index e49181cedf..bc6421692b 100644
--- a/test/parallel/test-http-client-readable.js
+++ b/test/parallel/test-http-client-readable.js
@@ -13,7 +13,7 @@ util.inherits(FakeAgent, http.Agent);
 
 FakeAgent.prototype.createConnection = function() {
   const s = new Duplex();
-  var once = false;
+  let once = false;
 
   s._read = function() {
     if (once)
@@ -38,7 +38,7 @@ FakeAgent.prototype.createConnection = function() {
   return s;
 };
 
-var received = '';
+let received = '';
 
 const req = http.request({
   agent: new FakeAgent()
diff --git a/test/parallel/test-http-client-response-domain.js b/test/parallel/test-http-client-response-domain.js
index 5355a301e8..eb8b226ace 100644
--- a/test/parallel/test-http-client-response-domain.js
+++ b/test/parallel/test-http-client-response-domain.js
@@ -4,12 +4,12 @@ const assert = require('assert');
 const http = require('http');
 const domain = require('domain');
 
-var d;
+let d;
 
 common.refreshTmpDir();
 
 // first fire up a simple HTTP server
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200);
   res.end();
   server.close();
@@ -26,7 +26,7 @@ function test() {
     assert.equal('should be caught by domain', err.message);
   }));
 
-  var req = http.get({
+  const req = http.get({
     socketPath: common.PIPE,
     headers: {'Content-Length': '1'},
     method: 'POST',
diff --git a/test/parallel/test-http-client-timeout-agent.js b/test/parallel/test-http-client-timeout-agent.js
index 84db25f7b0..16a1834246 100644
--- a/test/parallel/test-http-client-timeout-agent.js
+++ b/test/parallel/test-http-client-timeout-agent.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var requests_sent = 0;
-var requests_done = 0;
-var options = {
+let requests_sent = 0;
+let requests_done = 0;
+const options = {
   method: 'GET',
   port: undefined,
   host: '127.0.0.1',
@@ -13,7 +13,7 @@ var options = {
 
 //http.globalAgent.maxSockets = 15;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   const m = /\/(.*)/.exec(req.url);
   const reqid = parseInt(m[1], 10);
   if (reqid % 2) {
@@ -27,7 +27,7 @@ var server = http.createServer(function(req, res) {
 
 server.listen(0, options.host, function() {
   options.port = this.address().port;
-  var req;
+  let req;
 
   for (requests_sent = 0; requests_sent < 30; requests_sent += 1) {
     options.path = '/' + requests_sent;
diff --git a/test/parallel/test-http-client-timeout-option.js b/test/parallel/test-http-client-timeout-option.js
index 79aa6ba46b..6741fbe0bd 100644
--- a/test/parallel/test-http-client-timeout-option.js
+++ b/test/parallel/test-http-client-timeout-option.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var options = {
+const options = {
   method: 'GET',
   port: undefined,
   host: '127.0.0.1',
@@ -11,17 +11,17 @@ var options = {
   timeout: 1
 };
 
-var server = http.createServer();
+const server = http.createServer();
 
 server.listen(0, options.host, function() {
   options.port = this.address().port;
-  var req = http.request(options);
+  const req = http.request(options);
   req.on('error', function() {
     // this space is intentionally left blank
   });
   req.on('close', common.mustCall(() => server.close()));
 
-  var timeout_events = 0;
+  let timeout_events = 0;
   req.on('timeout', common.mustCall(() => timeout_events += 1));
   setTimeout(function() {
     req.destroy();
diff --git a/test/parallel/test-http-client-timeout-with-data.js b/test/parallel/test-http-client-timeout-with-data.js
index c82327beb2..a21a218175 100644
--- a/test/parallel/test-http-client-timeout-with-data.js
+++ b/test/parallel/test-http-client-timeout-with-data.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var nchunks = 0;
+let nchunks = 0;
 
 const options = {
   method: 'GET',
diff --git a/test/parallel/test-http-client-timeout.js b/test/parallel/test-http-client-timeout.js
index e26c93fe00..0caae0d04c 100644
--- a/test/parallel/test-http-client-timeout.js
+++ b/test/parallel/test-http-client-timeout.js
@@ -3,20 +3,20 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var options = {
+const options = {
   method: 'GET',
   port: undefined,
   host: '127.0.0.1',
   path: '/'
 };
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   // this space intentionally left blank
 });
 
 server.listen(0, options.host, function() {
   options.port = this.address().port;
-  var req = http.request(options, function(res) {
+  const req = http.request(options, function(res) {
     // this space intentionally left blank
   });
   req.on('close', function() {
@@ -25,7 +25,7 @@ server.listen(0, options.host, function() {
   function destroy() {
     req.destroy();
   }
-  var s = req.setTimeout(1, destroy);
+  const s = req.setTimeout(1, destroy);
   assert.ok(s instanceof http.ClientRequest);
   req.on('error', destroy);
   req.end();
diff --git a/test/parallel/test-http-client-upload-buf.js b/test/parallel/test-http-client-upload-buf.js
index e17a2c5ee5..338aab9cc5 100644
--- a/test/parallel/test-http-client-upload-buf.js
+++ b/test/parallel/test-http-client-upload-buf.js
@@ -3,12 +3,12 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var N = 1024;
+const N = 1024;
 
-var server = http.createServer(common.mustCall(function(req, res) {
+const server = http.createServer(common.mustCall(function(req, res) {
   assert.equal('POST', req.method);
 
-  var bytesReceived = 0;
+  let bytesReceived = 0;
 
   req.on('data', function(chunk) {
     bytesReceived += chunk.length;
@@ -25,7 +25,7 @@ var server = http.createServer(common.mustCall(function(req, res) {
 server.listen(0);
 
 server.on('listening', common.mustCall(function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'POST',
     path: '/'
diff --git a/test/parallel/test-http-client-upload.js b/test/parallel/test-http-client-upload.js
index ce2608fcfd..ba60681776 100644
--- a/test/parallel/test-http-client-upload.js
+++ b/test/parallel/test-http-client-upload.js
@@ -3,11 +3,11 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var server = http.createServer(common.mustCall(function(req, res) {
+const server = http.createServer(common.mustCall(function(req, res) {
   assert.equal('POST', req.method);
   req.setEncoding('utf8');
 
-  var sent_body = '';
+  let sent_body = '';
 
   req.on('data', function(chunk) {
     console.log('server got: ' + JSON.stringify(chunk));
@@ -25,7 +25,7 @@ var server = http.createServer(common.mustCall(function(req, res) {
 server.listen(0);
 
 server.on('listening', common.mustCall(function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'POST',
     path: '/'
diff --git a/test/parallel/test-http-conn-reset.js b/test/parallel/test-http-conn-reset.js
index f244838bc4..f7f08a6bf4 100644
--- a/test/parallel/test-http-conn-reset.js
+++ b/test/parallel/test-http-conn-reset.js
@@ -4,13 +4,13 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var options = {
+const options = {
   host: '127.0.0.1',
   port: undefined
 };
 
 // start a tcp server that closes incoming connections immediately
-var server = net.createServer(function(client) {
+const server = net.createServer(function(client) {
   client.destroy();
   server.close();
 });
@@ -19,7 +19,7 @@ server.listen(0, options.host, common.mustCall(onListen));
 // do a GET request, expect it to fail
 function onListen() {
   options.port = this.address().port;
-  var req = http.request(options, function(res) {
+  const req = http.request(options, function(res) {
     assert.ok(false, 'this should never run');
   });
   req.on('error', common.mustCall(function(err) {
diff --git a/test/parallel/test-http-connect-req-res.js b/test/parallel/test-http-connect-req-res.js
index 1cee61e4c2..0b1ca4630a 100644
--- a/test/parallel/test-http-connect-req-res.js
+++ b/test/parallel/test-http-connect-req-res.js
@@ -18,7 +18,7 @@ server.on('connect', common.mustCall(function(req, socket, firstBodyChunk) {
     'Head'
   );
 
-  var data = firstBodyChunk.toString();
+  let data = firstBodyChunk.toString();
   socket.on('data', function(buf) {
     data += buf.toString();
   });
@@ -49,7 +49,7 @@ server.listen(0, common.mustCall(function() {
     assert.equal(socket.listeners('connect').length, 0);
     assert.equal(socket.listeners('data').length, 0);
 
-    var data = firstBodyChunk.toString();
+    let data = firstBodyChunk.toString();
 
     // test that the firstBodyChunk was not parsed as HTTP
     assert.equal(data, 'Head');
diff --git a/test/parallel/test-http-content-length.js b/test/parallel/test-http-content-length.js
index fd1c8004ff..3d6882d92f 100644
--- a/test/parallel/test-http-content-length.js
+++ b/test/parallel/test-http-content-length.js
@@ -3,25 +3,25 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var expectedHeadersMultipleWrites = {
+const expectedHeadersMultipleWrites = {
   'connection': 'close',
   'transfer-encoding': 'chunked',
 };
 
-var expectedHeadersEndWithData = {
+const expectedHeadersEndWithData = {
   'connection': 'close',
   'content-length': String('hello world'.length)
 };
 
-var expectedHeadersEndNoData = {
+const expectedHeadersEndNoData = {
   'connection': 'close',
   'content-length': '0',
 };
 
-var receivedRequests = 0;
-var totalRequests = 3;
+let receivedRequests = 0;
+const totalRequests = 3;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.removeHeader('Date');
 
   switch (req.url.substr(1)) {
@@ -47,7 +47,7 @@ var server = http.createServer(function(req, res) {
 });
 
 server.listen(0, function() {
-  var req;
+  let req;
 
   req = http.request({
     port: this.address().port,
diff --git a/test/parallel/test-http-contentLength0.js b/test/parallel/test-http-contentLength0.js
index c4c8699df7..46eec45ed2 100644
--- a/test/parallel/test-http-contentLength0.js
+++ b/test/parallel/test-http-contentLength0.js
@@ -7,13 +7,13 @@ const http = require('http');
 // I.E. a space character after the 'Content-Length' throws an `error` event.
 
 
-var s = http.createServer(function(req, res) {
+const s = http.createServer(function(req, res) {
   res.writeHead(200, {'Content-Length': '0 '});
   res.end();
 });
 s.listen(0, function() {
 
-  var request = http.request({ port: this.address().port }, function(response) {
+  const request = http.request({ port: this.address().port }, (response) => {
     console.log('STATUS: ' + response.statusCode);
     s.close();
     response.resume();
diff --git a/test/parallel/test-http-date-header.js b/test/parallel/test-http-date-header.js
index 5b9df4c174..e14a92b310 100644
--- a/test/parallel/test-http-date-header.js
+++ b/test/parallel/test-http-date-header.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var testResBody = 'other stuff!\n';
+const testResBody = 'other stuff!\n';
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   assert.ok(!('date' in req.headers),
             'Request headers contained a Date.');
   res.writeHead(200, {
@@ -17,12 +17,12 @@ server.listen(0);
 
 
 server.addListener('listening', function() {
-  var options = {
+  const options = {
     port: this.address().port,
     path: '/',
     method: 'GET'
   };
-  var req = http.request(options, function(res) {
+  const req = http.request(options, function(res) {
     assert.ok('date' in res.headers,
               'Response headers didn\'t contain a Date.');
     res.addListener('end', function() {
diff --git a/test/parallel/test-http-default-encoding.js b/test/parallel/test-http-default-encoding.js
index 0026540373..4632708518 100644
--- a/test/parallel/test-http-default-encoding.js
+++ b/test/parallel/test-http-default-encoding.js
@@ -3,10 +3,10 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var expected = 'This is a unicode text: سلام';
-var result = '';
+const expected = 'This is a unicode text: سلام';
+let result = '';
 
-var server = http.Server(function(req, res) {
+const server = http.Server(function(req, res) {
   req.setEncoding('utf8');
   req.on('data', function(chunk) {
     result += chunk;
diff --git a/test/parallel/test-http-destroyed-socket-write2.js b/test/parallel/test-http-destroyed-socket-write2.js
index bb3f5d64cb..01023a0885 100644
--- a/test/parallel/test-http-destroyed-socket-write2.js
+++ b/test/parallel/test-http-destroyed-socket-write2.js
@@ -6,14 +6,14 @@ const assert = require('assert');
 // where the server has ended the socket.
 
 const http = require('http');
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   setImmediate(function() {
     res.destroy();
   });
 });
 
 server.listen(0, function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     path: '/',
     method: 'POST'
diff --git a/test/parallel/test-http-eof-on-connect.js b/test/parallel/test-http-eof-on-connect.js
index 765d8b35e7..f67e45ed40 100644
--- a/test/parallel/test-http-eof-on-connect.js
+++ b/test/parallel/test-http-eof-on-connect.js
@@ -7,7 +7,7 @@ const http = require('http');
 // It is separate from test-http-malformed-request.js because it is only
 // reproduceable on the first packet on the first connection to a server.
 
-var server = http.createServer(function(req, res) {});
+const server = http.createServer(function(req, res) {});
 server.listen(0);
 
 server.on('listening', function() {
diff --git a/test/parallel/test-http-exceptions.js b/test/parallel/test-http-exceptions.js
index 8ee46316e9..c003db1310 100644
--- a/test/parallel/test-http-exceptions.js
+++ b/test/parallel/test-http-exceptions.js
@@ -2,7 +2,7 @@
 require('../common');
 const http = require('http');
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   intentionally_not_defined(); // eslint-disable-line no-undef
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.write('Thank you, come again.');
@@ -10,12 +10,12 @@ var server = http.createServer(function(req, res) {
 });
 
 server.listen(0, function() {
-  for (var i = 0; i < 4; i += 1) {
+  for (let i = 0; i < 4; i += 1) {
     http.get({ port: this.address().port, path: '/busy/' + i });
   }
 });
 
-var exception_count = 0;
+let exception_count = 0;
 
 process.on('uncaughtException', function(err) {
   console.log('Caught an exception: ' + err);
diff --git a/test/parallel/test-http-expect-continue.js b/test/parallel/test-http-expect-continue.js
index 954d6b6211..0cf90e3b11 100644
--- a/test/parallel/test-http-expect-continue.js
+++ b/test/parallel/test-http-expect-continue.js
@@ -3,11 +3,11 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var outstanding_reqs = 0;
-var test_req_body = 'some stuff...\n';
-var test_res_body = 'other stuff!\n';
-var sent_continue = false;
-var got_continue = false;
+let outstanding_reqs = 0;
+const test_req_body = 'some stuff...\n';
+const test_res_body = 'other stuff!\n';
+let sent_continue = false;
+let got_continue = false;
 
 function handler(req, res) {
   assert.equal(sent_continue, true, 'Full response sent before 100 Continue');
@@ -19,7 +19,7 @@ function handler(req, res) {
   res.end(test_res_body);
 }
 
-var server = http.createServer(handler);
+const server = http.createServer(handler);
 server.on('checkContinue', function(req, res) {
   console.error('Server got Expect: 100-continue...');
   res.writeContinue();
@@ -32,7 +32,7 @@ server.listen(0);
 
 
 server.on('listening', function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'POST',
     path: '/world',
@@ -40,7 +40,7 @@ server.on('listening', function() {
   });
   console.error('Client sending request...');
   outstanding_reqs++;
-  var body = '';
+  let body = '';
   req.on('continue', function() {
     console.error('Client got 100 Continue...');
     got_continue = true;
diff --git a/test/parallel/test-http-extra-response.js b/test/parallel/test-http-extra-response.js
index 1866d88a33..6ebecaad13 100644
--- a/test/parallel/test-http-extra-response.js
+++ b/test/parallel/test-http-extra-response.js
@@ -8,8 +8,8 @@ const net = require('net');
 // node should ignore it and drop the connection.
 // Demos this bug: https://github.com/joyent/node/issues/680
 
-var body = 'hello world\r\n';
-var fullResponse =
+const body = 'hello world\r\n';
+const fullResponse =
     'HTTP/1.1 500 Internal Server Error\r\n' +
     'Content-Length: ' + body.length + '\r\n' +
     'Content-Type: text/plain\r\n' +
@@ -20,8 +20,8 @@ var fullResponse =
     '\r\n' +
     body;
 
-var server = net.createServer(function(socket) {
-  var postBody = '';
+const server = net.createServer(function(socket) {
+  let postBody = '';
 
   socket.setEncoding('utf8');
 
@@ -43,7 +43,7 @@ var server = net.createServer(function(socket) {
 
 server.listen(0, common.mustCall(function() {
   http.get({ port: this.address().port }, common.mustCall(function(res) {
-    var buffer = '';
+    let buffer = '';
     console.log('Got res code: ' + res.statusCode);
 
     res.setEncoding('utf8');
diff --git a/test/parallel/test-http-flush-response-headers.js b/test/parallel/test-http-flush-response-headers.js
index 1cd8402df3..ea7d2d97b4 100644
--- a/test/parallel/test-http-flush-response-headers.js
+++ b/test/parallel/test-http-flush-response-headers.js
@@ -11,7 +11,7 @@ server.on('request', function(req, res) {
   res.flushHeaders(); // Should be idempotent.
 });
 server.listen(0, common.localhostIPv4, function() {
-  var req = http.request({
+  const req = http.request({
     method: 'GET',
     host: common.localhostIPv4,
     port: this.address().port,
diff --git a/test/parallel/test-http-flush.js b/test/parallel/test-http-flush.js
index 524965d2d7..e8f5782c2f 100644
--- a/test/parallel/test-http-flush.js
+++ b/test/parallel/test-http-flush.js
@@ -6,7 +6,7 @@ http.createServer(function(req, res) {
   res.end('ok');
   this.close();
 }).listen(0, '127.0.0.1', function() {
-  var req = http.request({
+  const req = http.request({
     method: 'POST',
     host: '127.0.0.1',
     port: this.address().port,
diff --git a/test/parallel/test-http-full-response.js b/test/parallel/test-http-full-response.js
index be6749cc04..c834c6e654 100644
--- a/test/parallel/test-http-full-response.js
+++ b/test/parallel/test-http-full-response.js
@@ -5,11 +5,11 @@ const assert = require('assert');
 const http = require('http');
 const exec = require('child_process').exec;
 
-var bodyLength = 12345;
+const bodyLength = 12345;
 
-var body = 'c'.repeat(bodyLength);
+const body = 'c'.repeat(bodyLength);
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200, {
     'Content-Length': bodyLength,
     'Content-Type': 'text/plain'
@@ -18,7 +18,7 @@ var server = http.createServer(function(req, res) {
 });
 
 function runAb(opts, callback) {
-  var command = `ab ${opts} http://127.0.0.1:${server.address().port}/`;
+  const command = `ab ${opts} http://127.0.0.1:${server.address().port}/`;
   exec(command, function(err, stdout, stderr) {
     if (err) {
       if (/ab|apr/mi.test(stderr)) {
@@ -29,14 +29,14 @@ function runAb(opts, callback) {
       return;
     }
 
-    var m = /Document Length:\s*(\d+) bytes/mi.exec(stdout);
-    var documentLength = parseInt(m[1]);
+    let m = /Document Length:\s*(\d+) bytes/mi.exec(stdout);
+    const documentLength = parseInt(m[1]);
 
     m = /Complete requests:\s*(\d+)/mi.exec(stdout);
-    var completeRequests = parseInt(m[1]);
+    const completeRequests = parseInt(m[1]);
 
     m = /HTML transferred:\s*(\d+) bytes/mi.exec(stdout);
-    var htmlTransfered = parseInt(m[1]);
+    const htmlTransfered = parseInt(m[1]);
 
     assert.equal(bodyLength, documentLength);
     assert.equal(completeRequests * documentLength, htmlTransfered);
diff --git a/test/parallel/test-http-get-pipeline-problem.js b/test/parallel/test-http-get-pipeline-problem.js
index d4e075d229..15c96a9a4e 100644
--- a/test/parallel/test-http-get-pipeline-problem.js
+++ b/test/parallel/test-http-get-pipeline-problem.js
@@ -11,14 +11,14 @@ http.globalAgent.maxSockets = 1;
 
 common.refreshTmpDir();
 
-var image = fs.readFileSync(common.fixturesDir + '/person.jpg');
+const image = fs.readFileSync(common.fixturesDir + '/person.jpg');
 
 console.log('image.length = ' + image.length);
 
-var total = 10;
-var requests = 0, responses = 0;
+const total = 10;
+let requests = 0, responses = 0;
 
-var server = http.Server(function(req, res) {
+const server = http.Server(function(req, res) {
   if (++requests === total) {
     server.close();
   }
@@ -35,18 +35,18 @@ var server = http.Server(function(req, res) {
 
 
 server.listen(0, function() {
-  for (var i = 0; i < total; i++) {
+  for (let i = 0; i < total; i++) {
     (function() {
-      var x = i;
+      const x = i;
 
-      var opts = {
+      const opts = {
         port: server.address().port,
         headers: { connection: 'close' }
       };
 
       http.get(opts, function(res) {
         console.error('recv ' + x);
-        var s = fs.createWriteStream(common.tmpDir + '/' + x + '.jpg');
+        const s = fs.createWriteStream(common.tmpDir + '/' + x + '.jpg');
         res.pipe(s);
 
         s.on('finish', function() {
@@ -64,16 +64,16 @@ server.listen(0, function() {
 });
 
 
-var checkedFiles = false;
+let checkedFiles = false;
 function checkFiles() {
   // Should see 1.jpg, 2.jpg, ..., 100.jpg in tmpDir
-  var files = fs.readdirSync(common.tmpDir);
+  const files = fs.readdirSync(common.tmpDir);
   assert(total <= files.length);
 
-  for (var i = 0; i < total; i++) {
-    var fn = i + '.jpg';
+  for (let i = 0; i < total; i++) {
+    const fn = i + '.jpg';
     assert.ok(files.indexOf(fn) >= 0, "couldn't find '" + fn + "'");
-    var stat = fs.statSync(common.tmpDir + '/' + fn);
+    const stat = fs.statSync(common.tmpDir + '/' + fn);
     assert.equal(image.length, stat.size,
                  "size doesn't match on '" + fn +
                  "'. Got " + stat.size + ' bytes');
diff --git a/test/parallel/test-http-head-request.js b/test/parallel/test-http-head-request.js
index eb31930a34..de94dfb8b7 100644
--- a/test/parallel/test-http-head-request.js
+++ b/test/parallel/test-http-head-request.js
@@ -2,10 +2,10 @@
 const common = require('../common');
 const http = require('http');
 
-var body = 'hello world\n';
+const body = 'hello world\n';
 
 function test(headers) {
-  var server = http.createServer(function(req, res) {
+  const server = http.createServer(function(req, res) {
     console.error('req: %s headers: %j', req.method, headers);
     res.writeHead(200, headers);
     res.end();
@@ -13,7 +13,7 @@ function test(headers) {
   });
 
   server.listen(0, common.mustCall(function() {
-    var request = http.request({
+    const request = http.request({
       port: this.address().port,
       method: 'HEAD',
       path: '/'
diff --git a/test/parallel/test-http-head-response-has-no-body-end.js b/test/parallel/test-http-head-response-has-no-body-end.js
index cd2c22c8fa..de5ca976fc 100644
--- a/test/parallel/test-http-head-response-has-no-body-end.js
+++ b/test/parallel/test-http-head-response-has-no-body-end.js
@@ -6,14 +6,14 @@ const http = require('http');
 // responds to a HEAD request with data to res.end,
 // it does not send any body.
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200);
   res.end('FAIL'); // broken: sends FAIL from hot path.
 });
 server.listen(0);
 
 server.on('listening', common.mustCall(function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'HEAD',
     path: '/'
diff --git a/test/parallel/test-http-head-response-has-no-body.js b/test/parallel/test-http-head-response-has-no-body.js
index 17107d3b67..b2eb98b1ee 100644
--- a/test/parallel/test-http-head-response-has-no-body.js
+++ b/test/parallel/test-http-head-response-has-no-body.js
@@ -6,14 +6,14 @@ const http = require('http');
 // responds to a HEAD request, it does not send any body.
 // In this case it was sending '0\r\n\r\n'
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200); // broken: defaults to TE chunked
   res.end();
 });
 server.listen(0);
 
 server.on('listening', common.mustCall(function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'HEAD',
     path: '/'
diff --git a/test/parallel/test-http-header-read.js b/test/parallel/test-http-header-read.js
index 42d7e9af5f..3d9e756d50 100644
--- a/test/parallel/test-http-header-read.js
+++ b/test/parallel/test-http-header-read.js
@@ -6,9 +6,9 @@ const http = require('http');
 // Verify that ServerResponse.getHeader() works correctly even after
 // the response header has been sent. Issue 752 on github.
 
-var s = http.createServer(function(req, res) {
-  var contentType = 'Content-Type';
-  var plain = 'text/plain';
+const s = http.createServer(function(req, res) {
+  const contentType = 'Content-Type';
+  const plain = 'text/plain';
   res.setHeader(contentType, plain);
   assert.ok(!res.headersSent);
   res.writeHead(200);
diff --git a/test/parallel/test-http-hex-write.js b/test/parallel/test-http-hex-write.js
index 2fcabb0691..211f818e56 100644
--- a/test/parallel/test-http-hex-write.js
+++ b/test/parallel/test-http-hex-write.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 
 const http = require('http');
 
-var expect = 'hex\nutf8\n';
+const expect = 'hex\nutf8\n';
 
 http.createServer(function(q, s) {
   s.setHeader('content-length', expect.length);
@@ -15,7 +15,7 @@ http.createServer(function(q, s) {
 }).listen(0, common.mustCall(function() {
   http.request({ port: this.address().port })
     .on('response', common.mustCall(function(res) {
-      var data = '';
+      let data = '';
 
       res.setEncoding('ascii');
       res.on('data', function(c) {
diff --git a/test/parallel/test-http-host-header-ipv6-fail.js b/test/parallel/test-http-host-header-ipv6-fail.js
index 5f81234edb..94351bc5c1 100644
--- a/test/parallel/test-http-host-header-ipv6-fail.js
+++ b/test/parallel/test-http-host-header-ipv6-fail.js
@@ -16,7 +16,7 @@ const http = require('http');
 const hostname = '::1';
 
 function httpreq() {
-  var req = http.request({
+  const req = http.request({
     host: hostname,
     port: server.address().port,
     path: '/',
diff --git a/test/parallel/test-http-host-headers.js b/test/parallel/test-http-host-headers.js
index 0adf5716b7..0f6233190c 100644
--- a/test/parallel/test-http-host-headers.js
+++ b/test/parallel/test-http-host-headers.js
@@ -26,7 +26,7 @@ testHttp();
 
 function testHttp() {
 
-  var counter = 0;
+  let counter = 0;
 
   function cb(res) {
     counter--;
diff --git a/test/parallel/test-http-invalidheaderfield.js b/test/parallel/test-http-invalidheaderfield.js
index 9d4a12a952..aaed46c69c 100644
--- a/test/parallel/test-http-invalidheaderfield.js
+++ b/test/parallel/test-http-invalidheaderfield.js
@@ -5,7 +5,7 @@ const EventEmitter = require('events');
 const http = require('http');
 
 const ee = new EventEmitter();
-var count = 3;
+let count = 3;
 
 const server = http.createServer(function(req, res) {
   assert.doesNotThrow(function() {
@@ -24,7 +24,7 @@ server.listen(0, function() {
 
   assert.throws(
     function() {
-      var options = {
+      const options = {
         port: server.address().port,
         headers: {'testing 123': 123}
       };
@@ -38,7 +38,7 @@ server.listen(0, function() {
 
   assert.doesNotThrow(
     function() {
-      var options = {
+      const options = {
         port: server.address().port,
         headers: {'testing_123': 123}
       };
diff --git a/test/parallel/test-http-keep-alive-close-on-header.js b/test/parallel/test-http-keep-alive-close-on-header.js
index b69e90c038..15d5d62d42 100644
--- a/test/parallel/test-http-keep-alive-close-on-header.js
+++ b/test/parallel/test-http-keep-alive-close-on-header.js
@@ -3,22 +3,22 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var body = 'hello world\n';
-var headers = {'connection': 'keep-alive'};
+const body = 'hello world\n';
+const headers = {'connection': 'keep-alive'};
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200, {'Content-Length': body.length, 'Connection': 'close'});
   res.write(body);
   res.end();
 });
 
-var connectCount = 0;
+let connectCount = 0;
 
 
 server.listen(0, function() {
-  var agent = new http.Agent({ maxSockets: 1 });
-  var name = agent.getName({ port: this.address().port });
-  var request = http.request({
+  const agent = new http.Agent({ maxSockets: 1 });
+  const name = agent.getName({ port: this.address().port });
+  let request = http.request({
     method: 'GET',
     path: '/',
     headers: headers,
diff --git a/test/parallel/test-http-keepalive-client.js b/test/parallel/test-http-keepalive-client.js
index 55c3342a11..c9cf7bba96 100644
--- a/test/parallel/test-http-keepalive-client.js
+++ b/test/parallel/test-http-keepalive-client.js
@@ -5,8 +5,8 @@ const assert = require('assert');
 const http = require('http');
 
 
-var serverSocket = null;
-var server = http.createServer(function(req, res) {
+let serverSocket = null;
+const server = http.createServer(function(req, res) {
   // They should all come in on the same server socket.
   if (serverSocket) {
     assert.equal(req.socket, serverSocket);
@@ -20,12 +20,12 @@ server.listen(0, function() {
   makeRequest(expectRequests);
 });
 
-var agent = http.Agent({ keepAlive: true });
+const agent = http.Agent({ keepAlive: true });
 
 
-var clientSocket = null;
-var expectRequests = 10;
-var actualRequests = 0;
+let clientSocket = null;
+const expectRequests = 10;
+let actualRequests = 0;
 
 
 function makeRequest(n) {
@@ -35,7 +35,7 @@ function makeRequest(n) {
     return;
   }
 
-  var req = http.request({
+  const req = http.request({
     port: server.address().port,
     agent: agent,
     path: '/' + n
@@ -52,7 +52,7 @@ function makeRequest(n) {
   });
 
   req.on('response', function(res) {
-    var data = '';
+    let data = '';
     res.setEncoding('utf8');
     res.on('data', function(c) {
       data += c;
diff --git a/test/parallel/test-http-keepalive-maxsockets.js b/test/parallel/test-http-keepalive-maxsockets.js
index ffe1472a3c..14e9e88ef7 100644
--- a/test/parallel/test-http-keepalive-maxsockets.js
+++ b/test/parallel/test-http-keepalive-maxsockets.js
@@ -5,21 +5,21 @@ const assert = require('assert');
 const http = require('http');
 
 
-var serverSockets = [];
-var server = http.createServer(function(req, res) {
+const serverSockets = [];
+const server = http.createServer(function(req, res) {
   if (serverSockets.indexOf(req.socket) === -1) {
     serverSockets.push(req.socket);
   }
   res.end(req.url);
 });
 server.listen(0, function() {
-  var agent = http.Agent({
+  const agent = http.Agent({
     keepAlive: true,
     maxSockets: 5,
     maxFreeSockets: 2
   });
 
-  var closed = false;
+  let closed = false;
   makeReqs(10, function(er) {
     assert.ifError(er);
     assert.equal(count(agent.freeSockets), 2);
@@ -49,7 +49,7 @@ server.listen(0, function() {
   // make 10 requests in parallel,
   // then 10 more when they all finish.
   function makeReqs(n, cb) {
-    for (var i = 0; i < n; i++)
+    for (let i = 0; i < n; i++)
       makeReq(i, then);
 
     function then(er) {
@@ -66,7 +66,7 @@ server.listen(0, function() {
       path: '/' + i,
       agent: agent
     }, function(res) {
-      var data = '';
+      let data = '';
       res.setEncoding('ascii');
       res.on('data', function(c) {
         data += c;
diff --git a/test/parallel/test-http-keepalive-request.js b/test/parallel/test-http-keepalive-request.js
index 102f65bb08..dc9bdefb33 100644
--- a/test/parallel/test-http-keepalive-request.js
+++ b/test/parallel/test-http-keepalive-request.js
@@ -5,8 +5,8 @@ const assert = require('assert');
 const http = require('http');
 
 
-var serverSocket = null;
-var server = http.createServer(function(req, res) {
+let serverSocket = null;
+const server = http.createServer(function(req, res) {
   // They should all come in on the same server socket.
   if (serverSocket) {
     assert.equal(req.socket, serverSocket);
@@ -20,12 +20,12 @@ server.listen(0, function() {
   makeRequest(expectRequests);
 });
 
-var agent = http.Agent({ keepAlive: true });
+const agent = http.Agent({ keepAlive: true });
 
 
-var clientSocket = null;
-var expectRequests = 10;
-var actualRequests = 0;
+let clientSocket = null;
+const expectRequests = 10;
+let actualRequests = 0;
 
 
 function makeRequest(n) {
@@ -35,7 +35,7 @@ function makeRequest(n) {
     return;
   }
 
-  var req = http.request({
+  const req = http.request({
     port: server.address().port,
     path: '/' + n,
     agent: agent
@@ -52,7 +52,7 @@ function makeRequest(n) {
   });
 
   req.on('response', function(res) {
-    var data = '';
+    let data = '';
     res.setEncoding('utf8');
     res.on('data', function(c) {
       data += c;
diff --git a/test/parallel/test-http-localaddress-bind-error.js b/test/parallel/test-http-localaddress-bind-error.js
index 0290c6a584..a23c6f2a44 100644
--- a/test/parallel/test-http-localaddress-bind-error.js
+++ b/test/parallel/test-http-localaddress-bind-error.js
@@ -2,9 +2,9 @@
 const common = require('../common');
 const http = require('http');
 
-var invalidLocalAddress = '1.2.3.4';
+const invalidLocalAddress = '1.2.3.4';
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   console.log('Connect from: ' + req.connection.remoteAddress);
 
   req.on('end', function() {
diff --git a/test/parallel/test-http-localaddress.js b/test/parallel/test-http-localaddress.js
index d507d12ec8..e8df688886 100644
--- a/test/parallel/test-http-localaddress.js
+++ b/test/parallel/test-http-localaddress.js
@@ -8,7 +8,7 @@ if (!common.hasMultiLocalhost()) {
   return;
 }
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   console.log('Connect from: ' + req.connection.remoteAddress);
   assert.equal('127.0.0.2', req.connection.remoteAddress);
 
@@ -20,13 +20,13 @@ var server = http.createServer(function(req, res) {
 });
 
 server.listen(0, '127.0.0.1', function() {
-  var options = { host: 'localhost',
-                  port: this.address().port,
-                  path: '/',
-                  method: 'GET',
-                  localAddress: '127.0.0.2' };
+  const options = { host: 'localhost',
+                    port: this.address().port,
+                    path: '/',
+                    method: 'GET',
+                    localAddress: '127.0.0.2' };
 
-  var req = http.request(options, function(res) {
+  const req = http.request(options, function(res) {
     res.on('end', function() {
       server.close();
       process.exit();
diff --git a/test/parallel/test-http-malformed-request.js b/test/parallel/test-http-malformed-request.js
index 6739a44b6c..a36c9fdbe2 100644
--- a/test/parallel/test-http-malformed-request.js
+++ b/test/parallel/test-http-malformed-request.js
@@ -8,10 +8,10 @@ const url = require('url');
 // Make sure no exceptions are thrown when receiving malformed HTTP
 // requests.
 
-var nrequests_completed = 0;
-var nrequests_expected = 1;
+let nrequests_completed = 0;
+const nrequests_expected = 1;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   console.log('req: ' + JSON.stringify(url.parse(req.url)));
 
   res.writeHead(200, {'Content-Type': 'text/plain'});
@@ -23,7 +23,7 @@ var server = http.createServer(function(req, res) {
 server.listen(0);
 
 server.on('listening', function() {
-  var c = net.createConnection(this.address().port);
+  const c = net.createConnection(this.address().port);
   c.on('connect', function() {
     c.write('GET /hello?foo=%99bar HTTP/1.1\r\n\r\n');
     c.end();
diff --git a/test/parallel/test-http-many-ended-pipelines.js b/test/parallel/test-http-many-ended-pipelines.js
index 5fa8e53e01..b4af99ef00 100644
--- a/test/parallel/test-http-many-ended-pipelines.js
+++ b/test/parallel/test-http-many-ended-pipelines.js
@@ -2,7 +2,7 @@
 require('../common');
 
 // no warnings should happen!
-var trace = console.trace;
+const trace = console.trace;
 console.trace = function() {
   trace.apply(console, arguments);
   throw new Error('no tracing should happen here');
@@ -11,10 +11,10 @@ console.trace = function() {
 const http = require('http');
 const net = require('net');
 
-var numRequests = 20;
-var first = false;
+const numRequests = 20;
+let first = false;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   if (!first) {
     first = true;
     req.socket.on('close', function() {
@@ -28,8 +28,9 @@ var server = http.createServer(function(req, res) {
 });
 
 server.listen(0, function() {
-  var client = net.connect({ port: this.address().port, allowHalfOpen: true });
-  for (var i = 0; i < numRequests; i++) {
+  const client = net.connect({ port: this.address().port,
+                               allowHalfOpen: true });
+  for (let i = 0; i < numRequests; i++) {
     client.write('GET / HTTP/1.1\r\n' +
                  'Host: some.host.name\r\n' +
                  '\r\n\r\n');
diff --git a/test/parallel/test-http-max-headers-count.js b/test/parallel/test-http-max-headers-count.js
index 0abddbd2cb..018ed33c1b 100644
--- a/test/parallel/test-http-max-headers-count.js
+++ b/test/parallel/test-http-max-headers-count.js
@@ -3,24 +3,24 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var requests = 0;
-var responses = 0;
+let requests = 0;
+let responses = 0;
 
-var headers = {};
-var N = 2000;
-for (var i = 0; i < N; ++i) {
+const headers = {};
+const N = 2000;
+for (let i = 0; i < N; ++i) {
   headers['key' + i] = i;
 }
 
-var maxAndExpected = [ // for server
+const maxAndExpected = [ // for server
   [50, 50],
   [1500, 1500],
   [0, N + 2] // Host and Connection
 ];
-var max = maxAndExpected[requests][0];
-var expected = maxAndExpected[requests][1];
+let max = maxAndExpected[requests][0];
+let expected = maxAndExpected[requests][1];
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   assert.equal(Object.keys(req.headers).length, expected);
   if (++requests < maxAndExpected.length) {
     max = maxAndExpected[requests][0];
@@ -33,7 +33,7 @@ var server = http.createServer(function(req, res) {
 server.maxHeadersCount = max;
 
 server.listen(0, function() {
-  var maxAndExpected = [ // for client
+  const maxAndExpected = [ // for client
     [20, 20],
     [1200, 1200],
     [0, N + 3] // Connection, Date and Transfer-Encoding
@@ -41,9 +41,9 @@ server.listen(0, function() {
   doRequest();
 
   function doRequest() {
-    var max = maxAndExpected[responses][0];
-    var expected = maxAndExpected[responses][1];
-    var req = http.request({
+    const max = maxAndExpected[responses][0];
+    const expected = maxAndExpected[responses][1];
+    const req = http.request({
       port: server.address().port,
       headers: headers
     }, function(res) {
diff --git a/test/parallel/test-http-multi-line-headers.js b/test/parallel/test-http-multi-line-headers.js
index 9c0d758ed5..2096f533a1 100644
--- a/test/parallel/test-http-multi-line-headers.js
+++ b/test/parallel/test-http-multi-line-headers.js
@@ -5,10 +5,10 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var server = net.createServer(function(conn) {
-  var body = 'Yet another node.js server.';
+const server = net.createServer(function(conn) {
+  const body = 'Yet another node.js server.';
 
-  var response =
+  const response =
       'HTTP/1.1 200 OK\r\n' +
       'Connection: close\r\n' +
       'Content-Length: ' + body.length + '\r\n' +
diff --git a/test/parallel/test-http-no-content-length.js b/test/parallel/test-http-no-content-length.js
index a393d58d57..a6cfad13aa 100644
--- a/test/parallel/test-http-no-content-length.js
+++ b/test/parallel/test-http-no-content-length.js
@@ -4,12 +4,12 @@ const assert = require('assert');
 const net = require('net');
 const http = require('http');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   // Neither Content-Length nor Connection
   socket.end('HTTP/1.1 200 ok\r\n\r\nHello');
 }).listen(0, common.mustCall(function() {
   http.get({port: this.address().port}, common.mustCall(function(res) {
-    var body = '';
+    let body = '';
 
     res.setEncoding('utf8');
     res.on('data', function(chunk) {
diff --git a/test/parallel/test-http-outgoing-finish.js b/test/parallel/test-http-outgoing-finish.js
index 06630e6178..73654909d8 100644
--- a/test/parallel/test-http-outgoing-finish.js
+++ b/test/parallel/test-http-outgoing-finish.js
@@ -11,7 +11,7 @@ http.createServer(function(req, res) {
   });
   this.close();
 }).listen(0, function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'PUT'
   });
@@ -21,11 +21,11 @@ http.createServer(function(req, res) {
   });
 });
 
-var buf = Buffer.alloc(1024 * 16, 'x');
+const buf = Buffer.alloc(1024 * 16, 'x');
 function write(out) {
-  var name = out.constructor.name;
-  var finishEvent = false;
-  var endCb = false;
+  const name = out.constructor.name;
+  let finishEvent = false;
+  let endCb = false;
 
   // first, write until it gets some backpressure
   while (out.write(buf)) {}
diff --git a/test/parallel/test-http-parser-bad-ref.js b/test/parallel/test-http-parser-bad-ref.js
index 9eddb2bb3b..69682128fa 100644
--- a/test/parallel/test-http-parser-bad-ref.js
+++ b/test/parallel/test-http-parser-bad-ref.js
@@ -6,15 +6,15 @@
 
 require('../common');
 const assert = require('assert');
-var HTTPParser = process.binding('http_parser').HTTPParser;
+const HTTPParser = process.binding('http_parser').HTTPParser;
 
-var kOnHeaders = HTTPParser.kOnHeaders | 0;
-var kOnHeadersComplete = HTTPParser.kOnHeadersComplete | 0;
-var kOnBody = HTTPParser.kOnBody | 0;
-var kOnMessageComplete = HTTPParser.kOnMessageComplete | 0;
+const kOnHeaders = HTTPParser.kOnHeaders | 0;
+const kOnHeadersComplete = HTTPParser.kOnHeadersComplete | 0;
+const kOnBody = HTTPParser.kOnBody | 0;
+const kOnMessageComplete = HTTPParser.kOnMessageComplete | 0;
 
-var headersComplete = 0;
-var messagesComplete = 0;
+let headersComplete = 0;
+let messagesComplete = 0;
 
 function flushPool() {
   Buffer.allocUnsafe(Buffer.poolSize - 1);
@@ -24,7 +24,7 @@ function flushPool() {
 function demoBug(part1, part2) {
   flushPool();
 
-  var parser = new HTTPParser('REQUEST');
+  const parser = new HTTPParser('REQUEST');
 
   parser.headers = [];
   parser.url = '';
@@ -49,7 +49,7 @@ function demoBug(part1, part2) {
   // We use a function to eliminate references to the Buffer b
   // We want b to be GCed. The parser will hold a bad reference to it.
   (function() {
-    var b = Buffer.from(part1);
+    const b = Buffer.from(part1);
     flushPool();
 
     console.log('parse the first part of the message');
@@ -59,7 +59,7 @@ function demoBug(part1, part2) {
   flushPool();
 
   (function() {
-    var b = Buffer.from(part2);
+    const b = Buffer.from(part2);
 
     console.log('parse the second part of the message');
     parser.execute(b, 0, b.length);
diff --git a/test/parallel/test-http-parser-free.js b/test/parallel/test-http-parser-free.js
index e9ddfa27e5..80cc3cc64e 100644
--- a/test/parallel/test-http-parser-free.js
+++ b/test/parallel/test-http-parser-free.js
@@ -2,19 +2,19 @@
 require('../common');
 const assert = require('assert');
 const http = require('http');
-var N = 100;
-var responses = 0;
+const N = 100;
+let responses = 0;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.end('Hello');
 });
 
 server.listen(0, function() {
   http.globalAgent.maxSockets = 1;
-  var parser;
-  for (var i = 0; i < N; ++i) {
+  let parser;
+  for (let i = 0; i < N; ++i) {
     (function makeRequest(i) {
-      var req = http.get({port: server.address().port}, function(res) {
+      const req = http.get({port: server.address().port}, function(res) {
         if (!parser) {
           parser = req.parser;
         } else {
diff --git a/test/parallel/test-http-parser.js b/test/parallel/test-http-parser.js
index 8617a093fb..e1215fbd13 100644
--- a/test/parallel/test-http-parser.js
+++ b/test/parallel/test-http-parser.js
@@ -6,14 +6,14 @@ const binding = process.binding('http_parser');
 const methods = binding.methods;
 const HTTPParser = binding.HTTPParser;
 
-var CRLF = '\r\n';
-var REQUEST = HTTPParser.REQUEST;
-var RESPONSE = HTTPParser.RESPONSE;
+const CRLF = '\r\n';
+const REQUEST = HTTPParser.REQUEST;
+const RESPONSE = HTTPParser.RESPONSE;
 
-var kOnHeaders = HTTPParser.kOnHeaders | 0;
-var kOnHeadersComplete = HTTPParser.kOnHeadersComplete | 0;
-var kOnBody = HTTPParser.kOnBody | 0;
-var kOnMessageComplete = HTTPParser.kOnMessageComplete | 0;
+const kOnHeaders = HTTPParser.kOnHeaders | 0;
+const kOnHeadersComplete = HTTPParser.kOnHeadersComplete | 0;
+const kOnBody = HTTPParser.kOnBody | 0;
+const kOnMessageComplete = HTTPParser.kOnMessageComplete | 0;
 
 // The purpose of this test is not to check HTTP compliance but to test the
 // binding. Tests for pathological http messages should be submitted
@@ -22,7 +22,7 @@ var kOnMessageComplete = HTTPParser.kOnMessageComplete | 0;
 
 
 function newParser(type) {
-  var parser = new HTTPParser(type);
+  const parser = new HTTPParser(type);
 
   parser.headers = [];
   parser.url = '';
@@ -47,7 +47,7 @@ function newParser(type) {
 
 
 function mustCall(f, times) {
-  var actual = 0;
+  let actual = 0;
 
   process.setMaxListeners(256);
   process.on('exit', function() {
@@ -63,7 +63,7 @@ function mustCall(f, times) {
 
 function expectBody(expected) {
   return mustCall(function(buf, start, len) {
-    var body = '' + buf.slice(start, start + len);
+    const body = '' + buf.slice(start, start + len);
     assert.equal(body, expected);
   });
 }
@@ -447,11 +447,11 @@ function expectBody(expected) {
     assert.strictEqual(expected_body, '');
   }
 
-  for (var i = 1; i < request.length - 1; ++i) {
-    var a = request.slice(0, i);
+  for (let i = 1; i < request.length - 1; ++i) {
+    const a = request.slice(0, i);
     console.error('request.slice(0, ' + i + ') = ',
                   JSON.stringify(a.toString()));
-    var b = request.slice(i);
+    const b = request.slice(i);
     console.error('request.slice(' + i + ') = ',
                   JSON.stringify(b.toString()));
     test(a, b);
@@ -571,11 +571,11 @@ function expectBody(expected) {
 // Test parser 'this' safety
 // https://github.com/joyent/node/issues/6690
 assert.throws(function() {
-  var request = Buffer.from(
+  const request = Buffer.from(
       'GET /hello HTTP/1.1' + CRLF +
       CRLF);
 
-  var parser = newParser(REQUEST);
-  var notparser = { execute: parser.execute };
+  const parser = newParser(REQUEST);
+  const notparser = { execute: parser.execute };
   notparser.execute(request, 0, request.length);
 }, TypeError);
diff --git a/test/parallel/test-http-pause.js b/test/parallel/test-http-pause.js
index 3c96dc6792..8bb8dd4b5d 100644
--- a/test/parallel/test-http-pause.js
+++ b/test/parallel/test-http-pause.js
@@ -3,12 +3,12 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var expectedServer = 'Request Body from Client';
-var resultServer = '';
-var expectedClient = 'Response Body from Server';
-var resultClient = '';
+const expectedServer = 'Request Body from Client';
+let resultServer = '';
+const expectedClient = 'Response Body from Server';
+let resultClient = '';
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   console.error('pause server request');
   req.pause();
   setTimeout(function() {
@@ -27,7 +27,7 @@ var server = http.createServer(function(req, res) {
 });
 
 server.listen(0, function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     path: '/',
     method: 'POST'
diff --git a/test/parallel/test-http-pipe-fs.js b/test/parallel/test-http-pipe-fs.js
index 5b3eb87a14..5348637bd0 100644
--- a/test/parallel/test-http-pipe-fs.js
+++ b/test/parallel/test-http-pipe-fs.js
@@ -6,10 +6,10 @@ const path = require('path');
 
 common.refreshTmpDir();
 
-var file = path.join(common.tmpDir, 'http-pipe-fs-test.txt');
+const file = path.join(common.tmpDir, 'http-pipe-fs-test.txt');
 
-var server = http.createServer(common.mustCall(function(req, res) {
-  var stream = fs.createWriteStream(file);
+const server = http.createServer(common.mustCall(function(req, res) {
+  const stream = fs.createWriteStream(file);
   req.pipe(stream);
   stream.on('close', function() {
     res.writeHead(200);
@@ -18,9 +18,9 @@ var server = http.createServer(common.mustCall(function(req, res) {
 }, 2)).listen(0, function() {
   http.globalAgent.maxSockets = 1;
 
-  for (var i = 0; i < 2; ++i) {
+  for (let i = 0; i < 2; ++i) {
     (function(i) {
-      var req = http.request({
+      const req = http.request({
         port: server.address().port,
         method: 'POST',
         headers: {
diff --git a/test/parallel/test-http-pipeline-flood.js b/test/parallel/test-http-pipeline-flood.js
index 4f6775656e..99e4c3e7b5 100644
--- a/test/parallel/test-http-pipeline-flood.js
+++ b/test/parallel/test-http-pipeline-flood.js
@@ -26,7 +26,7 @@ switch (process.argv[2]) {
 function parent() {
   const http = require('http');
   const bigResponse = Buffer.alloc(10240, 'x');
-  var backloggedReqs = 0;
+  let backloggedReqs = 0;
 
   const server = http.createServer(function(req, res) {
     res.setHeader('content-length', bigResponse.length);
@@ -68,7 +68,7 @@ function child() {
   const port = +process.argv[3];
   const conn = net.connect({ port: port });
 
-  var req = `GET / HTTP/1.1\r\nHost: localhost:${port}\r\nAccept: */*\r\n\r\n`;
+  let req = `GET / HTTP/1.1\r\nHost: localhost:${port}\r\nAccept: */*\r\n\r\n`;
 
   req = new Array(10241).join(req);
 
diff --git a/test/parallel/test-http-pipeline-regr-2639.js b/test/parallel/test-http-pipeline-regr-2639.js
index f67803ba15..7933e419d3 100644
--- a/test/parallel/test-http-pipeline-regr-2639.js
+++ b/test/parallel/test-http-pipeline-regr-2639.js
@@ -6,9 +6,9 @@ const net = require('net');
 
 const COUNT = 10;
 
-var received = 0;
+let received = 0;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   // Close the server, we have only one TCP connection anyway
   if (received++ === 0)
     server.close();
@@ -22,7 +22,7 @@ var server = http.createServer(function(req, res) {
 }).listen(0, function() {
   const s = net.connect(this.address().port);
 
-  var big = 'GET / HTTP/1.0\r\n\r\n'.repeat(COUNT);
+  const big = 'GET / HTTP/1.0\r\n\r\n'.repeat(COUNT);
 
   s.write(big);
   s.resume();
diff --git a/test/parallel/test-http-pipeline-regr-3332.js b/test/parallel/test-http-pipeline-regr-3332.js
index fdbb76bf8d..3ab9e6104c 100644
--- a/test/parallel/test-http-pipeline-regr-3332.js
+++ b/test/parallel/test-http-pipeline-regr-3332.js
@@ -8,9 +8,9 @@ const big = Buffer.alloc(16 * 1024, 'A');
 
 const COUNT = 1e4;
 
-var received = 0;
+let received = 0;
 
-var client;
+let client;
 const server = http.createServer(function(req, res) {
   res.end(big, function() {
     if (++received === COUNT) {
@@ -19,7 +19,7 @@ const server = http.createServer(function(req, res) {
     }
   });
 }).listen(0, function() {
-  var req = new Array(COUNT + 1).join('GET / HTTP/1.1\r\n\r\n');
+  const req = new Array(COUNT + 1).join('GET / HTTP/1.1\r\n\r\n');
   client = net.connect(this.address().port, function() {
     client.write(req);
   });
diff --git a/test/parallel/test-http-pipeline-regr-3508.js b/test/parallel/test-http-pipeline-regr-3508.js
index b368bae0d7..ff735818f6 100644
--- a/test/parallel/test-http-pipeline-regr-3508.js
+++ b/test/parallel/test-http-pipeline-regr-3508.js
@@ -3,18 +3,18 @@ require('../common');
 const http = require('http');
 const net = require('net');
 
-var once = false;
-var first = null;
-var second = null;
+let once = false;
+let first = null;
+let second = null;
 
 const chunk = Buffer.alloc(1024, 'X');
 
-var size = 0;
+let size = 0;
 
-var more;
-var done;
+let more;
+let done;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   if (!once)
     server.close();
   once = true;
@@ -41,7 +41,7 @@ var server = http.createServer(function(req, res) {
   });
   first.end('hello');
 }).listen(0, function() {
-  var s = net.connect(this.address().port);
+  const s = net.connect(this.address().port);
   more = function() {
     s.write('GET / HTTP/1.1\r\n\r\n');
   };
diff --git a/test/parallel/test-http-proxy.js b/test/parallel/test-http-proxy.js
index bfa928afb3..8e7fd55a0e 100644
--- a/test/parallel/test-http-proxy.js
+++ b/test/parallel/test-http-proxy.js
@@ -4,23 +4,23 @@ const assert = require('assert');
 const http = require('http');
 const url = require('url');
 
-var cookies = [
+const cookies = [
   'session_token=; path=/; expires=Sun, 15-Sep-2030 13:48:52 GMT',
   'prefers_open_id=; path=/; expires=Thu, 01-Jan-1970 00:00:00 GMT'
 ];
 
-var headers = {'content-type': 'text/plain',
-               'set-cookie': cookies,
-               'hello': 'world' };
+const headers = {'content-type': 'text/plain',
+                 'set-cookie': cookies,
+                 'hello': 'world' };
 
-var backend = http.createServer(function(req, res) {
+const backend = http.createServer(function(req, res) {
   console.error('backend request');
   res.writeHead(200, headers);
   res.write('hello world\n');
   res.end();
 });
 
-var proxy = http.createServer(function(req, res) {
+const proxy = http.createServer(function(req, res) {
   console.error('proxy req headers: ' + JSON.stringify(req.headers));
   http.get({
     port: backend.address().port,
@@ -46,9 +46,9 @@ var proxy = http.createServer(function(req, res) {
   });
 });
 
-var body = '';
+let body = '';
 
-var nlistening = 0;
+let nlistening = 0;
 function startReq() {
   nlistening++;
   if (nlistening < 2) return;
diff --git a/test/parallel/test-http-raw-headers.js b/test/parallel/test-http-raw-headers.js
index 0c7cc18415..7ae0c35e3d 100644
--- a/test/parallel/test-http-raw-headers.js
+++ b/test/parallel/test-http-raw-headers.js
@@ -5,7 +5,7 @@ const assert = require('assert');
 const http = require('http');
 
 http.createServer(function(req, res) {
-  var expectRawHeaders = [
+  const expectRawHeaders = [
     'Host',
     `localhost:${this.address().port}`,
     'transfer-ENCODING',
@@ -15,13 +15,13 @@ http.createServer(function(req, res) {
     'Connection',
     'close'
   ];
-  var expectHeaders = {
+  const expectHeaders = {
     host: `localhost:${this.address().port}`,
     'transfer-encoding': 'CHUNKED',
     'x-bar': 'yoyoyo',
     connection: 'close'
   };
-  var expectRawTrailers = [
+  const expectRawTrailers = [
     'x-bAr',
     'yOyOyOy',
     'x-baR',
@@ -31,7 +31,7 @@ http.createServer(function(req, res) {
     'X-baR',
     'OyOyOyO'
   ];
-  var expectTrailers = { 'x-bar': 'yOyOyOy, OyOyOyO, yOyOyOy, OyOyOyO' };
+  const expectTrailers = { 'x-bar': 'yOyOyOy, OyOyOyO, yOyOyOy, OyOyOyO' };
 
   this.close();
 
@@ -53,7 +53,7 @@ http.createServer(function(req, res) {
   ]);
   res.end('x f o o');
 }).listen(0, function() {
-  var req = http.request({ port: this.address().port, path: '/' });
+  const req = http.request({ port: this.address().port, path: '/' });
   req.addTrailers([
     ['x-bAr', 'yOyOyOy'],
     ['x-baR', 'OyOyOyO'],
@@ -64,7 +64,7 @@ http.createServer(function(req, res) {
   req.setHeader('x-BaR', 'yoyoyo');
   req.end('y b a r');
   req.on('response', function(res) {
-    var expectRawHeaders = [
+    const expectRawHeaders = [
       'Trailer',
       'x-foo',
       'Date',
@@ -74,7 +74,7 @@ http.createServer(function(req, res) {
       'Transfer-Encoding',
       'chunked'
     ];
-    var expectHeaders = {
+    const expectHeaders = {
       trailer: 'x-foo',
       date: null,
       connection: 'close',
@@ -85,7 +85,7 @@ http.createServer(function(req, res) {
     assert.deepStrictEqual(res.rawHeaders, expectRawHeaders);
     assert.deepStrictEqual(res.headers, expectHeaders);
     res.on('end', function() {
-      var expectRawTrailers = [
+      const expectRawTrailers = [
         'x-fOo',
         'xOxOxOx',
         'x-foO',
@@ -95,7 +95,7 @@ http.createServer(function(req, res) {
         'X-foO',
         'OxOxOxO'
       ];
-      var expectTrailers = { 'x-foo': 'xOxOxOx, OxOxOxO, xOxOxOx, OxOxOxO' };
+      const expectTrailers = { 'x-foo': 'xOxOxOx, OxOxOxO, xOxOxOx, OxOxOxO' };
 
       assert.deepStrictEqual(res.rawTrailers, expectRawTrailers);
       assert.deepStrictEqual(res.trailers, expectTrailers);
diff --git a/test/parallel/test-http-remove-header-stays-removed.js b/test/parallel/test-http-remove-header-stays-removed.js
index bf417597b1..077ff0f5b3 100644
--- a/test/parallel/test-http-remove-header-stays-removed.js
+++ b/test/parallel/test-http-remove-header-stays-removed.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 
 const http = require('http');
 
-var server = http.createServer(function(request, response) {
+const server = http.createServer(function(request, response) {
   // removed headers should stay removed, even if node automatically adds them
   // to the output:
   response.removeHeader('connection');
@@ -20,7 +20,7 @@ var server = http.createServer(function(request, response) {
   this.close();
 });
 
-var response = '';
+let response = '';
 
 process.on('exit', function() {
   assert.equal('beep boop\n', response);
diff --git a/test/parallel/test-http-request-dont-override-options.js b/test/parallel/test-http-request-dont-override-options.js
index 5570dd49a2..04235fd0e4 100644
--- a/test/parallel/test-http-request-dont-override-options.js
+++ b/test/parallel/test-http-request-dont-override-options.js
@@ -4,7 +4,7 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var requests = 0;
+let requests = 0;
 
 http.createServer(function(req, res) {
   res.writeHead(200);
@@ -12,13 +12,13 @@ http.createServer(function(req, res) {
 
   requests++;
 }).listen(0, function() {
-  var agent = new http.Agent();
+  const agent = new http.Agent();
   agent.defaultPort = this.address().port;
 
   // options marked as explicitly undefined for readability
   // in this test, they should STAY undefined as options should not
   // be mutable / modified
-  var options = {
+  const options = {
     host: undefined,
     hostname: common.localhostIPv4,
     port: undefined,
diff --git a/test/parallel/test-http-request-end-twice.js b/test/parallel/test-http-request-end-twice.js
index 5dd21d06de..ab30c0a1ee 100644
--- a/test/parallel/test-http-request-end-twice.js
+++ b/test/parallel/test-http-request-end-twice.js
@@ -3,12 +3,12 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var server = http.Server(function(req, res) {
+const server = http.Server(function(req, res) {
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.end('hello world\n');
 });
 server.listen(0, function() {
-  var req = http.get({port: this.address().port}, function(res) {
+  const req = http.get({port: this.address().port}, function(res) {
     res.on('end', function() {
       assert.ok(!req.end());
       server.close();
diff --git a/test/parallel/test-http-request-end.js b/test/parallel/test-http-request-end.js
index 207c3564fb..dd4a8caf10 100644
--- a/test/parallel/test-http-request-end.js
+++ b/test/parallel/test-http-request-end.js
@@ -3,10 +3,10 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var expected = 'Post Body For Test';
+const expected = 'Post Body For Test';
 
-var server = http.Server(function(req, res) {
-  var result = '';
+const server = http.Server(function(req, res) {
+  let result = '';
 
   req.setEncoding('utf8');
   req.on('data', function(chunk) {
diff --git a/test/parallel/test-http-request-methods.js b/test/parallel/test-http-request-methods.js
index b24779adf8..e4579791ef 100644
--- a/test/parallel/test-http-request-methods.js
+++ b/test/parallel/test-http-request-methods.js
@@ -7,7 +7,7 @@ const http = require('http');
 // Test that the DELETE, PATCH and PURGE verbs get passed through correctly
 
 ['DELETE', 'PATCH', 'PURGE'].forEach(function(method, index) {
-  var server = http.createServer(common.mustCall(function(req, res) {
+  const server = http.createServer(common.mustCall(function(req, res) {
     assert.strictEqual(req.method, method);
     res.writeHead(200, {'Content-Type': 'text/plain'});
     res.write('hello ');
@@ -17,8 +17,8 @@ const http = require('http');
   server.listen(0);
 
   server.on('listening', common.mustCall(function() {
-    var c = net.createConnection(this.address().port);
-    var server_response = '';
+    const c = net.createConnection(this.address().port);
+    let server_response = '';
 
     c.setEncoding('utf8');
 
diff --git a/test/parallel/test-http-res-write-after-end.js b/test/parallel/test-http-res-write-after-end.js
index d01753b88a..2ee0e4e29d 100644
--- a/test/parallel/test-http-res-write-after-end.js
+++ b/test/parallel/test-http-res-write-after-end.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var server = http.Server(common.mustCall(function(req, res) {
+const server = http.Server(common.mustCall(function(req, res) {
   res.on('error', common.mustCall(function onResError(err) {
     assert.strictEqual(err.message, 'write after end');
   }));
@@ -11,7 +11,7 @@ var server = http.Server(common.mustCall(function(req, res) {
   res.write('This should write.');
   res.end();
 
-  var r = res.write('This should raise an error.');
+  const r = res.write('This should raise an error.');
   assert.equal(r, true, 'write after end should return true');
 }));
 
diff --git a/test/parallel/test-http-res-write-end-dont-take-array.js b/test/parallel/test-http-res-write-end-dont-take-array.js
index cbae755b4f..6964ec1596 100644
--- a/test/parallel/test-http-res-write-end-dont-take-array.js
+++ b/test/parallel/test-http-res-write-end-dont-take-array.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var test = 1;
+let test = 1;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200, {'Content-Type': 'text/plain'});
   if (test === 1) {
     // write should accept string
diff --git a/test/parallel/test-http-response-close.js b/test/parallel/test-http-response-close.js
index 6b5966735a..de179c113e 100644
--- a/test/parallel/test-http-response-close.js
+++ b/test/parallel/test-http-response-close.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const http = require('http');
 
-var server = http.createServer(common.mustCall(function(req, res) {
+const server = http.createServer(common.mustCall(function(req, res) {
   res.writeHead(200);
   res.write('a');
 
diff --git a/test/parallel/test-http-response-multi-content-length.js b/test/parallel/test-http-response-multi-content-length.js
index 3290f74c16..7cd802a74a 100644
--- a/test/parallel/test-http-response-multi-content-length.js
+++ b/test/parallel/test-http-response-multi-content-length.js
@@ -24,7 +24,7 @@ const server = http.createServer((req, res) => {
   res.end('ok');
 });
 
-var count = 0;
+let count = 0;
 
 server.listen(0, common.mustCall(() => {
   for (let n = 1; n <= MAX_COUNT; n++) {
diff --git a/test/parallel/test-http-response-multiheaders.js b/test/parallel/test-http-response-multiheaders.js
index f4c1fd4116..8f48dc2377 100644
--- a/test/parallel/test-http-response-multiheaders.js
+++ b/test/parallel/test-http-response-multiheaders.js
@@ -29,7 +29,7 @@ const norepeat = [
 ];
 
 const server = http.createServer(function(req, res) {
-  var num = req.headers['x-num'];
+  const num = req.headers['x-num'];
   if (num === '1') {
     for (const name of norepeat) {
       res.setHeader(name, ['A', 'B']);
@@ -47,7 +47,7 @@ const server = http.createServer(function(req, res) {
 });
 
 server.listen(0, common.mustCall(function() {
-  var count = 0;
+  let count = 0;
   for (let n = 1; n <= 2; n++) {
     // this runs twice, the first time, the server will use
     // setHeader, the second time it uses writeHead. The
diff --git a/test/parallel/test-http-response-no-headers.js b/test/parallel/test-http-response-no-headers.js
index 4c261fc2ad..3f44b4ef5f 100644
--- a/test/parallel/test-http-response-no-headers.js
+++ b/test/parallel/test-http-response-no-headers.js
@@ -4,28 +4,28 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var expected = {
+const expected = {
   '0.9': 'I AM THE WALRUS',
   '1.0': 'I AM THE WALRUS',
   '1.1': ''
 };
 
 function test(httpVersion, callback) {
-  var server = net.createServer(function(conn) {
-    var reply = 'HTTP/' + httpVersion + ' 200 OK\r\n\r\n' +
-                expected[httpVersion];
+  const server = net.createServer(function(conn) {
+    const reply = 'HTTP/' + httpVersion + ' 200 OK\r\n\r\n' +
+                  expected[httpVersion];
 
     conn.end(reply);
   });
 
   server.listen(0, '127.0.0.1', common.mustCall(function() {
-    var options = {
+    const options = {
       host: '127.0.0.1',
       port: this.address().port
     };
 
-    var req = http.get(options, common.mustCall(function(res) {
-      var body = '';
+    const req = http.get(options, common.mustCall(function(res) {
+      let body = '';
 
       res.on('data', function(data) {
         body += data;
diff --git a/test/parallel/test-http-response-readable.js b/test/parallel/test-http-response-readable.js
index f8ecd5bab5..07c0a40e4b 100644
--- a/test/parallel/test-http-response-readable.js
+++ b/test/parallel/test-http-response-readable.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var testServer = new http.Server(function(req, res) {
+const testServer = new http.Server(function(req, res) {
   res.writeHead(200);
   res.end('Hello world');
 });
diff --git a/test/parallel/test-http-response-splitting.js b/test/parallel/test-http-response-splitting.js
index e4021e7831..f6c3a04064 100644
--- a/test/parallel/test-http-response-splitting.js
+++ b/test/parallel/test-http-response-splitting.js
@@ -17,7 +17,7 @@ const str = '/welcome?lang=bar%c4%8d%c4%8aContent­Length:%200%c4%8d%c4%8a%c' +
 const x = 'fooഊSet-Cookie: foo=barഊഊ<script>alert("Hi!")</script>';
 const y = 'foo⠊Set-Cookie: foo=bar';
 
-var count = 0;
+let count = 0;
 
 const server = http.createServer((req, res) => {
   switch (count++) {
diff --git a/test/parallel/test-http-response-status-message.js b/test/parallel/test-http-response-status-message.js
index cfc9e64209..d17dfb24c6 100644
--- a/test/parallel/test-http-response-status-message.js
+++ b/test/parallel/test-http-response-status-message.js
@@ -4,9 +4,9 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var testsComplete = 0;
+let testsComplete = 0;
 
-var testCases = [
+const testCases = [
   { path: '/200', statusMessage: 'OK',
     response: 'HTTP/1.1 200 OK\r\n\r\n' },
   { path: '/500', statusMessage: 'Internal Server Error',
@@ -19,7 +19,7 @@ var testCases = [
     response: 'HTTP/1.1 200\r\n\r\n' }
 ];
 testCases.findByPath = function(path) {
-  var matching = this.filter(function(testCase) {
+  const matching = this.filter(function(testCase) {
     return testCase.path === path;
   });
   if (matching.length === 0) {
@@ -28,18 +28,18 @@ testCases.findByPath = function(path) {
   return matching[0];
 };
 
-var server = net.createServer(function(connection) {
+const server = net.createServer(function(connection) {
   connection.on('data', function(data) {
-    var path = data.toString().match(/GET (.*) HTTP.1.1/)[1];
-    var testCase = testCases.findByPath(path);
+    const path = data.toString().match(/GET (.*) HTTP.1.1/)[1];
+    const testCase = testCases.findByPath(path);
 
     connection.write(testCase.response);
     connection.end();
   });
 });
 
-var runTest = function(testCaseIndex) {
-  var testCase = testCases[testCaseIndex];
+const runTest = function(testCaseIndex) {
+  const testCase = testCases[testCaseIndex];
 
   http.get({
     port: server.address().port,
diff --git a/test/parallel/test-http-response-statuscode.js b/test/parallel/test-http-response-statuscode.js
index 57256db603..4bd75647ae 100644
--- a/test/parallel/test-http-response-statuscode.js
+++ b/test/parallel/test-http-response-statuscode.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const http = require('http');
 
 const MAX_REQUESTS = 12;
-var reqNum = 0;
+let reqNum = 0;
 
 const server = http.Server(common.mustCall(function(req, res) {
   switch (reqNum) {
diff --git a/test/parallel/test-http-server-client-error.js b/test/parallel/test-http-server-client-error.js
index cd20b7321a..260963dc24 100644
--- a/test/parallel/test-http-server-client-error.js
+++ b/test/parallel/test-http-server-client-error.js
@@ -21,7 +21,7 @@ server.listen(0, function() {
     const client = net.connect(server.address().port);
     client.end('Oopsie-doopsie\r\n');
 
-    var chunks = '';
+    let chunks = '';
     client.on('data', function(chunk) {
       chunks += chunk;
     });
diff --git a/test/parallel/test-http-server-multiheaders.js b/test/parallel/test-http-server-multiheaders.js
index b978cb3ad4..189ab0657b 100644
--- a/test/parallel/test-http-server-multiheaders.js
+++ b/test/parallel/test-http-server-multiheaders.js
@@ -7,7 +7,7 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var srv = http.createServer(function(req, res) {
+const srv = http.createServer(function(req, res) {
   assert.equal(req.headers.accept, 'abc, def, ghijklmnopqrst');
   assert.equal(req.headers.host, 'foo');
   assert.equal(req.headers['www-authenticate'], 'foo, bar, baz');
diff --git a/test/parallel/test-http-server-multiheaders2.js b/test/parallel/test-http-server-multiheaders2.js
index ba0710f70c..82cc996032 100644
--- a/test/parallel/test-http-server-multiheaders2.js
+++ b/test/parallel/test-http-server-multiheaders2.js
@@ -7,7 +7,7 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var multipleAllowed = [
+const multipleAllowed = [
   'Accept',
   'Accept-Charset',
   'Accept-Encoding',
@@ -31,7 +31,7 @@ var multipleAllowed = [
   'X-Some-Random-Header',
 ];
 
-var multipleForbidden = [
+const multipleForbidden = [
   'Content-Type',
   'User-Agent',
   'Referer',
@@ -48,7 +48,7 @@ var multipleForbidden = [
   //'Content-Length',
 ];
 
-var srv = http.createServer(function(req, res) {
+const srv = http.createServer(function(req, res) {
   multipleForbidden.forEach(function(header) {
     assert.equal(req.headers[header.toLowerCase()],
                  'foo', 'header parsed incorrectly: ' + header);
@@ -70,7 +70,7 @@ function makeHeader(value) {
   };
 }
 
-var headers = []
+const headers = []
   .concat(multipleAllowed.map(makeHeader('foo')))
   .concat(multipleForbidden.map(makeHeader('foo')))
   .concat(multipleAllowed.map(makeHeader('bar')))
diff --git a/test/parallel/test-http-server-stale-close.js b/test/parallel/test-http-server-stale-close.js
index 04e9635403..3728453c03 100644
--- a/test/parallel/test-http-server-stale-close.js
+++ b/test/parallel/test-http-server-stale-close.js
@@ -5,7 +5,7 @@ const util = require('util');
 const fork = require('child_process').fork;
 
 if (process.env.NODE_TEST_FORK_PORT) {
-  var req = http.request({
+  const req = http.request({
     headers: {'Content-Length': '42'},
     method: 'POST',
     host: '127.0.0.1',
@@ -14,7 +14,7 @@ if (process.env.NODE_TEST_FORK_PORT) {
   req.write('BAM');
   req.end();
 } else {
-  var server = http.createServer(function(req, res) {
+  const server = http.createServer(function(req, res) {
     res.writeHead(200, {'Content-Length': '42'});
     req.pipe(res);
     req.on('close', function() {
diff --git a/test/parallel/test-http-server-unconsume.js b/test/parallel/test-http-server-unconsume.js
index fd5486a289..fca2691aac 100644
--- a/test/parallel/test-http-server-unconsume.js
+++ b/test/parallel/test-http-server-unconsume.js
@@ -4,9 +4,9 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var received = '';
+let received = '';
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200);
   res.end();
 
@@ -16,7 +16,7 @@ var server = http.createServer(function(req, res) {
 
   server.close();
 }).listen(0, function() {
-  var socket = net.connect(this.address().port, function() {
+  const socket = net.connect(this.address().port, function() {
     socket.write('PUT / HTTP/1.1\r\n\r\n');
 
     socket.once('data', function() {
diff --git a/test/parallel/test-http-server.js b/test/parallel/test-http-server.js
index c915272af2..90c2709ebd 100644
--- a/test/parallel/test-http-server.js
+++ b/test/parallel/test-http-server.js
@@ -6,12 +6,12 @@ const http = require('http');
 const url = require('url');
 const qs = require('querystring');
 
-var request_number = 0;
-var requests_sent = 0;
-var server_response = '';
-var client_got_eof = false;
+let request_number = 0;
+let requests_sent = 0;
+let server_response = '';
+let client_got_eof = false;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.id = request_number;
   req.id = request_number++;
 
@@ -48,7 +48,7 @@ server.listen(0);
 server.httpAllowHalfOpen = true;
 
 server.on('listening', function() {
-  var c = net.createConnection(this.address().port);
+  const c = net.createConnection(this.address().port);
 
   c.setEncoding('utf8');
 
@@ -94,10 +94,10 @@ process.on('exit', function() {
   assert.strictEqual(4, request_number);
   assert.strictEqual(4, requests_sent);
 
-  var hello = new RegExp('/hello');
+  const hello = new RegExp('/hello');
   assert.notStrictEqual(null, hello.exec(server_response));
 
-  var quit = new RegExp('/quit');
+  const quit = new RegExp('/quit');
   assert.notStrictEqual(null, quit.exec(server_response));
 
   assert.strictEqual(true, client_got_eof);
diff --git a/test/parallel/test-http-set-cookies.js b/test/parallel/test-http-set-cookies.js
index 3fa01655b4..cb06352b3c 100644
--- a/test/parallel/test-http-set-cookies.js
+++ b/test/parallel/test-http-set-cookies.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var nresponses = 0;
+let nresponses = 0;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   if (req.url === '/one') {
     res.writeHead(200, [['set-cookie', 'A'],
                         ['content-type', 'text/plain']]);
diff --git a/test/parallel/test-http-set-timeout-server.js b/test/parallel/test-http-set-timeout-server.js
index 03fca8f219..7a2447763e 100644
--- a/test/parallel/test-http-set-timeout-server.js
+++ b/test/parallel/test-http-set-timeout-server.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var tests = [];
+const tests = [];
 
 function test(fn) {
   if (!tests.length)
@@ -13,7 +13,7 @@ function test(fn) {
 }
 
 function run() {
-  var fn = tests.shift();
+  const fn = tests.shift();
   if (fn) {
     console.log('# %s', fn.name);
     fn(run);
@@ -23,17 +23,17 @@ function run() {
 }
 
 test(function serverTimeout(cb) {
-  var caughtTimeout = false;
+  let caughtTimeout = false;
   process.on('exit', function() {
     assert(caughtTimeout);
   });
-  var server = http.createServer(function(req, res) {
+  const server = http.createServer(function(req, res) {
     // just do nothing, we should get a timeout event.
   });
   server.listen(common.mustCall(function() {
     http.get({ port: server.address().port }).on('error', function() {});
   }));
-  var s = server.setTimeout(50, function(socket) {
+  const s = server.setTimeout(50, function(socket) {
     caughtTimeout = true;
     socket.destroy();
     server.close();
@@ -43,13 +43,13 @@ test(function serverTimeout(cb) {
 });
 
 test(function serverRequestTimeout(cb) {
-  var caughtTimeout = false;
+  let caughtTimeout = false;
   process.on('exit', function() {
     assert(caughtTimeout);
   });
-  var server = http.createServer(function(req, res) {
+  const server = http.createServer(function(req, res) {
     // just do nothing, we should get a timeout event.
-    var s = req.setTimeout(50, function() {
+    const s = req.setTimeout(50, function() {
       caughtTimeout = true;
       req.socket.destroy();
       server.close();
@@ -58,8 +58,8 @@ test(function serverRequestTimeout(cb) {
     assert.ok(s instanceof http.IncomingMessage);
   });
   server.listen(common.mustCall(function() {
-    var port = server.address().port;
-    var req = http.request({ port: port, method: 'POST' });
+    const port = server.address().port;
+    const req = http.request({ port: port, method: 'POST' });
     req.on('error', function() {});
     req.write('Hello');
     // req is in progress
@@ -67,13 +67,13 @@ test(function serverRequestTimeout(cb) {
 });
 
 test(function serverResponseTimeout(cb) {
-  var caughtTimeout = false;
+  let caughtTimeout = false;
   process.on('exit', function() {
     assert(caughtTimeout);
   });
-  var server = http.createServer(function(req, res) {
+  const server = http.createServer(function(req, res) {
     // just do nothing, we should get a timeout event.
-    var s = res.setTimeout(50, function() {
+    const s = res.setTimeout(50, function() {
       caughtTimeout = true;
       res.socket.destroy();
       server.close();
@@ -82,21 +82,21 @@ test(function serverResponseTimeout(cb) {
     assert.ok(s instanceof http.OutgoingMessage);
   });
   server.listen(common.mustCall(function() {
-    var port = server.address().port;
+    const port = server.address().port;
     http.get({ port: port }).on('error', function() {});
   }));
 });
 
 test(function serverRequestNotTimeoutAfterEnd(cb) {
-  var caughtTimeoutOnRequest = false;
-  var caughtTimeoutOnResponse = false;
+  let caughtTimeoutOnRequest = false;
+  let caughtTimeoutOnResponse = false;
   process.on('exit', function() {
     assert(!caughtTimeoutOnRequest);
     assert(caughtTimeoutOnResponse);
   });
-  var server = http.createServer(function(req, res) {
+  const server = http.createServer(function(req, res) {
     // just do nothing, we should get a timeout event.
-    var s = req.setTimeout(50, function(socket) {
+    const s = req.setTimeout(50, function(socket) {
       caughtTimeoutOnRequest = true;
     });
     assert.ok(s instanceof http.IncomingMessage);
@@ -110,18 +110,18 @@ test(function serverRequestNotTimeoutAfterEnd(cb) {
     cb();
   });
   server.listen(common.mustCall(function() {
-    var port = server.address().port;
+    const port = server.address().port;
     http.get({ port: port }).on('error', function() {});
   }));
 });
 
 test(function serverResponseTimeoutWithPipeline(cb) {
-  var caughtTimeout = '';
+  let caughtTimeout = '';
   process.on('exit', function() {
     assert.equal(caughtTimeout, '/2');
   });
-  var server = http.createServer(function(req, res) {
-    var s = res.setTimeout(50, function() {
+  const server = http.createServer(function(req, res) {
+    const s = res.setTimeout(50, function() {
       caughtTimeout += req.url;
     });
     assert.ok(s instanceof http.OutgoingMessage);
@@ -133,8 +133,8 @@ test(function serverResponseTimeoutWithPipeline(cb) {
     cb();
   });
   server.listen(common.mustCall(function() {
-    var port = server.address().port;
-    var c = net.connect({ port: port, allowHalfOpen: true }, function() {
+    const port = server.address().port;
+    const c = net.connect({ port: port, allowHalfOpen: true }, function() {
       c.write('GET /1 HTTP/1.1\r\nHost: localhost\r\n\r\n');
       c.write('GET /2 HTTP/1.1\r\nHost: localhost\r\n\r\n');
       c.write('GET /3 HTTP/1.1\r\nHost: localhost\r\n\r\n');
@@ -143,15 +143,15 @@ test(function serverResponseTimeoutWithPipeline(cb) {
 });
 
 test(function idleTimeout(cb) {
-  var caughtTimeoutOnRequest = false;
-  var caughtTimeoutOnResponse = false;
-  var caughtTimeoutOnServer = false;
+  let caughtTimeoutOnRequest = false;
+  let caughtTimeoutOnResponse = false;
+  let caughtTimeoutOnServer = false;
   process.on('exit', function() {
     assert(!caughtTimeoutOnRequest);
     assert(!caughtTimeoutOnResponse);
     assert(caughtTimeoutOnServer);
   });
-  var server = http.createServer(function(req, res) {
+  const server = http.createServer(function(req, res) {
     req.on('timeout', function(socket) {
       caughtTimeoutOnRequest = true;
     });
@@ -160,7 +160,7 @@ test(function idleTimeout(cb) {
     });
     res.end();
   });
-  var s = server.setTimeout(50, function(socket) {
+  const s = server.setTimeout(50, function(socket) {
     caughtTimeoutOnServer = true;
     socket.destroy();
     server.close();
@@ -168,8 +168,8 @@ test(function idleTimeout(cb) {
   });
   assert.ok(s instanceof http.Server);
   server.listen(common.mustCall(function() {
-    var port = server.address().port;
-    var c = net.connect({ port: port, allowHalfOpen: true }, function() {
+    const port = server.address().port;
+    const c = net.connect({ port: port, allowHalfOpen: true }, function() {
       c.write('GET /1 HTTP/1.1\r\nHost: localhost\r\n\r\n');
       // Keep-Alive
     });
diff --git a/test/parallel/test-http-set-timeout.js b/test/parallel/test-http-set-timeout.js
index e8df29e0cc..965b3486fa 100644
--- a/test/parallel/test-http-set-timeout.js
+++ b/test/parallel/test-http-set-timeout.js
@@ -4,9 +4,9 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   console.log('got request. setting 500ms timeout');
-  var socket = req.connection.setTimeout(500);
+  const socket = req.connection.setTimeout(500);
   assert.ok(socket instanceof net.Socket);
   req.connection.on('timeout', common.mustCall(function() {
     req.connection.destroy();
@@ -18,7 +18,7 @@ var server = http.createServer(function(req, res) {
 server.listen(0, function() {
   console.log(`Server running at http://127.0.0.1:${this.address().port}/`);
 
-  var request = http.get({port: this.address().port, path: '/'});
+  const request = http.get({port: this.address().port, path: '/'});
   request.on('error', common.mustCall(function() {
     console.log('HTTP REQUEST COMPLETE (this is good)');
   }));
diff --git a/test/parallel/test-http-set-trailers.js b/test/parallel/test-http-set-trailers.js
index e34ea889b0..7b66933ecd 100644
--- a/test/parallel/test-http-set-trailers.js
+++ b/test/parallel/test-http-set-trailers.js
@@ -4,9 +4,9 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var outstanding_reqs = 0;
+let outstanding_reqs = 0;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200, [['content-type', 'text/plain']]);
   res.addTrailers({'x-foo': 'bar'});
   res.end('stuff' + '\n');
@@ -16,8 +16,8 @@ server.listen(0);
 
 // first, we test an HTTP/1.0 request.
 server.on('listening', function() {
-  var c = net.createConnection(this.address().port);
-  var res_buffer = '';
+  const c = net.createConnection(this.address().port);
+  let res_buffer = '';
 
   c.setEncoding('utf8');
 
@@ -44,9 +44,9 @@ server.on('listening', function() {
 
 // now, we test an HTTP/1.1 request.
 server.on('listening', function() {
-  var c = net.createConnection(this.address().port);
-  var res_buffer = '';
-  var tid;
+  const c = net.createConnection(this.address().port);
+  let res_buffer = '';
+  let tid;
 
   c.setEncoding('utf8');
 
diff --git a/test/parallel/test-http-should-keep-alive.js b/test/parallel/test-http-should-keep-alive.js
index 5cfbd51e76..77a3c3922a 100644
--- a/test/parallel/test-http-should-keep-alive.js
+++ b/test/parallel/test-http-should-keep-alive.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var SERVER_RESPONSES = [
+const SERVER_RESPONSES = [
   'HTTP/1.0 200 ok\r\nContent-Length: 0\r\n\r\n',
   'HTTP/1.0 200 ok\r\nContent-Length: 0\r\nConnection: keep-alive\r\n\r\n',
   'HTTP/1.0 200 ok\r\nContent-Length: 0\r\nConnection: close\r\n\r\n',
@@ -12,7 +12,7 @@ var SERVER_RESPONSES = [
   'HTTP/1.1 200 ok\r\nContent-Length: 0\r\nConnection: keep-alive\r\n\r\n',
   'HTTP/1.1 200 ok\r\nContent-Length: 0\r\nConnection: close\r\n\r\n'
 ];
-var SHOULD_KEEP_ALIVE = [
+const SHOULD_KEEP_ALIVE = [
   false, // HTTP/1.0, default
   true,  // HTTP/1.0, Connection: keep-alive
   false, // HTTP/1.0, Connection: close
@@ -20,16 +20,16 @@ var SHOULD_KEEP_ALIVE = [
   true,  // HTTP/1.1, Connection: keep-alive
   false  // HTTP/1.1, Connection: close
 ];
-var requests = 0;
-var responses = 0;
+let requests = 0;
+let responses = 0;
 http.globalAgent.maxSockets = 5;
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.write(SERVER_RESPONSES[requests]);
   ++requests;
 }).listen(0, function() {
   function makeRequest() {
-    var req = http.get({port: server.address().port}, function(res) {
+    const req = http.get({port: server.address().port}, function(res) {
       assert.equal(req.shouldKeepAlive, SHOULD_KEEP_ALIVE[responses],
                    SERVER_RESPONSES[responses] + ' should ' +
                    (SHOULD_KEEP_ALIVE[responses] ? '' : 'not ') +
diff --git a/test/parallel/test-http-status-code.js b/test/parallel/test-http-status-code.js
index 0b3d204c43..b5060109bf 100644
--- a/test/parallel/test-http-status-code.js
+++ b/test/parallel/test-http-status-code.js
@@ -6,12 +6,12 @@ const http = require('http');
 // Simple test of Node's HTTP ServerResponse.statusCode
 // ServerResponse.prototype.statusCode
 
-var testsComplete = 0;
-var tests = [200, 202, 300, 404, 451, 500];
-var testIdx = 0;
+let testsComplete = 0;
+const tests = [200, 202, 300, 404, 451, 500];
+let testIdx = 0;
 
-var s = http.createServer(function(req, res) {
-  var t = tests[testIdx];
+const s = http.createServer(function(req, res) {
+  const t = tests[testIdx];
   res.writeHead(t, {'Content-Type': 'text/plain'});
   console.log('--\nserver: statusCode after writeHead: ' + res.statusCode);
   assert.equal(res.statusCode, t);
@@ -25,7 +25,7 @@ function nextTest() {
   if (testIdx + 1 === tests.length) {
     return s.close();
   }
-  var test = tests[testIdx];
+  const test = tests[testIdx];
 
   http.get({ port: s.address().port }, function(response) {
     console.log('client: expected status: ' + test);
diff --git a/test/parallel/test-http-status-message.js b/test/parallel/test-http-status-message.js
index 9a086dcd46..668b02ee07 100644
--- a/test/parallel/test-http-status-message.js
+++ b/test/parallel/test-http-status-message.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var s = http.createServer(function(req, res) {
+const s = http.createServer(function(req, res) {
   res.statusCode = 200;
   res.statusMessage = 'Custom Message';
   res.end('');
@@ -14,15 +14,15 @@ s.listen(0, test);
 
 
 function test() {
-  var bufs = [];
-  var client = net.connect(this.address().port, function() {
+  const bufs = [];
+  const client = net.connect(this.address().port, function() {
     client.write('GET / HTTP/1.1\r\nConnection: close\r\n\r\n');
   });
   client.on('data', function(chunk) {
     bufs.push(chunk);
   });
   client.on('end', function() {
-    var head = Buffer.concat(bufs).toString('latin1').split('\r\n')[0];
+    const head = Buffer.concat(bufs).toString('latin1').split('\r\n')[0];
     assert.equal('HTTP/1.1 200 Custom Message', head);
     console.log('ok');
     s.close();
diff --git a/test/parallel/test-http-timeout-overflow.js b/test/parallel/test-http-timeout-overflow.js
index c045a28ce7..39b4550d57 100644
--- a/test/parallel/test-http-timeout-overflow.js
+++ b/test/parallel/test-http-timeout-overflow.js
@@ -4,10 +4,10 @@ const assert = require('assert');
 
 const http = require('http');
 
-var serverRequests = 0;
-var clientRequests = 0;
+let serverRequests = 0;
+let clientRequests = 0;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   serverRequests++;
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.end('OK');
@@ -16,7 +16,7 @@ var server = http.createServer(function(req, res) {
 server.listen(0, function() {
   function callback() {}
 
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     path: '/',
     agent: false
diff --git a/test/parallel/test-http-timeout.js b/test/parallel/test-http-timeout.js
index 128ca14883..63f87ed630 100644
--- a/test/parallel/test-http-timeout.js
+++ b/test/parallel/test-http-timeout.js
@@ -3,22 +3,22 @@ require('../common');
 
 const http = require('http');
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.end('OK');
 });
 
-var agent = new http.Agent({maxSockets: 1});
+const agent = new http.Agent({maxSockets: 1});
 
 server.listen(0, function() {
 
-  for (var i = 0; i < 11; ++i) {
+  for (let i = 0; i < 11; ++i) {
     createRequest().end();
   }
 
   function callback() {}
 
-  var count = 0;
+  let count = 0;
 
   function createRequest() {
     const req = http.request(
diff --git a/test/parallel/test-http-unix-socket.js b/test/parallel/test-http-unix-socket.js
index d2b99bde95..f9308d8632 100644
--- a/test/parallel/test-http-unix-socket.js
+++ b/test/parallel/test-http-unix-socket.js
@@ -17,12 +17,12 @@ common.refreshTmpDir();
 
 server.listen(common.PIPE, common.mustCall(function() {
 
-  var options = {
+  const options = {
     socketPath: common.PIPE,
     path: '/'
   };
 
-  var req = http.get(options, common.mustCall(function(res) {
+  const req = http.get(options, common.mustCall(function(res) {
     assert.strictEqual(res.statusCode, 200);
     assert.strictEqual(res.headers['content-type'], 'text/plain');
 
diff --git a/test/parallel/test-http-upgrade-agent.js b/test/parallel/test-http-upgrade-agent.js
index 27f21649a7..a196ba9049 100644
--- a/test/parallel/test-http-upgrade-agent.js
+++ b/test/parallel/test-http-upgrade-agent.js
@@ -10,7 +10,7 @@ const http = require('http');
 const net = require('net');
 
 // Create a TCP server
-var srv = net.createServer(function(c) {
+const srv = net.createServer(function(c) {
   c.on('data', function(d) {
     c.write('HTTP/1.1 101\r\n');
     c.write('hello: world\r\n');
@@ -27,7 +27,7 @@ var srv = net.createServer(function(c) {
 
 srv.listen(0, '127.0.0.1', common.mustCall(function() {
 
-  var options = {
+  const options = {
     port: this.address().port,
     host: '127.0.0.1',
     headers: {
@@ -35,13 +35,13 @@ srv.listen(0, '127.0.0.1', common.mustCall(function() {
       'upgrade': 'websocket'
     }
   };
-  var name = options.host + ':' + options.port;
+  const name = options.host + ':' + options.port;
 
-  var req = http.request(options);
+  const req = http.request(options);
   req.end();
 
   req.on('upgrade', common.mustCall(function(res, socket, upgradeHead) {
-    var recvData = upgradeHead;
+    let recvData = upgradeHead;
     socket.on('data', function(d) {
       recvData += d;
     });
@@ -51,9 +51,9 @@ srv.listen(0, '127.0.0.1', common.mustCall(function() {
     }));
 
     console.log(res.headers);
-    var expectedHeaders = { 'hello': 'world',
-                            'connection': 'upgrade',
-                            'upgrade': 'websocket' };
+    const expectedHeaders = { 'hello': 'world',
+                              'connection': 'upgrade',
+                              'upgrade': 'websocket' };
     assert.deepStrictEqual(expectedHeaders, res.headers);
 
     // Make sure this request got removed from the pool.
diff --git a/test/parallel/test-http-upgrade-client.js b/test/parallel/test-http-upgrade-client.js
index f5c25f15d4..34e3e8f241 100644
--- a/test/parallel/test-http-upgrade-client.js
+++ b/test/parallel/test-http-upgrade-client.js
@@ -10,7 +10,7 @@ const http = require('http');
 const net = require('net');
 
 // Create a TCP server
-var srv = net.createServer(function(c) {
+const srv = net.createServer(function(c) {
   c.on('data', function(d) {
     c.write('HTTP/1.1 101\r\n');
     c.write('hello: world\r\n');
@@ -39,16 +39,16 @@ srv.listen(0, '127.0.0.1', common.mustCall(function() {
       ['Origin', 'http://www.websocket.org']
     ]
   ];
-  var left = headers.length;
+  let left = headers.length;
   headers.forEach(function(h) {
-    var req = http.get({
+    const req = http.get({
       port: port,
       headers: h
     });
-    var sawUpgrade = false;
+    let sawUpgrade = false;
     req.on('upgrade', common.mustCall(function(res, socket, upgradeHead) {
       sawUpgrade = true;
-      var recvData = upgradeHead;
+      let recvData = upgradeHead;
       socket.on('data', function(d) {
         recvData += d;
       });
diff --git a/test/parallel/test-http-upgrade-client2.js b/test/parallel/test-http-upgrade-client2.js
index 03cb36981a..fa14f8ca51 100644
--- a/test/parallel/test-http-upgrade-client2.js
+++ b/test/parallel/test-http-upgrade-client2.js
@@ -2,9 +2,9 @@
 const common = require('../common');
 const http = require('http');
 
-var CRLF = '\r\n';
+const CRLF = '\r\n';
 
-var server = http.createServer();
+const server = http.createServer();
 server.on('upgrade', function(req, socket, head) {
   socket.write('HTTP/1.1 101 Ok' + CRLF +
                'Connection: Upgrade' + CRLF +
@@ -18,12 +18,12 @@ server.listen(0, common.mustCall(function() {
 
   function upgradeRequest(fn) {
     console.log('req');
-    var header = { 'Connection': 'Upgrade', 'Upgrade': 'Test' };
-    var request = http.request({
+    const header = { 'Connection': 'Upgrade', 'Upgrade': 'Test' };
+    const request = http.request({
       port: server.address().port,
       headers: header
     });
-    var wasUpgrade = false;
+    let wasUpgrade = false;
 
     function onUpgrade(res, socket, head) {
       console.log('client upgraded');
diff --git a/test/parallel/test-http-upgrade-server.js b/test/parallel/test-http-upgrade-server.js
index 50183dffe1..221263e98c 100644
--- a/test/parallel/test-http-upgrade-server.js
+++ b/test/parallel/test-http-upgrade-server.js
@@ -7,9 +7,9 @@ const net = require('net');
 const http = require('http');
 
 
-var requests_recv = 0;
-var requests_sent = 0;
-var request_upgradeHead = null;
+let requests_recv = 0;
+let requests_sent = 0;
+let request_upgradeHead = null;
 
 function createTestServer() {
   return new testServer();
@@ -37,7 +37,7 @@ function testServer() {
     request_upgradeHead = upgradeHead;
 
     socket.on('data', function(d) {
-      var data = d.toString('utf8');
+      const data = d.toString('utf8');
       if (data === 'kill') {
         socket.end();
       } else {
@@ -60,9 +60,9 @@ function writeReq(socket, data, encoding) {
   connection: Upgrade with listener
 -----------------------------------------------*/
 function test_upgrade_with_listener() {
-  var conn = net.createConnection(server.address().port);
+  const conn = net.createConnection(server.address().port);
   conn.setEncoding('utf8');
-  var state = 0;
+  let state = 0;
 
   conn.on('connect', function() {
     writeReq(conn,
@@ -99,10 +99,10 @@ function test_upgrade_with_listener() {
 /*-----------------------------------------------
   connection: Upgrade, no listener
 -----------------------------------------------*/
-var test_upgrade_no_listener_ended = false;
+let test_upgrade_no_listener_ended = false;
 
 function test_upgrade_no_listener() {
-  var conn = net.createConnection(server.address().port);
+  const conn = net.createConnection(server.address().port);
   conn.setEncoding('utf8');
 
   conn.on('connect', function() {
@@ -127,7 +127,7 @@ function test_upgrade_no_listener() {
   connection: normal
 -----------------------------------------------*/
 function test_standard_http() {
-  var conn = net.createConnection(server.address().port);
+  const conn = net.createConnection(server.address().port);
   conn.setEncoding('utf8');
 
   conn.on('connect', function() {
@@ -146,7 +146,7 @@ function test_standard_http() {
 }
 
 
-var server = createTestServer();
+const server = createTestServer();
 
 server.listen(0, function() {
   // All tests get chained after this:
diff --git a/test/parallel/test-http-upgrade-server2.js b/test/parallel/test-http-upgrade-server2.js
index e7533b194d..c64b2e8436 100644
--- a/test/parallel/test-http-upgrade-server2.js
+++ b/test/parallel/test-http-upgrade-server2.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const http = require('http');
 const net = require('net');
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   throw new Error('This shouldn\'t happen.');
 });
 
@@ -21,7 +21,7 @@ process.on('uncaughtException', common.mustCall(function(e) {
 
 
 server.listen(0, function() {
-  var c = net.createConnection(this.address().port);
+  const c = net.createConnection(this.address().port);
 
   c.on('connect', function() {
     c.write('GET /blah HTTP/1.1\r\n' +
diff --git a/test/parallel/test-http-url.parse-auth-with-header-in-request.js b/test/parallel/test-http-url.parse-auth-with-header-in-request.js
index 9781cefcbd..d91ec75f60 100644
--- a/test/parallel/test-http-url.parse-auth-with-header-in-request.js
+++ b/test/parallel/test-http-url.parse-auth-with-header-in-request.js
@@ -9,7 +9,7 @@ function check(request) {
   assert.strictEqual(request.headers.authorization, 'NoAuthForYOU');
 }
 
-var server = http.createServer(function(request, response) {
+const server = http.createServer(function(request, response) {
   // run the check function
   check.call(this, request, response);
   response.writeHead(200, {});
@@ -18,7 +18,8 @@ var server = http.createServer(function(request, response) {
 });
 
 server.listen(0, function() {
-  var testURL = url.parse(`http://asdf:qwer@localhost:${this.address().port}`);
+  const testURL = url.parse('http://asdf:qwer@localhost:' +
+      `${this.address().port}`);
   // the test here is if you set a specific authorization header in the
   // request we should not override that with basic auth
   testURL.headers = {
diff --git a/test/parallel/test-http-url.parse-auth.js b/test/parallel/test-http-url.parse-auth.js
index 72e69b14a9..25913f4e49 100644
--- a/test/parallel/test-http-url.parse-auth.js
+++ b/test/parallel/test-http-url.parse-auth.js
@@ -9,7 +9,7 @@ function check(request) {
   assert.strictEqual(request.headers.authorization, 'Basic dXNlcjpwYXNzOg==');
 }
 
-var server = http.createServer(function(request, response) {
+const server = http.createServer(function(request, response) {
   // run the check function
   check.call(this, request, response);
   response.writeHead(200, {});
@@ -20,7 +20,7 @@ var server = http.createServer(function(request, response) {
 server.listen(0, function() {
   const port = this.address().port;
   // username = "user", password = "pass:"
-  var testURL = url.parse(`http://user:pass%3A@localhost:${port}`);
+  const testURL = url.parse(`http://user:pass%3A@localhost:${port}`);
 
   // make the request
   http.request(testURL).end();
diff --git a/test/parallel/test-http-url.parse-basic.js b/test/parallel/test-http-url.parse-basic.js
index 0fa8f10465..ac22b6e38d 100644
--- a/test/parallel/test-http-url.parse-basic.js
+++ b/test/parallel/test-http-url.parse-basic.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const http = require('http');
 const url = require('url');
 
-var testURL;
+let testURL;
 
 // make sure the basics work
 function check(request) {
@@ -17,7 +17,7 @@ function check(request) {
                      testURL.hostname + ':' + testURL.port);
 }
 
-var server = http.createServer(function(request, response) {
+const server = http.createServer(function(request, response) {
   // run the check function
   check.call(this, request, response);
   response.writeHead(200, {});
@@ -29,7 +29,7 @@ server.listen(0, function() {
   testURL = url.parse(`http://localhost:${this.address().port}`);
 
   // make the request
-  var clientRequest = http.request(testURL);
+  const clientRequest = http.request(testURL);
   // since there is a little magic with the agent
   // make sure that an http request uses the http.Agent
   assert.ok(clientRequest.agent instanceof http.Agent);
diff --git a/test/parallel/test-http-url.parse-https.request.js b/test/parallel/test-http-url.parse-https.request.js
index ddd1d51475..ad40e76deb 100644
--- a/test/parallel/test-http-url.parse-https.request.js
+++ b/test/parallel/test-http-url.parse-https.request.js
@@ -12,7 +12,7 @@ const url = require('url');
 const fs = require('fs');
 
 // https options
-var httpsOptions = {
+const httpsOptions = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
@@ -22,7 +22,7 @@ function check(request) {
   assert.ok(request.socket._secureEstablished);
 }
 
-var server = https.createServer(httpsOptions, function(request, response) {
+const server = https.createServer(httpsOptions, function(request, response) {
   // run the check function
   check.call(this, request, response);
   response.writeHead(200, {});
@@ -31,11 +31,11 @@ var server = https.createServer(httpsOptions, function(request, response) {
 });
 
 server.listen(0, function() {
-  var testURL = url.parse(`https://localhost:${this.address().port}`);
+  const testURL = url.parse(`https://localhost:${this.address().port}`);
   testURL.rejectUnauthorized = false;
 
   // make the request
-  var clientRequest = https.request(testURL);
+  const clientRequest = https.request(testURL);
   // since there is a little magic with the agent
   // make sure that the request uses the https.Agent
   assert.ok(clientRequest.agent instanceof https.Agent);
diff --git a/test/parallel/test-http-url.parse-path.js b/test/parallel/test-http-url.parse-path.js
index e5a1306f8e..6fab8bb0eb 100644
--- a/test/parallel/test-http-url.parse-path.js
+++ b/test/parallel/test-http-url.parse-path.js
@@ -9,7 +9,7 @@ function check(request) {
   assert.strictEqual(request.url, '/asdf');
 }
 
-var server = http.createServer(function(request, response) {
+const server = http.createServer(function(request, response) {
   // run the check function
   check.call(this, request, response);
   response.writeHead(200, {});
@@ -18,7 +18,7 @@ var server = http.createServer(function(request, response) {
 });
 
 server.listen(0, function() {
-  var testURL = url.parse(`http://localhost:${this.address().port}/asdf`);
+  const testURL = url.parse(`http://localhost:${this.address().port}/asdf`);
 
   // make the request
   http.request(testURL).end();
diff --git a/test/parallel/test-http-url.parse-post.js b/test/parallel/test-http-url.parse-post.js
index 3ea40de6ec..d63f404c54 100644
--- a/test/parallel/test-http-url.parse-post.js
+++ b/test/parallel/test-http-url.parse-post.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const http = require('http');
 const url = require('url');
 
-var testURL;
+let testURL;
 
 function check(request) {
   //url.parse should not mess with the method
@@ -16,7 +16,7 @@ function check(request) {
                      testURL.hostname + ':' + testURL.port);
 }
 
-var server = http.createServer(function(request, response) {
+const server = http.createServer(function(request, response) {
   // run the check function
   check.call(this, request, response);
   response.writeHead(200, {});
diff --git a/test/parallel/test-http-url.parse-search.js b/test/parallel/test-http-url.parse-search.js
index 47cc0c03a6..f2a5d1806d 100644
--- a/test/parallel/test-http-url.parse-search.js
+++ b/test/parallel/test-http-url.parse-search.js
@@ -9,7 +9,7 @@ function check(request) {
   assert.strictEqual(request.url, '/asdf?qwer=zxcv');
 }
 
-var server = http.createServer(function(request, response) {
+const server = http.createServer(function(request, response) {
   // run the check function
   check.call(this, request, response);
   response.writeHead(200, {});
@@ -19,7 +19,7 @@ var server = http.createServer(function(request, response) {
 
 server.listen(0, function() {
   const port = this.address().port;
-  var testURL = url.parse(`http://localhost:${port}/asdf?qwer=zxcv`);
+  const testURL = url.parse(`http://localhost:${port}/asdf?qwer=zxcv`);
 
   // make the request
   http.request(testURL).end();
diff --git a/test/parallel/test-http-wget.js b/test/parallel/test-http-wget.js
index 795f179477..a633873c8b 100644
--- a/test/parallel/test-http-wget.js
+++ b/test/parallel/test-http-wget.js
@@ -19,7 +19,7 @@ const http = require('http');
 // content-length is not provided, that the connection is in fact
 // closed.
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.write('hello ');
   res.write('world\n');
@@ -28,8 +28,8 @@ var server = http.createServer(function(req, res) {
 server.listen(0);
 
 server.on('listening', common.mustCall(function() {
-  var c = net.createConnection(this.address().port);
-  var server_response = '';
+  const c = net.createConnection(this.address().port);
+  let server_response = '';
 
   c.setEncoding('utf8');
 
diff --git a/test/parallel/test-http-write-callbacks.js b/test/parallel/test-http-write-callbacks.js
index 5901025655..3783dc305e 100644
--- a/test/parallel/test-http-write-callbacks.js
+++ b/test/parallel/test-http-write-callbacks.js
@@ -4,13 +4,13 @@ const assert = require('assert');
 
 const http = require('http');
 
-var serverEndCb = false;
-var serverIncoming = '';
-var serverIncomingExpect = 'bazquuxblerg';
+let serverEndCb = false;
+let serverIncoming = '';
+const serverIncomingExpect = 'bazquuxblerg';
 
-var clientEndCb = false;
-var clientIncoming = '';
-var clientIncomingExpect = 'asdffoobar';
+let clientEndCb = false;
+let clientIncoming = '';
+const clientIncomingExpect = 'asdffoobar';
 
 process.on('exit', function() {
   assert(serverEndCb);
@@ -21,7 +21,7 @@ process.on('exit', function() {
 });
 
 // Verify that we get a callback when we do res.write(..., cb)
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.statusCode = 400;
   res.end('Bad Request.\nMust send Expect:100-continue\n');
 });
@@ -51,7 +51,7 @@ server.on('checkContinue', function(req, res) {
 });
 
 server.listen(0, function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'PUT',
     headers: { 'expect': '100-continue' }
diff --git a/test/parallel/test-http-write-empty-string.js b/test/parallel/test-http-write-empty-string.js
index c659c47fa4..69463fb3a5 100644
--- a/test/parallel/test-http-write-empty-string.js
+++ b/test/parallel/test-http-write-empty-string.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 
 const http = require('http');
 
-var server = http.createServer(function(request, response) {
+const server = http.createServer(function(request, response) {
   console.log('responding to ' + request.url);
 
   response.writeHead(200, {'Content-Type': 'text/plain'});
@@ -19,7 +19,7 @@ var server = http.createServer(function(request, response) {
 
 server.listen(0, common.mustCall(function() {
   http.get({ port: this.address().port }, common.mustCall(function(res) {
-    var response = '';
+    let response = '';
 
     assert.equal(200, res.statusCode);
     res.setEncoding('ascii');
diff --git a/test/parallel/test-http-write-head.js b/test/parallel/test-http-write-head.js
index 9807dc6943..12f5e00b0d 100644
--- a/test/parallel/test-http-write-head.js
+++ b/test/parallel/test-http-write-head.js
@@ -6,11 +6,11 @@ const http = require('http');
 // Verify that ServerResponse.writeHead() works as setHeader.
 // Issue 5036 on github.
 
-var s = http.createServer(function(req, res) {
+const s = http.createServer(function(req, res) {
   res.setHeader('test', '1');
 
   // toLowerCase() is used on the name argument, so it must be a string.
-  var threw = false;
+  let threw = false;
   try {
     res.setHeader(0xf00, 'bar');
   } catch (e) {
diff --git a/test/parallel/test-http-zero-length-write.js b/test/parallel/test-http-zero-length-write.js
index b446fd7786..63446e4604 100644
--- a/test/parallel/test-http-zero-length-write.js
+++ b/test/parallel/test-http-zero-length-write.js
@@ -9,10 +9,10 @@ const Stream = require('stream');
 function getSrc() {
   // An old-style readable stream.
   // The Readable class prevents this behavior.
-  var src = new Stream();
+  const src = new Stream();
 
   // start out paused, just so we don't miss anything yet.
-  var paused = false;
+  let paused = false;
   src.pause = function() {
     paused = true;
   };
@@ -20,12 +20,12 @@ function getSrc() {
     paused = false;
   };
 
-  var chunks = [ '', 'asdf', '', 'foo', '', 'bar', '' ];
-  var interval = setInterval(function() {
+  const chunks = [ '', 'asdf', '', 'foo', '', 'bar', '' ];
+  const interval = setInterval(function() {
     if (paused)
       return;
 
-    var chunk = chunks.shift();
+    const chunk = chunks.shift();
     if (chunk !== undefined) {
       src.emit('data', chunk);
     } else {
@@ -38,10 +38,10 @@ function getSrc() {
 }
 
 
-var expect = 'asdffoobar';
+const expect = 'asdffoobar';
 
-var server = http.createServer(function(req, res) {
-  var actual = '';
+const server = http.createServer(function(req, res) {
+  let actual = '';
   req.setEncoding('utf8');
   req.on('data', function(c) {
     actual += c;
@@ -54,8 +54,8 @@ var server = http.createServer(function(req, res) {
 });
 
 server.listen(0, function() {
-  var req = http.request({ port: this.address().port, method: 'POST' });
-  var actual = '';
+  const req = http.request({ port: this.address().port, method: 'POST' });
+  let actual = '';
   req.on('response', function(res) {
     res.setEncoding('utf8');
     res.on('data', function(c) {
diff --git a/test/parallel/test-http.js b/test/parallel/test-http.js
index f755bca8bc..a80a2b8856 100644
--- a/test/parallel/test-http.js
+++ b/test/parallel/test-http.js
@@ -4,10 +4,10 @@ const assert = require('assert');
 const http = require('http');
 const url = require('url');
 
-var responses_sent = 0;
-var responses_recvd = 0;
-var body0 = '';
-var body1 = '';
+let responses_sent = 0;
+let responses_recvd = 0;
+let body0 = '';
+let body1 = '';
 
 const server = http.Server(function(req, res) {
   if (responses_sent === 0) {
diff --git a/test/parallel/test-https-agent-disable-session-reuse.js b/test/parallel/test-https-agent-disable-session-reuse.js
index 2f59118e36..8e694a61b7 100644
--- a/test/parallel/test-https-agent-disable-session-reuse.js
+++ b/test/parallel/test-https-agent-disable-session-reuse.js
@@ -19,7 +19,7 @@ const options = {
 };
 
 const clientSessions = [];
-var serverRequests = 0;
+let serverRequests = 0;
 
 const agent = new https.Agent({
   maxCachedSessions: 0
@@ -29,7 +29,7 @@ const server = https.createServer(options, function(req, res) {
   serverRequests++;
   res.end('ok');
 }).listen(0, function() {
-  var waiting = TOTAL_REQS;
+  let waiting = TOTAL_REQS;
   function request() {
     const options = {
       agent: agent,
diff --git a/test/parallel/test-https-agent-servername.js b/test/parallel/test-https-agent-servername.js
index 09a0f6bdd6..a321f71403 100644
--- a/test/parallel/test-https-agent-servername.js
+++ b/test/parallel/test-https-agent-servername.js
@@ -9,14 +9,14 @@ if (!common.hasCrypto) {
 const https = require('https');
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem'),
   ca:  fs.readFileSync(common.fixturesDir + '/keys/ca1-cert.pem')
 };
 
 
-var server = https.Server(options, function(req, res) {
+const server = https.Server(options, function(req, res) {
   res.writeHead(200);
   res.end('hello world\n');
 });
diff --git a/test/parallel/test-https-agent-sni.js b/test/parallel/test-https-agent-sni.js
index 4174fd0b9c..ca15dc3db3 100644
--- a/test/parallel/test-https-agent-sni.js
+++ b/test/parallel/test-https-agent-sni.js
@@ -16,7 +16,7 @@ const options = {
 };
 
 const TOTAL = 4;
-var waiting = TOTAL;
+let waiting = TOTAL;
 
 const server = https.Server(options, function(req, res) {
   if (--waiting === 0) server.close();
@@ -35,10 +35,10 @@ server.listen(0, function() {
     });
   }
 
-  var agent = new https.Agent({
+  const agent = new https.Agent({
     maxSockets: 1
   });
-  for (var j = 0; j < TOTAL; j++) {
+  for (let j = 0; j < TOTAL; j++) {
     https.get({
       agent: agent,
 
diff --git a/test/parallel/test-https-agent.js b/test/parallel/test-https-agent.js
index 221ed2bbd2..392bdcbf03 100644
--- a/test/parallel/test-https-agent.js
+++ b/test/parallel/test-https-agent.js
@@ -22,15 +22,15 @@ const server = https.Server(options, function(req, res) {
 });
 
 
-var responses = 0;
+let responses = 0;
 const N = 4;
 const M = 4;
 
 
 server.listen(0, function() {
-  for (var i = 0; i < N; i++) {
+  for (let i = 0; i < N; i++) {
     setTimeout(function() {
-      for (var j = 0; j < M; j++) {
+      for (let j = 0; j < M; j++) {
         https.get({
           path: '/',
           port: server.address().port,
diff --git a/test/parallel/test-https-byteswritten.js b/test/parallel/test-https-byteswritten.js
index 51f59bc311..cdea4d1ee9 100644
--- a/test/parallel/test-https-byteswritten.js
+++ b/test/parallel/test-https-byteswritten.js
@@ -9,14 +9,14 @@ if (!common.hasCrypto) {
 }
 const https = require('https');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var body = 'hello world\n';
+const body = 'hello world\n';
 
-var httpsServer = https.createServer(options, function(req, res) {
+const httpsServer = https.createServer(options, function(req, res) {
   res.on('finish', function() {
     assert.strictEqual(typeof req.connection.bytesWritten, 'number');
     assert(req.connection.bytesWritten > 0);
diff --git a/test/parallel/test-https-client-checkServerIdentity.js b/test/parallel/test-https-client-checkServerIdentity.js
index 82f03320c4..688c7e2270 100644
--- a/test/parallel/test-https-client-checkServerIdentity.js
+++ b/test/parallel/test-https-client-checkServerIdentity.js
@@ -11,12 +11,12 @@ const https = require('https');
 const fs = require('fs');
 const path = require('path');
 
-var options = {
+const options = {
   key: fs.readFileSync(path.join(common.fixturesDir, 'keys/agent3-key.pem')),
   cert: fs.readFileSync(path.join(common.fixturesDir, 'keys/agent3-cert.pem'))
 };
 
-var server = https.createServer(options, common.mustCall(function(req, res) {
+const server = https.createServer(options, common.mustCall(function(req, res) {
   res.writeHead(200);
   res.end();
   req.resume();
@@ -25,7 +25,7 @@ var server = https.createServer(options, common.mustCall(function(req, res) {
 });
 
 function authorized() {
-  var req = https.request({
+  const req = https.request({
     port: server.address().port,
     rejectUnauthorized: true,
     ca: [fs.readFileSync(path.join(common.fixturesDir, 'keys/ca2-cert.pem'))]
@@ -37,7 +37,7 @@ function authorized() {
 }
 
 function override() {
-  var options = {
+  const options = {
     port: server.address().port,
     rejectUnauthorized: true,
     ca: [fs.readFileSync(path.join(common.fixturesDir, 'keys/ca2-cert.pem'))],
@@ -46,7 +46,7 @@ function override() {
     }
   };
   options.agent = new https.Agent(options);
-  var req = https.request(options, function(res) {
+  const req = https.request(options, function(res) {
     assert(req.socket.authorized);
     server.close();
   });
diff --git a/test/parallel/test-https-client-get-url.js b/test/parallel/test-https-client-get-url.js
index 6852967848..1f4f6ff748 100644
--- a/test/parallel/test-https-client-get-url.js
+++ b/test/parallel/test-https-client-get-url.js
@@ -15,12 +15,12 @@ const fs = require('fs');
 const url = require('url');
 const URL = url.URL;
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var server = https.createServer(options, common.mustCall(function(req, res) {
+const server = https.createServer(options, common.mustCall(function(req, res) {
   assert.equal('GET', req.method);
   assert.equal('/foo?bar', req.url);
   res.writeHead(200, {'Content-Type': 'text/plain'});
diff --git a/test/parallel/test-https-client-reject.js b/test/parallel/test-https-client-reject.js
index 32626b0a05..92bbabe8b3 100644
--- a/test/parallel/test-https-client-reject.js
+++ b/test/parallel/test-https-client-reject.js
@@ -11,12 +11,12 @@ const https = require('https');
 const fs = require('fs');
 const path = require('path');
 
-var options = {
+const options = {
   key: fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem')),
   cert: fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'))
 };
 
-var server = https.createServer(options, common.mustCall(function(req, res) {
+const server = https.createServer(options, common.mustCall(function(req, res) {
   res.writeHead(200);
   res.end();
   req.resume();
@@ -25,7 +25,7 @@ var server = https.createServer(options, common.mustCall(function(req, res) {
 });
 
 function unauthorized() {
-  var req = https.request({
+  const req = https.request({
     port: server.address().port,
     rejectUnauthorized: false
   }, function(res) {
@@ -40,11 +40,11 @@ function unauthorized() {
 }
 
 function rejectUnauthorized() {
-  var options = {
+  const options = {
     port: server.address().port
   };
   options.agent = new https.Agent(options);
-  var req = https.request(options, common.fail);
+  const req = https.request(options, common.fail);
   req.on('error', function(err) {
     authorized();
   });
@@ -52,12 +52,12 @@ function rejectUnauthorized() {
 }
 
 function authorized() {
-  var options = {
+  const options = {
     port: server.address().port,
     ca: [fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'))]
   };
   options.agent = new https.Agent(options);
-  var req = https.request(options, function(res) {
+  const req = https.request(options, function(res) {
     res.resume();
     assert(req.socket.authorized);
     server.close();
diff --git a/test/parallel/test-https-client-resume.js b/test/parallel/test-https-client-resume.js
index 58a06c70e2..79e1a716bd 100644
--- a/test/parallel/test-https-client-resume.js
+++ b/test/parallel/test-https-client-resume.js
@@ -14,21 +14,21 @@ const https = require('https');
 const tls = require('tls');
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem')
 };
 
 // create server
-var server = https.createServer(options, common.mustCall(function(req, res) {
+const server = https.createServer(options, common.mustCall(function(req, res) {
   res.end('Goodbye');
 }, 2));
 
 // start listening
 server.listen(0, function() {
 
-  var session1 = null;
-  var client1 = tls.connect({
+  let session1 = null;
+  const client1 = tls.connect({
     port: this.address().port,
     rejectUnauthorized: false
   }, function() {
@@ -43,13 +43,13 @@ server.listen(0, function() {
   client1.on('close', function() {
     console.log('close1');
 
-    var opts = {
+    const opts = {
       port: server.address().port,
       rejectUnauthorized: false,
       session: session1
     };
 
-    var client2 = tls.connect(opts, function() {
+    const client2 = tls.connect(opts, function() {
       console.log('connect2');
       assert.ok(client2.isSessionReused(), 'Session *should* be reused.');
       client2.write('GET / HTTP/1.0\r\n' +
diff --git a/test/parallel/test-https-close.js b/test/parallel/test-https-close.js
index f77726ceb6..cf950f29d6 100644
--- a/test/parallel/test-https-close.js
+++ b/test/parallel/test-https-close.js
@@ -13,17 +13,17 @@ const options = {
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var connections = {};
+const connections = {};
 
-var server = https.createServer(options, function(req, res) {
-  var interval = setInterval(function() {
+const server = https.createServer(options, function(req, res) {
+  const interval = setInterval(function() {
     res.write('data');
   }, 1000);
   interval.unref();
 });
 
 server.on('connection', function(connection) {
-  var key = connection.remoteAddress + ':' + connection.remotePort;
+  const key = connection.remoteAddress + ':' + connection.remotePort;
   connection.on('close', function() {
     delete connections[key];
   });
@@ -33,14 +33,14 @@ server.on('connection', function(connection) {
 function shutdown() {
   server.close(common.mustCall(function() {}));
 
-  for (var key in connections) {
+  for (const key in connections) {
     connections[key].destroy();
     delete connections[key];
   }
 }
 
 server.listen(0, function() {
-  var requestOptions = {
+  const requestOptions = {
     hostname: '127.0.0.1',
     port: this.address().port,
     path: '/',
@@ -48,7 +48,7 @@ server.listen(0, function() {
     rejectUnauthorized: false
   };
 
-  var req = https.request(requestOptions, function(res) {
+  const req = https.request(requestOptions, function(res) {
     res.on('data', function(d) {});
     setImmediate(shutdown);
   });
diff --git a/test/parallel/test-https-connecting-to-http.js b/test/parallel/test-https-connecting-to-http.js
index 3bfab5f626..ae8507f99c 100644
--- a/test/parallel/test-https-connecting-to-http.js
+++ b/test/parallel/test-https-connecting-to-http.js
@@ -10,10 +10,10 @@ if (!common.hasCrypto) {
 }
 const https = require('https');
 
-var server = http.createServer(common.fail);
+const server = http.createServer(common.fail);
 
 server.listen(0, common.mustCall(function() {
-  var req = https.get({ port: this.address().port }, common.fail);
+  const req = https.get({ port: this.address().port }, common.fail);
 
   req.on('error', common.mustCall(function(e) {
     console.log('Got expected error: ', e.message);
diff --git a/test/parallel/test-https-drain.js b/test/parallel/test-https-drain.js
index 0f6501e8ef..0b18795078 100644
--- a/test/parallel/test-https-drain.js
+++ b/test/parallel/test-https-drain.js
@@ -11,28 +11,28 @@ const https = require('https');
 const fs = require('fs');
 const path = require('path');
 
-var options = {
+const options = {
   key: fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem')),
   cert: fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'))
 };
 
-var bufSize = 1024 * 1024;
-var sent = 0;
-var received = 0;
+const bufSize = 1024 * 1024;
+let sent = 0;
+let received = 0;
 
-var server = https.createServer(options, function(req, res) {
+const server = https.createServer(options, function(req, res) {
   res.writeHead(200);
   req.pipe(res);
 });
 
 server.listen(0, function() {
-  var resumed = false;
-  var req = https.request({
+  let resumed = false;
+  const req = https.request({
     method: 'POST',
     port: this.address().port,
     rejectUnauthorized: false
   }, function(res) {
-    var timer;
+    let timer;
     res.pause();
     console.error('paused');
     send();
diff --git a/test/parallel/test-https-eof-for-eom.js b/test/parallel/test-https-eof-for-eom.js
index 81d4e978a8..d5e896c769 100644
--- a/test/parallel/test-https-eof-for-eom.js
+++ b/test/parallel/test-https-eof-for-eom.js
@@ -19,13 +19,13 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
 
-var server = tls.Server(options, function(socket) {
+const server = tls.Server(options, function(socket) {
   console.log('2) Server got request');
   socket.write('HTTP/1.1 200 OK\r\n' +
                'Date: Tue, 15 Feb 2011 22:14:54 GMT\r\n' +
@@ -52,7 +52,7 @@ server.listen(0, common.mustCall(function() {
     port: this.address().port,
     rejectUnauthorized: false
   }, common.mustCall(function(res) {
-    var bodyBuffer = '';
+    let bodyBuffer = '';
 
     server.close();
     console.log('3) Client got response headers.');
diff --git a/test/parallel/test-https-foafssl.js b/test/parallel/test-https-foafssl.js
index 6be850a11e..f1b8d442e2 100644
--- a/test/parallel/test-https-foafssl.js
+++ b/test/parallel/test-https-foafssl.js
@@ -18,7 +18,7 @@ if (!common.hasCrypto) {
 }
 const https = require('https');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/agent.key'),
   cert: fs.readFileSync(common.fixturesDir + '/agent.crt'),
   requestCert: true
@@ -34,11 +34,11 @@ const modulus = 'A6F44A9C25791431214F5C87AF9E040177A8BB89AC803F7E09BBC3A5519F' +
                 'EBDAF1191F4A4E26D71879C4C7867B62FCD508E8CE66E82D128A71E91580' +
                 '9FCF44E8DE774067F1DE5D70B9C03687';
 
-var CRLF = '\r\n';
-var body = 'hello world\n';
-var cert;
+const CRLF = '\r\n';
+const body = 'hello world\n';
+let cert;
 
-var server = https.createServer(options, common.mustCall(function(req, res) {
+const server = https.createServer(options, common.mustCall(function(req, res) {
   console.log('got request');
 
   cert = req.connection.getPeerCertificate();
@@ -51,21 +51,21 @@ var server = https.createServer(options, common.mustCall(function(req, res) {
 }));
 
 server.listen(0, function() {
-  var args = ['s_client',
-              '-quiet',
-              '-connect', `127.0.0.1:${this.address().port}`,
-              '-cert', join(common.fixturesDir, 'foafssl.crt'),
-              '-key', join(common.fixturesDir, 'foafssl.key')];
+  const args = ['s_client',
+                '-quiet',
+                '-connect', `127.0.0.1:${this.address().port}`,
+                '-cert', join(common.fixturesDir, 'foafssl.crt'),
+                '-key', join(common.fixturesDir, 'foafssl.key')];
 
   // for the performance and stability issue in s_client on Windows
   if (common.isWindows)
     args.push('-no_rand_screen');
 
-  var client = spawn(common.opensslCli, args);
+  const client = spawn(common.opensslCli, args);
 
   client.stdout.on('data', function(data) {
-    var message = data.toString();
-    var contents = message.split(CRLF + CRLF).pop();
+    const message = data.toString();
+    const contents = message.split(CRLF + CRLF).pop();
     assert.equal(body, contents);
     server.close();
   });
diff --git a/test/parallel/test-https-host-headers.js b/test/parallel/test-https-host-headers.js
index 25de4435ea..190f15f29a 100644
--- a/test/parallel/test-https-host-headers.js
+++ b/test/parallel/test-https-host-headers.js
@@ -37,7 +37,7 @@ testHttps();
 
 function testHttps() {
 
-  var counter = 0;
+  let counter = 0;
 
   function cb(res) {
     counter--;
diff --git a/test/parallel/test-https-localaddress-bind-error.js b/test/parallel/test-https-localaddress-bind-error.js
index 2382d59a7c..662d6f080a 100644
--- a/test/parallel/test-https-localaddress-bind-error.js
+++ b/test/parallel/test-https-localaddress-bind-error.js
@@ -8,14 +8,14 @@ if (!common.hasCrypto) {
 }
 const https = require('https');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var invalidLocalAddress = '1.2.3.4';
+const invalidLocalAddress = '1.2.3.4';
 
-var server = https.createServer(options, function(req, res) {
+const server = https.createServer(options, function(req, res) {
   console.log('Connect from: ' + req.connection.remoteAddress);
 
   req.on('end', function() {
diff --git a/test/parallel/test-https-localaddress.js b/test/parallel/test-https-localaddress.js
index 22e96eb467..78583b38ec 100644
--- a/test/parallel/test-https-localaddress.js
+++ b/test/parallel/test-https-localaddress.js
@@ -14,12 +14,12 @@ if (!common.hasMultiLocalhost()) {
   return;
 }
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var server = https.createServer(options, function(req, res) {
+const server = https.createServer(options, function(req, res) {
   console.log('Connect from: ' + req.connection.remoteAddress);
   assert.equal('127.0.0.2', req.connection.remoteAddress);
 
@@ -31,7 +31,7 @@ var server = https.createServer(options, function(req, res) {
 });
 
 server.listen(0, '127.0.0.1', function() {
-  var options = {
+  const options = {
     host: 'localhost',
     port: this.address().port,
     path: '/',
@@ -40,7 +40,7 @@ server.listen(0, '127.0.0.1', function() {
     rejectUnauthorized: false
   };
 
-  var req = https.request(options, function(res) {
+  const req = https.request(options, function(res) {
     res.on('end', function() {
       server.close();
       process.exit();
diff --git a/test/parallel/test-https-pfx.js b/test/parallel/test-https-pfx.js
index 8d0f51b6a5..c30bbc0848 100644
--- a/test/parallel/test-https-pfx.js
+++ b/test/parallel/test-https-pfx.js
@@ -9,9 +9,9 @@ if (!common.hasCrypto) {
 }
 const https = require('https');
 
-var pfx = fs.readFileSync(common.fixturesDir + '/test_cert.pfx');
+const pfx = fs.readFileSync(common.fixturesDir + '/test_cert.pfx');
 
-var options = {
+const options = {
   host: '127.0.0.1',
   port: undefined,
   path: '/',
@@ -21,7 +21,7 @@ var options = {
   rejectUnauthorized: false
 };
 
-var server = https.createServer(options, function(req, res) {
+const server = https.createServer(options, function(req, res) {
   assert.equal(req.socket.authorized, false); // not a client cert
   assert.equal(req.socket.authorizationError, 'DEPTH_ZERO_SELF_SIGNED_CERT');
   res.writeHead(200);
@@ -32,7 +32,7 @@ server.listen(0, options.host, common.mustCall(function() {
   options.port = this.address().port;
 
   https.get(options, common.mustCall(function(res) {
-    var data = '';
+    let data = '';
 
     res.on('data', function(data_) { data += data_; });
     res.on('end', common.mustCall(function() {
diff --git a/test/parallel/test-https-req-split.js b/test/parallel/test-https-req-split.js
index d0d20a73ad..4298abe5ef 100644
--- a/test/parallel/test-https-req-split.js
+++ b/test/parallel/test-https-req-split.js
@@ -12,7 +12,7 @@ const https = require('https');
 const tls = require('tls');
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
@@ -20,7 +20,7 @@ var options = {
 // Force splitting incoming data
 tls.SLAB_BUFFER_SIZE = 1;
 
-var server = https.createServer(options);
+const server = https.createServer(options);
 server.on('upgrade', common.mustCall(function(req, socket, upgrade) {
   socket.on('data', function(data) {
     throw new Error('Unexpected data: ' + data);
@@ -29,7 +29,7 @@ server.on('upgrade', common.mustCall(function(req, socket, upgrade) {
 }));
 
 server.listen(0, function() {
-  var req = https.request({
+  const req = https.request({
     host: '127.0.0.1',
     port: this.address().port,
     agent: false,
diff --git a/test/parallel/test-https-resume-after-renew.js b/test/parallel/test-https-resume-after-renew.js
index 72d7710425..cc712b3124 100644
--- a/test/parallel/test-https-resume-after-renew.js
+++ b/test/parallel/test-https-resume-after-renew.js
@@ -9,24 +9,25 @@ const fs = require('fs');
 const https = require('https');
 const crypto = require('crypto');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem'),
   ca: fs.readFileSync(common.fixturesDir + '/keys/ca1-cert.pem')
 };
 
-var server = https.createServer(options, function(req, res) {
+const server = https.createServer(options, function(req, res) {
   res.end('hello');
 });
 
-var aes = Buffer.alloc(16, 'S');
-var hmac = Buffer.alloc(16, 'H');
+const aes = Buffer.alloc(16, 'S');
+const hmac = Buffer.alloc(16, 'H');
 
 server._sharedCreds.context.enableTicketKeyCallback();
 server._sharedCreds.context.onticketkeycallback = function(name, iv, enc) {
+  let newName, newIV;
   if (enc) {
-    var newName = Buffer.alloc(16, 'A');
-    var newIV = crypto.randomBytes(16);
+    newName = Buffer.alloc(16, 'A');
+    newIV = crypto.randomBytes(16);
   } else {
     // Renew
     return [ 2, hmac, aes ];
@@ -36,7 +37,7 @@ server._sharedCreds.context.onticketkeycallback = function(name, iv, enc) {
 };
 
 server.listen(0, function() {
-  var addr = this.address();
+  const addr = this.address();
 
   function doReq(callback) {
     https.request({
diff --git a/test/parallel/test-https-set-timeout-server.js b/test/parallel/test-https-set-timeout-server.js
index fbf16a0ce3..4cf999667d 100644
--- a/test/parallel/test-https-set-timeout-server.js
+++ b/test/parallel/test-https-set-timeout-server.js
@@ -11,9 +11,9 @@ const https = require('https');
 const tls = require('tls');
 const fs = require('fs');
 
-var tests = [];
+const tests = [];
 
-var serverOptions = {
+const serverOptions = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
@@ -25,7 +25,7 @@ function test(fn) {
 }
 
 function run() {
-  var fn = tests.shift();
+  const fn = tests.shift();
   if (fn) {
     console.log('# %s', fn.name);
     fn(run);
@@ -35,11 +35,11 @@ function run() {
 }
 
 test(function serverTimeout(cb) {
-  var server = https.createServer(serverOptions, function(req, res) {
+  const server = https.createServer(serverOptions, function(req, res) {
     // just do nothing, we should get a timeout event.
   });
   server.listen(0, common.mustCall(function() {
-    var s = server.setTimeout(50, common.mustCall(function(socket) {
+    const s = server.setTimeout(50, common.mustCall(function(socket) {
       socket.destroy();
       server.close();
       cb();
@@ -62,9 +62,9 @@ test(function serverRequestTimeout(cb) {
     }));
   }
 
-  var server = https.createServer(serverOptions, common.mustCall(handler));
+  const server = https.createServer(serverOptions, common.mustCall(handler));
   server.listen(0, function() {
-    var req = https.request({
+    const req = https.request({
       port: this.address().port,
       method: 'POST',
       rejectUnauthorized: false
@@ -85,7 +85,7 @@ test(function serverResponseTimeout(cb) {
     }));
   }
 
-  var server = https.createServer(serverOptions, common.mustCall(handler));
+  const server = https.createServer(serverOptions, common.mustCall(handler));
   server.listen(0, function() {
     https.get({
       port: this.address().port,
@@ -100,7 +100,7 @@ test(function serverRequestNotTimeoutAfterEnd(cb) {
     req.setTimeout(50, common.fail);
     res.on('timeout', common.mustCall(function(socket) {}));
   }
-  var server = https.createServer(serverOptions, common.mustCall(handler));
+  const server = https.createServer(serverOptions, common.mustCall(handler));
   server.on('timeout', function(socket) {
     socket.destroy();
     server.close();
@@ -115,11 +115,11 @@ test(function serverRequestNotTimeoutAfterEnd(cb) {
 });
 
 test(function serverResponseTimeoutWithPipeline(cb) {
-  var caughtTimeout = '';
+  let caughtTimeout = '';
   process.on('exit', function() {
     assert.equal(caughtTimeout, '/2');
   });
-  var server = https.createServer(serverOptions, function(req, res) {
+  const server = https.createServer(serverOptions, function(req, res) {
     res.setTimeout(50, function() {
       caughtTimeout += req.url;
     });
@@ -131,12 +131,12 @@ test(function serverResponseTimeoutWithPipeline(cb) {
     cb();
   });
   server.listen(0, function() {
-    var options = {
+    const options = {
       port: this.address().port,
       allowHalfOpen: true,
       rejectUnauthorized: false
     };
-    var c = tls.connect(options, function() {
+    const c = tls.connect(options, function() {
       c.write('GET /1 HTTP/1.1\r\nHost: localhost\r\n\r\n');
       c.write('GET /2 HTTP/1.1\r\nHost: localhost\r\n\r\n');
       c.write('GET /3 HTTP/1.1\r\nHost: localhost\r\n\r\n');
@@ -145,19 +145,19 @@ test(function serverResponseTimeoutWithPipeline(cb) {
 });
 
 test(function idleTimeout(cb) {
-  var server = https.createServer(serverOptions,
-                                  common.mustCall(function(req, res) {
-                                    req.on('timeout', common.fail);
-                                    res.on('timeout', common.fail);
-                                    res.end();
-                                  }));
+  const server = https.createServer(serverOptions,
+                                    common.mustCall(function(req, res) {
+                                      req.on('timeout', common.fail);
+                                      res.on('timeout', common.fail);
+                                      res.end();
+                                    }));
   server.setTimeout(50, common.mustCall(function(socket) {
     socket.destroy();
     server.close();
     cb();
   }));
   server.listen(0, function() {
-    var options = {
+    const options = {
       port: this.address().port,
       allowHalfOpen: true,
       rejectUnauthorized: false
diff --git a/test/parallel/test-https-socket-options.js b/test/parallel/test-https-socket-options.js
index 337cc264a2..05137b35c4 100644
--- a/test/parallel/test-https-socket-options.js
+++ b/test/parallel/test-https-socket-options.js
@@ -11,16 +11,16 @@ const fs = require('fs');
 
 const http = require('http');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var body = 'hello world\n';
+const body = 'hello world\n';
 
 // Try first with http server
 
-var server_http = http.createServer(function(req, res) {
+const server_http = http.createServer(function(req, res) {
   console.log('got HTTP request');
   res.writeHead(200, { 'content-type': 'text/plain' });
   res.end(body);
@@ -28,7 +28,7 @@ var server_http = http.createServer(function(req, res) {
 
 
 server_http.listen(0, function() {
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     rejectUnauthorized: false
   }, function(res) {
@@ -45,14 +45,14 @@ server_http.listen(0, function() {
 // Then try https server (requires functions to be
 // mirroed in tls.js's CryptoStream)
 
-var server_https = https.createServer(options, function(req, res) {
+const server_https = https.createServer(options, function(req, res) {
   console.log('got HTTPS request');
   res.writeHead(200, { 'content-type': 'text/plain' });
   res.end(body);
 });
 
 server_https.listen(0, function() {
-  var req = https.request({
+  const req = https.request({
     port: this.address().port,
     rejectUnauthorized: false
   }, function(res) {
diff --git a/test/parallel/test-https-timeout-server-2.js b/test/parallel/test-https-timeout-server-2.js
index a17abec539..8729af1ae8 100644
--- a/test/parallel/test-https-timeout-server-2.js
+++ b/test/parallel/test-https-timeout-server-2.js
@@ -12,15 +12,15 @@ const https = require('https');
 const tls = require('tls');
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var server = https.createServer(options, common.fail);
+const server = https.createServer(options, common.fail);
 
 server.on('secureConnection', function(cleartext) {
-  var s = cleartext.setTimeout(50, function() {
+  const s = cleartext.setTimeout(50, function() {
     cleartext.destroy();
     server.close();
   });
diff --git a/test/parallel/test-icu-transcode.js b/test/parallel/test-icu-transcode.js
index 8e4176547d..783bc00699 100644
--- a/test/parallel/test-icu-transcode.js
+++ b/test/parallel/test-icu-transcode.js
@@ -18,7 +18,7 @@ const tests = {
 for (const test in tests) {
   const dest = buffer.transcode(orig, 'utf8', test);
   assert.strictEqual(dest.length, tests[test].length);
-  for (var n = 0; n < tests[test].length; n++)
+  for (let n = 0; n < tests[test].length; n++)
     assert.strictEqual(dest[n], tests[test][n]);
 }
 
diff --git a/test/parallel/test-listen-fd-cluster.js b/test/parallel/test-listen-fd-cluster.js
index 001eb007f2..de1fbed7b5 100644
--- a/test/parallel/test-listen-fd-cluster.js
+++ b/test/parallel/test-listen-fd-cluster.js
@@ -22,7 +22,7 @@ switch (process.argv[2]) {
   case 'worker': return worker();
 }
 
-var ok;
+let ok;
 
 process.on('exit', function() {
   assert.ok(ok);
@@ -41,7 +41,7 @@ test(function(parent, port) {
     port: port,
     path: '/',
   }).on('response', function(res) {
-    var s = '';
+    let s = '';
     res.on('data', function(c) {
       s += c.toString();
     });
@@ -61,7 +61,7 @@ test(function(parent, port) {
 
 function test(cb) {
   console.error('about to listen in parent');
-  var server = net.createServer(function(conn) {
+  const server = net.createServer(function(conn) {
     console.error('connection on parent');
     conn.end('hello from parent\n');
   }).listen(0, function() {
@@ -69,7 +69,7 @@ function test(cb) {
     console.error('server listening on %d', port);
 
     const spawn = require('child_process').spawn;
-    var master = spawn(process.execPath, [__filename, 'master'], {
+    const master = spawn(process.execPath, [__filename, 'master'], {
       stdio: [ 0, 'pipe', 2, server._handle, 'ipc' ],
       detached: true
     });
@@ -100,7 +100,7 @@ function master() {
   cluster.setupMaster({
     args: [ 'worker' ]
   });
-  var worker = cluster.fork();
+  const worker = cluster.fork();
   worker.on('message', function(msg) {
     if (msg === 'worker ready') {
       process.send('started worker');
diff --git a/test/parallel/test-listen-fd-detached-inherit.js b/test/parallel/test-listen-fd-detached-inherit.js
index 9699232d2e..2ac4d83dba 100644
--- a/test/parallel/test-listen-fd-detached-inherit.js
+++ b/test/parallel/test-listen-fd-detached-inherit.js
@@ -23,24 +23,24 @@ switch (process.argv[2]) {
 // concurrency in HTTP servers!  Use the cluster module, or if you want
 // a more low-level approach, use child process IPC manually.
 function test() {
-  var parent = spawn(process.execPath, [__filename, 'parent'], {
+  const parent = spawn(process.execPath, [__filename, 'parent'], {
     stdio: [ 0, 'pipe', 2 ]
   });
-  var json = '';
+  let json = '';
   parent.stdout.on('data', function(c) {
     json += c.toString();
     if (json.indexOf('\n') !== -1) next();
   });
   function next() {
     console.error('output from parent = %s', json);
-    var child = JSON.parse(json);
+    const child = JSON.parse(json);
     // now make sure that we can request to the child, then kill it.
     http.get({
       server: 'localhost',
       port: child.port,
       path: '/',
     }).on('response', function(res) {
-      var s = '';
+      let s = '';
       res.on('data', function(c) {
         s += c.toString();
       });
@@ -62,13 +62,13 @@ function test() {
 // Listen on port, and then pass the handle to the detached child.
 // Then output the child's pid, and immediately exit.
 function parent() {
-  var server = net.createServer(function(conn) {
+  const server = net.createServer(function(conn) {
     conn.end('HTTP/1.1 403 Forbidden\r\n\r\nI got problems.\r\n');
     throw new Error('Should not see connections on parent');
   }).listen(0, function() {
     console.error('server listening on %d', this.address().port);
 
-    var child = spawn(process.execPath, [__filename, 'child'], {
+    const child = spawn(process.execPath, [__filename, 'child'], {
       stdio: [ 0, 1, 2, server._handle ],
       detached: true
     });
diff --git a/test/parallel/test-listen-fd-detached.js b/test/parallel/test-listen-fd-detached.js
index 4ec3654a21..1194d00161 100644
--- a/test/parallel/test-listen-fd-detached.js
+++ b/test/parallel/test-listen-fd-detached.js
@@ -23,24 +23,24 @@ switch (process.argv[2]) {
 // concurrency in HTTP servers!  Use the cluster module, or if you want
 // a more low-level approach, use child process IPC manually.
 function test() {
-  var parent = spawn(process.execPath, [__filename, 'parent'], {
+  const parent = spawn(process.execPath, [__filename, 'parent'], {
     stdio: [ 0, 'pipe', 2 ]
   });
-  var json = '';
+  let json = '';
   parent.stdout.on('data', function(c) {
     json += c.toString();
     if (json.indexOf('\n') !== -1) next();
   });
   function next() {
     console.error('output from parent = %s', json);
-    var child = JSON.parse(json);
+    const child = JSON.parse(json);
     // now make sure that we can request to the child, then kill it.
     http.get({
       server: 'localhost',
       port: child.port,
       path: '/',
     }).on('response', function(res) {
-      var s = '';
+      let s = '';
       res.on('data', function(c) {
         s += c.toString();
       });
@@ -60,14 +60,14 @@ function test() {
 }
 
 function parent() {
-  var server = net.createServer(function(conn) {
+  const server = net.createServer(function(conn) {
     console.error('connection on parent');
     conn.end('hello from parent\n');
   }).listen(0, function() {
     console.error('server listening on %d', this.address().port);
 
     const spawn = require('child_process').spawn;
-    var child = spawn(process.execPath, [__filename, 'child'], {
+    const child = spawn(process.execPath, [__filename, 'child'], {
       stdio: [ 'ignore', 'ignore', 'ignore', server._handle ],
       detached: true
     });
diff --git a/test/parallel/test-listen-fd-server.js b/test/parallel/test-listen-fd-server.js
index 87a624f713..60aa8717a8 100644
--- a/test/parallel/test-listen-fd-server.js
+++ b/test/parallel/test-listen-fd-server.js
@@ -13,7 +13,7 @@ switch (process.argv[2]) {
   case 'child': return child();
 }
 
-var ok;
+let ok;
 
 process.on('exit', function() {
   assert.ok(ok);
@@ -31,7 +31,7 @@ test(function(child, port) {
     port: port,
     path: '/',
   }).on('response', function(res) {
-    var s = '';
+    let s = '';
     res.on('data', function(c) {
       s += c.toString();
     });
@@ -67,7 +67,7 @@ function child() {
 }
 
 function test(cb) {
-  var server = net.createServer(function(conn) {
+  const server = net.createServer(function(conn) {
     console.error('connection on parent');
     conn.end('hello from parent\n');
   }).listen(0, function() {
@@ -75,7 +75,7 @@ function test(cb) {
     console.error('server listening on %d', port);
 
     const spawn = require('child_process').spawn;
-    var child = spawn(process.execPath, [__filename, 'child'], {
+    const child = spawn(process.execPath, [__filename, 'child'], {
       stdio: [ 0, 1, 2, server._handle, 'ipc' ]
     });
 
diff --git a/test/parallel/test-memory-usage-emfile.js b/test/parallel/test-memory-usage-emfile.js
index ce74dd3be3..09ab270b70 100644
--- a/test/parallel/test-memory-usage-emfile.js
+++ b/test/parallel/test-memory-usage-emfile.js
@@ -4,10 +4,10 @@ const assert = require('assert');
 
 const fs = require('fs');
 
-var files = [];
+const files = [];
 
 while (files.length < 256)
   files.push(fs.openSync(__filename, 'r'));
 
-var r = process.memoryUsage();
+const r = process.memoryUsage();
 assert.equal(true, r['rss'] > 0);
diff --git a/test/parallel/test-memory-usage.js b/test/parallel/test-memory-usage.js
index 400e17aa00..cba3f9a517 100644
--- a/test/parallel/test-memory-usage.js
+++ b/test/parallel/test-memory-usage.js
@@ -2,7 +2,7 @@
 require('../common');
 const assert = require('assert');
 
-var r = process.memoryUsage();
+const r = process.memoryUsage();
 assert.ok(r.rss > 0);
 assert.ok(r.heapTotal > 0);
 assert.ok(r.heapUsed > 0);
diff --git a/test/parallel/test-microtask-queue-integration-domain.js b/test/parallel/test-microtask-queue-integration-domain.js
index 2e865041df..7110af7648 100644
--- a/test/parallel/test-microtask-queue-integration-domain.js
+++ b/test/parallel/test-microtask-queue-integration-domain.js
@@ -9,21 +9,21 @@ const assert = require('assert');
 // removed.
 require('domain');
 
-var implementations = [
+const implementations = [
   function(fn) {
     Promise.resolve().then(fn);
   }
 ];
 
-var expected = 0;
-var done = 0;
+let expected = 0;
+let done = 0;
 
 process.on('exit', function() {
   assert.equal(done, expected);
 });
 
 function test(scheduleMicrotask) {
-  var nextTickCalled = false;
+  let nextTickCalled = false;
   expected++;
 
   scheduleMicrotask(function() {
diff --git a/test/parallel/test-microtask-queue-integration.js b/test/parallel/test-microtask-queue-integration.js
index f4f518caa3..d212bbae96 100644
--- a/test/parallel/test-microtask-queue-integration.js
+++ b/test/parallel/test-microtask-queue-integration.js
@@ -2,21 +2,21 @@
 require('../common');
 const assert = require('assert');
 
-var implementations = [
+const implementations = [
   function(fn) {
     Promise.resolve().then(fn);
   }
 ];
 
-var expected = 0;
-var done = 0;
+let expected = 0;
+let done = 0;
 
 process.on('exit', function() {
   assert.equal(done, expected);
 });
 
 function test(scheduleMicrotask) {
-  var nextTickCalled = false;
+  let nextTickCalled = false;
   expected++;
 
   scheduleMicrotask(function() {
diff --git a/test/parallel/test-microtask-queue-run-domain.js b/test/parallel/test-microtask-queue-run-domain.js
index 57ba4543a3..7d34ec829a 100644
--- a/test/parallel/test-microtask-queue-run-domain.js
+++ b/test/parallel/test-microtask-queue-run-domain.js
@@ -13,7 +13,7 @@ function enqueueMicrotask(fn) {
   Promise.resolve().then(fn);
 }
 
-var done = 0;
+let done = 0;
 
 process.on('exit', function() {
   assert.equal(done, 2);
@@ -29,7 +29,7 @@ setTimeout(function() {
 
 // no nextTick, microtask with nextTick
 setTimeout(function() {
-  var called = false;
+  let called = false;
 
   enqueueMicrotask(function() {
     process.nextTick(function() {
diff --git a/test/parallel/test-microtask-queue-run-immediate-domain.js b/test/parallel/test-microtask-queue-run-immediate-domain.js
index 5241b6c4a9..e09087f74e 100644
--- a/test/parallel/test-microtask-queue-run-immediate-domain.js
+++ b/test/parallel/test-microtask-queue-run-immediate-domain.js
@@ -13,7 +13,7 @@ function enqueueMicrotask(fn) {
   Promise.resolve().then(fn);
 }
 
-var done = 0;
+let done = 0;
 
 process.on('exit', function() {
   assert.equal(done, 2);
@@ -29,7 +29,7 @@ setImmediate(function() {
 
 // no nextTick, microtask with nextTick
 setImmediate(function() {
-  var called = false;
+  let called = false;
 
   enqueueMicrotask(function() {
     process.nextTick(function() {
diff --git a/test/parallel/test-microtask-queue-run-immediate.js b/test/parallel/test-microtask-queue-run-immediate.js
index b21af670ff..c5ed6c6dcb 100644
--- a/test/parallel/test-microtask-queue-run-immediate.js
+++ b/test/parallel/test-microtask-queue-run-immediate.js
@@ -6,7 +6,7 @@ function enqueueMicrotask(fn) {
   Promise.resolve().then(fn);
 }
 
-var done = 0;
+let done = 0;
 
 process.on('exit', function() {
   assert.equal(done, 2);
@@ -22,7 +22,7 @@ setImmediate(function() {
 
 // no nextTick, microtask with nextTick
 setImmediate(function() {
-  var called = false;
+  let called = false;
 
   enqueueMicrotask(function() {
     process.nextTick(function() {
diff --git a/test/parallel/test-microtask-queue-run.js b/test/parallel/test-microtask-queue-run.js
index 3150c9ebda..71e1fbedba 100644
--- a/test/parallel/test-microtask-queue-run.js
+++ b/test/parallel/test-microtask-queue-run.js
@@ -6,7 +6,7 @@ function enqueueMicrotask(fn) {
   Promise.resolve().then(fn);
 }
 
-var done = 0;
+let done = 0;
 
 process.on('exit', function() {
   assert.equal(done, 2);
@@ -22,7 +22,7 @@ setTimeout(function() {
 
 // no nextTick, microtask with nextTick
 setTimeout(function() {
-  var called = false;
+  let called = false;
 
   enqueueMicrotask(function() {
     process.nextTick(function() {
diff --git a/test/parallel/test-module-globalpaths-nodepath.js b/test/parallel/test-module-globalpaths-nodepath.js
index 66511e51ed..ec90f1a3e2 100644
--- a/test/parallel/test-module-globalpaths-nodepath.js
+++ b/test/parallel/test-module-globalpaths-nodepath.js
@@ -3,8 +3,8 @@ const common = require('../common');
 const assert = require('assert');
 const mod = require('module');
 
-var partA, partB;
-var partC = '';
+let partA, partB;
+const partC = '';
 
 if (common.isWindows) {
   partA = 'C:\\Users\\Rocko Artischocko\\AppData\\Roaming\\npm';
diff --git a/test/parallel/test-module-loading-error.js b/test/parallel/test-module-loading-error.js
index b8454f2ae9..d78f61de60 100644
--- a/test/parallel/test-module-loading-error.js
+++ b/test/parallel/test-module-loading-error.js
@@ -4,13 +4,13 @@ const assert = require('assert');
 
 console.error('load test-module-loading-error.js');
 
-var error_desc = {
+const error_desc = {
   win32: ['%1 is not a valid Win32 application'],
   linux: ['file too short', 'Exec format error'],
   sunos: ['unknown file type', 'not an ELF file'],
   darwin: ['file too short']
 };
-var dlerror_msg = error_desc[process.platform];
+const dlerror_msg = error_desc[process.platform];
 
 if (!dlerror_msg) {
   common.skip('platform not supported.');
diff --git a/test/parallel/test-net-after-close.js b/test/parallel/test-net-after-close.js
index db658bcb03..d46f096116 100644
--- a/test/parallel/test-net-after-close.js
+++ b/test/parallel/test-net-after-close.js
@@ -3,13 +3,13 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server = net.createServer(function(s) {
+const server = net.createServer(function(s) {
   console.error('SERVER: got connection');
   s.end();
 });
 
 server.listen(0, common.mustCall(function() {
-  var c = net.createConnection(this.address().port);
+  const c = net.createConnection(this.address().port);
   c.on('close', common.mustCall(function() {
     console.error('connection closed');
     assert.strictEqual(c._handle, null);
diff --git a/test/parallel/test-net-better-error-messages-path.js b/test/parallel/test-net-better-error-messages-path.js
index bf18e14898..2f821ef5d7 100644
--- a/test/parallel/test-net-better-error-messages-path.js
+++ b/test/parallel/test-net-better-error-messages-path.js
@@ -2,8 +2,8 @@
 const common = require('../common');
 const net = require('net');
 const assert = require('assert');
-var fp = '/tmp/fadagagsdfgsdf';
-var c = net.connect(fp);
+const fp = '/tmp/fadagagsdfgsdf';
+const c = net.connect(fp);
 
 c.on('connect', common.fail);
 
diff --git a/test/parallel/test-net-binary.js b/test/parallel/test-net-binary.js
index 47e0be8d04..bcc554ac40 100644
--- a/test/parallel/test-net-binary.js
+++ b/test/parallel/test-net-binary.js
@@ -3,8 +3,8 @@ require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var binaryString = '';
-for (var i = 255; i >= 0; i--) {
+let binaryString = '';
+for (let i = 255; i >= 0; i--) {
   const s = `'\\${i.toString(8)}'`;
   const S = eval(s);
   assert.strictEqual(S.charCodeAt(0), i);
@@ -13,7 +13,7 @@ for (var i = 255; i >= 0; i--) {
 }
 
 // safe constructor
-var echoServer = net.Server(function(connection) {
+const echoServer = net.Server(function(connection) {
   connection.setEncoding('latin1');
   connection.on('data', function(chunk) {
     connection.write(chunk, 'latin1');
@@ -24,10 +24,10 @@ var echoServer = net.Server(function(connection) {
 });
 echoServer.listen(0);
 
-var recv = '';
+let recv = '';
 
 echoServer.on('listening', function() {
-  var j = 0;
+  let j = 0;
   const c = net.createConnection({
     port: this.address().port
   });
diff --git a/test/parallel/test-net-bind-twice.js b/test/parallel/test-net-bind-twice.js
index 68e93a75f0..17fdc90cb3 100644
--- a/test/parallel/test-net-bind-twice.js
+++ b/test/parallel/test-net-bind-twice.js
@@ -3,9 +3,9 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server1 = net.createServer(common.fail);
+const server1 = net.createServer(common.fail);
 server1.listen(0, '127.0.0.1', common.mustCall(function() {
-  var server2 = net.createServer(common.fail);
+  const server2 = net.createServer(common.fail);
   server2.listen(this.address().port, '127.0.0.1', common.fail);
 
   server2.on('error', common.mustCall(function(e) {
diff --git a/test/parallel/test-net-buffersize.js b/test/parallel/test-net-buffersize.js
index dfaf5dd50d..e8ba17a76e 100644
--- a/test/parallel/test-net-buffersize.js
+++ b/test/parallel/test-net-buffersize.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var iter = 10;
+const iter = 10;
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.on('readable', function() {
     socket.read();
   });
@@ -16,13 +16,13 @@ var server = net.createServer(function(socket) {
 });
 
 server.listen(0, function() {
-  var client = net.connect(this.address().port);
+  const client = net.connect(this.address().port);
 
   client.on('finish', function() {
     assert.strictEqual(client.bufferSize, 0);
   });
 
-  for (var i = 1; i < iter; i++) {
+  for (let i = 1; i < iter; i++) {
     client.write('a');
     assert.strictEqual(client.bufferSize, i);
   }
diff --git a/test/parallel/test-net-bytes-stats.js b/test/parallel/test-net-bytes-stats.js
index 518b2f10ba..4cc6eee837 100644
--- a/test/parallel/test-net-bytes-stats.js
+++ b/test/parallel/test-net-bytes-stats.js
@@ -3,11 +3,11 @@ require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var bytesRead = 0;
-var bytesWritten = 0;
-var count = 0;
+let bytesRead = 0;
+let bytesWritten = 0;
+let count = 0;
 
-var tcp = net.Server(function(s) {
+const tcp = net.Server(function(s) {
   console.log('tcp server connection');
 
   // trigger old mode.
@@ -21,7 +21,7 @@ var tcp = net.Server(function(s) {
 
 tcp.listen(0, function doTest() {
   console.error('listening');
-  var socket = net.createConnection(this.address().port);
+  const socket = net.createConnection(this.address().port);
 
   socket.on('connect', function() {
     count++;
diff --git a/test/parallel/test-net-can-reset-timeout.js b/test/parallel/test-net-can-reset-timeout.js
index c5333e2522..faa460e847 100644
--- a/test/parallel/test-net-can-reset-timeout.js
+++ b/test/parallel/test-net-can-reset-timeout.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const net = require('net');
 
-var server = net.createServer(common.mustCall(function(stream) {
+const server = net.createServer(common.mustCall(function(stream) {
   stream.setTimeout(100);
 
   stream.resume();
@@ -20,7 +20,7 @@ var server = net.createServer(common.mustCall(function(stream) {
 }));
 
 server.listen(0, function() {
-  var c = net.createConnection(this.address().port);
+  const c = net.createConnection(this.address().port);
 
   c.on('data', function() {
     c.end();
diff --git a/test/parallel/test-net-connect-buffer.js b/test/parallel/test-net-connect-buffer.js
index 187cbf1942..c43ee1abd0 100644
--- a/test/parallel/test-net-connect-buffer.js
+++ b/test/parallel/test-net-connect-buffer.js
@@ -3,15 +3,15 @@ require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var dataWritten = false;
-var connectHappened = false;
+let dataWritten = false;
+let connectHappened = false;
 
-var tcp = net.Server(function(s) {
+const tcp = net.Server(function(s) {
   tcp.close();
 
   console.log('tcp server connection');
 
-  var buf = '';
+  let buf = '';
   s.on('data', function(d) {
     buf += d;
   });
@@ -30,7 +30,7 @@ var tcp = net.Server(function(s) {
 });
 
 tcp.listen(0, function() {
-  var socket = net.Stream({ highWaterMark: 0 });
+  const socket = net.Stream({ highWaterMark: 0 });
 
   console.log('Connecting to socket ');
 
@@ -65,13 +65,13 @@ tcp.listen(0, function() {
 
   // Write a string that contains a multi-byte character sequence to test that
   // `bytesWritten` is incremented with the # of bytes, not # of characters.
-  var a = "L'État, c'est ";
-  var b = 'moi';
+  const a = "L'État, c'est ";
+  const b = 'moi';
 
   // We're still connecting at this point so the datagram is first pushed onto
   // the connect queue. Make sure that it's not added to `bytesWritten` again
   // when the actual write happens.
-  var r = socket.write(a, function(er) {
+  const r = socket.write(a, function(er) {
     console.error('write cb');
     dataWritten = true;
     assert.ok(connectHappened);
diff --git a/test/parallel/test-net-connect-local-error.js b/test/parallel/test-net-connect-local-error.js
index 197b283bdf..8f1784565d 100644
--- a/test/parallel/test-net-connect-local-error.js
+++ b/test/parallel/test-net-connect-local-error.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var client = net.connect({
+const client = net.connect({
   port: common.PORT + 1,
   localPort: common.PORT,
   localAddress: common.localhostIPv4
diff --git a/test/parallel/test-net-connect-options-ipv6.js b/test/parallel/test-net-connect-options-ipv6.js
index 7f638d66f5..e9d8a78cbb 100644
--- a/test/parallel/test-net-connect-options-ipv6.js
+++ b/test/parallel/test-net-connect-options-ipv6.js
@@ -9,9 +9,9 @@ if (!common.hasIPv6) {
 }
 
 const hosts = common.localIPv6Hosts;
-var hostIdx = 0;
-var host = hosts[hostIdx];
-var localhostTries = 10;
+let hostIdx = 0;
+let host = hosts[hostIdx];
+let localhostTries = 10;
 
 const server = net.createServer({allowHalfOpen: true}, function(socket) {
   socket.resume();
diff --git a/test/parallel/test-net-connect-options.js b/test/parallel/test-net-connect-options.js
index 727504b06f..e794c40859 100644
--- a/test/parallel/test-net-connect-options.js
+++ b/test/parallel/test-net-connect-options.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server = net.createServer({
+const server = net.createServer({
   allowHalfOpen: true
 }, common.mustCall(function(socket) {
   socket.resume();
@@ -12,7 +12,7 @@ var server = net.createServer({
 }));
 
 server.listen(0, function() {
-  var client = net.connect({
+  const client = net.connect({
     host: '127.0.0.1',
     port: this.address().port,
     allowHalfOpen: true
diff --git a/test/parallel/test-net-create-connection.js b/test/parallel/test-net-create-connection.js
index a21f92a3a5..a5d8cd101d 100644
--- a/test/parallel/test-net-create-connection.js
+++ b/test/parallel/test-net-create-connection.js
@@ -5,8 +5,8 @@ const dns = require('dns');
 const net = require('net');
 
 const expectedConnections = 7;
-var clientConnected = 0;
-var serverConnected = 0;
+let clientConnected = 0;
+let serverConnected = 0;
 
 const server = net.createServer(function(socket) {
   socket.end();
diff --git a/test/parallel/test-net-dns-custom-lookup.js b/test/parallel/test-net-dns-custom-lookup.js
index d643910d62..c7a01f5fa6 100644
--- a/test/parallel/test-net-dns-custom-lookup.js
+++ b/test/parallel/test-net-dns-custom-lookup.js
@@ -4,13 +4,13 @@ const assert = require('assert');
 const net = require('net');
 
 function check(addressType, cb) {
-  var server = net.createServer(function(client) {
+  const server = net.createServer(function(client) {
     client.end();
     server.close();
     cb && cb();
   });
 
-  var address = addressType === 4 ? common.localhostIPv4 : '::1';
+  const address = addressType === 4 ? common.localhostIPv4 : '::1';
   server.listen(0, address, common.mustCall(function() {
     net.connect({
       port: this.address().port,
diff --git a/test/parallel/test-net-dns-error.js b/test/parallel/test-net-dns-error.js
index f5a1dcc6b1..aa288c5fde 100644
--- a/test/parallel/test-net-dns-error.js
+++ b/test/parallel/test-net-dns-error.js
@@ -4,13 +4,13 @@ const assert = require('assert');
 
 const net = require('net');
 
-var host = '*'.repeat(256);
+const host = '*'.repeat(256);
 
 function do_not_call() {
   throw new Error('This function should not have been called.');
 }
 
-var socket = net.connect(42, host, do_not_call);
+const socket = net.connect(42, host, do_not_call);
 socket.on('error', common.mustCall(function(err) {
   assert.equal(err.code, 'ENOTFOUND');
 }));
diff --git a/test/parallel/test-net-dns-lookup-skip.js b/test/parallel/test-net-dns-lookup-skip.js
index 60674053f7..0997e6cc6c 100644
--- a/test/parallel/test-net-dns-lookup-skip.js
+++ b/test/parallel/test-net-dns-lookup-skip.js
@@ -3,12 +3,12 @@ const common = require('../common');
 const net = require('net');
 
 function check(addressType) {
-  var server = net.createServer(function(client) {
+  const server = net.createServer(function(client) {
     client.end();
     server.close();
   });
 
-  var address = addressType === 4 ? '127.0.0.1' : '::1';
+  const address = addressType === 4 ? '127.0.0.1' : '::1';
   server.listen(0, address, function() {
     net.connect(this.address().port, address).on('lookup', common.fail);
   });
diff --git a/test/parallel/test-net-dns-lookup.js b/test/parallel/test-net-dns-lookup.js
index c0aebf0ee9..8d9930f4b7 100644
--- a/test/parallel/test-net-dns-lookup.js
+++ b/test/parallel/test-net-dns-lookup.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server = net.createServer(function(client) {
+const server = net.createServer(function(client) {
   client.end();
   server.close();
 });
diff --git a/test/parallel/test-net-during-close.js b/test/parallel/test-net-during-close.js
index f6a5455ee7..e8723fb1a6 100644
--- a/test/parallel/test-net-during-close.js
+++ b/test/parallel/test-net-during-close.js
@@ -3,12 +3,12 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.end();
 });
 
 server.listen(0, common.mustCall(function() {
-  var client = net.createConnection(this.address().port);
+  const client = net.createConnection(this.address().port);
   server.close();
   // server connection event has not yet fired
   // client is still attempting to connect
diff --git a/test/parallel/test-net-eaddrinuse.js b/test/parallel/test-net-eaddrinuse.js
index 53ae1a0589..3d7ceb8d5e 100644
--- a/test/parallel/test-net-eaddrinuse.js
+++ b/test/parallel/test-net-eaddrinuse.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server1 = net.createServer(function(socket) {
+const server1 = net.createServer(function(socket) {
 });
-var server2 = net.createServer(function(socket) {
+const server2 = net.createServer(function(socket) {
 });
 server1.listen(0, function() {
   server2.on('error', function(error) {
diff --git a/test/parallel/test-net-end-without-connect.js b/test/parallel/test-net-end-without-connect.js
index 0bbd283362..99324b071d 100644
--- a/test/parallel/test-net-end-without-connect.js
+++ b/test/parallel/test-net-end-without-connect.js
@@ -2,5 +2,5 @@
 require('../common');
 const net = require('net');
 
-var sock = new net.Socket();
+const sock = new net.Socket();
 sock.end();  // Should not throw.
diff --git a/test/parallel/test-net-error-twice.js b/test/parallel/test-net-error-twice.js
index 36c33b4b1c..98a2c33755 100644
--- a/test/parallel/test-net-error-twice.js
+++ b/test/parallel/test-net-error-twice.js
@@ -6,8 +6,8 @@ const net = require('net');
 const buf = Buffer.alloc(10 * 1024 * 1024, 0x62);
 
 const errs = [];
-var clientSocket;
-var serverSocket;
+let clientSocket;
+let serverSocket;
 
 function ready() {
   if (clientSocket && serverSocket) {
@@ -16,7 +16,7 @@ function ready() {
   }
 }
 
-var srv = net.createServer(function onConnection(conn) {
+const srv = net.createServer(function onConnection(conn) {
   conn.on('error', function(err) {
     errs.push(err);
     if (errs.length > 1 && errs[0] === errs[1])
@@ -28,7 +28,7 @@ var srv = net.createServer(function onConnection(conn) {
   serverSocket = conn;
   ready();
 }).listen(0, function() {
-  var client = net.connect({ port: this.address().port });
+  const client = net.connect({ port: this.address().port });
 
   client.on('connect', function() {
     clientSocket = client;
diff --git a/test/parallel/test-net-internal.js b/test/parallel/test-net-internal.js
index 0c8d1cf9e1..a79d022a5d 100644
--- a/test/parallel/test-net-internal.js
+++ b/test/parallel/test-net-internal.js
@@ -6,7 +6,7 @@ require('../common');
 const assert = require('assert');
 const isLegalPort = require('internal/net').isLegalPort;
 
-for (var n = 0; n <= 0xFFFF; n++) {
+for (let n = 0; n <= 0xFFFF; n++) {
   assert(isLegalPort(n));
   assert(isLegalPort('' + n));
   assert(`0x${n.toString(16)}`);
diff --git a/test/parallel/test-net-keepalive.js b/test/parallel/test-net-keepalive.js
index 8b6998fa83..68ed5f842f 100644
--- a/test/parallel/test-net-keepalive.js
+++ b/test/parallel/test-net-keepalive.js
@@ -3,9 +3,9 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var serverConnection;
-var clientConnection;
-var echoServer = net.createServer(function(connection) {
+let serverConnection;
+let clientConnection;
+const echoServer = net.createServer(function(connection) {
   serverConnection = connection;
   setTimeout(common.mustCall(function() {
     // make sure both connections are still open
diff --git a/test/parallel/test-net-large-string.js b/test/parallel/test-net-large-string.js
index ba70275eb0..83612111e2 100644
--- a/test/parallel/test-net-large-string.js
+++ b/test/parallel/test-net-large-string.js
@@ -3,12 +3,12 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var kPoolSize = 40 * 1024;
-var data = 'あ'.repeat(kPoolSize);
-var encoding = 'UTF-8';
+const kPoolSize = 40 * 1024;
+const data = 'あ'.repeat(kPoolSize);
+const encoding = 'UTF-8';
 
-var server = net.createServer(common.mustCall(function(socket) {
-  var receivedSize = 0;
+const server = net.createServer(common.mustCall(function(socket) {
+  let receivedSize = 0;
 
   socket.setEncoding(encoding);
   socket.on('data', function(data) {
@@ -21,7 +21,7 @@ var server = net.createServer(common.mustCall(function(socket) {
 }));
 
 server.listen(0, function() {
-  var client = net.createConnection(this.address().port);
+  const client = net.createConnection(this.address().port);
   client.on('end', function() {
     server.close();
   });
diff --git a/test/parallel/test-net-listen-close-server-callback-is-not-function.js b/test/parallel/test-net-listen-close-server-callback-is-not-function.js
index 18234a816e..472ea37e71 100644
--- a/test/parallel/test-net-listen-close-server-callback-is-not-function.js
+++ b/test/parallel/test-net-listen-close-server-callback-is-not-function.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const net = require('net');
 
-var server = net.createServer(common.fail);
+const server = net.createServer(common.fail);
 
 server.on('close', common.mustCall(function() {}));
 
diff --git a/test/parallel/test-net-listen-close-server.js b/test/parallel/test-net-listen-close-server.js
index b6ab99c60e..0a45adecbe 100644
--- a/test/parallel/test-net-listen-close-server.js
+++ b/test/parallel/test-net-listen-close-server.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const net = require('net');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
 });
 server.listen(0, common.fail);
 server.on('error', common.fail);
diff --git a/test/parallel/test-net-listen-error.js b/test/parallel/test-net-listen-error.js
index 009d12b09e..7b721d1004 100644
--- a/test/parallel/test-net-listen-error.js
+++ b/test/parallel/test-net-listen-error.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const net = require('net');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
 });
 server.listen(1, '1.1.1.1', common.fail); // EACCESS or EADDRNOTAVAIL
 server.on('error', common.mustCall(function(error) {}));
diff --git a/test/parallel/test-net-listen-exclusive-random-ports.js b/test/parallel/test-net-listen-exclusive-random-ports.js
index 974af6aaa6..cc1ab4d805 100644
--- a/test/parallel/test-net-listen-exclusive-random-ports.js
+++ b/test/parallel/test-net-listen-exclusive-random-ports.js
@@ -7,11 +7,11 @@ const net = require('net');
 function noop() {}
 
 if (cluster.isMaster) {
-  var worker1 = cluster.fork();
+  const worker1 = cluster.fork();
 
   worker1.on('message', function(port1) {
     assert.equal(port1, port1 | 0, 'first worker could not listen');
-    var worker2 = cluster.fork();
+    const worker2 = cluster.fork();
 
     worker2.on('message', function(port2) {
       assert.equal(port2, port2 | 0, 'second worker could not listen');
@@ -21,7 +21,7 @@ if (cluster.isMaster) {
     });
   });
 } else {
-  var server = net.createServer(noop);
+  const server = net.createServer(noop);
 
   server.on('error', function(err) {
     process.send(err.code);
diff --git a/test/parallel/test-net-listen-shared-ports.js b/test/parallel/test-net-listen-shared-ports.js
index 4cd26ac330..ce344be4a6 100644
--- a/test/parallel/test-net-listen-shared-ports.js
+++ b/test/parallel/test-net-listen-shared-ports.js
@@ -7,11 +7,11 @@ const net = require('net');
 function noop() {}
 
 if (cluster.isMaster) {
-  var worker1 = cluster.fork();
+  const worker1 = cluster.fork();
 
   worker1.on('message', function(msg) {
     assert.equal(msg, 'success');
-    var worker2 = cluster.fork();
+    const worker2 = cluster.fork();
 
     worker2.on('message', function(msg) {
       assert.equal(msg, 'server2:EADDRINUSE');
@@ -20,8 +20,8 @@ if (cluster.isMaster) {
     });
   });
 } else {
-  var server1 = net.createServer(noop);
-  var server2 = net.createServer(noop);
+  const server1 = net.createServer(noop);
+  const server2 = net.createServer(noop);
 
   server1.on('error', function(err) {
     // no errors expected
diff --git a/test/parallel/test-net-local-address-port.js b/test/parallel/test-net-local-address-port.js
index 56f8eab585..b256d93b5f 100644
--- a/test/parallel/test-net-local-address-port.js
+++ b/test/parallel/test-net-local-address-port.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server = net.createServer(common.mustCall(function(socket) {
+const server = net.createServer(common.mustCall(function(socket) {
   assert.strictEqual(socket.localAddress, common.localhostIPv4);
   assert.strictEqual(socket.localPort, this.address().port);
   socket.on('end', function() {
@@ -13,7 +13,8 @@ var server = net.createServer(common.mustCall(function(socket) {
 }));
 
 server.listen(0, common.localhostIPv4, function() {
-  var client = net.createConnection(this.address().port, common.localhostIPv4);
+  const client = net.createConnection(this.address()
+                    .port, common.localhostIPv4);
   client.on('connect', function() {
     client.end();
   });
diff --git a/test/parallel/test-net-localport.js b/test/parallel/test-net-localport.js
index 3c105ff287..b100e4ae76 100644
--- a/test/parallel/test-net-localport.js
+++ b/test/parallel/test-net-localport.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   console.log(socket.remotePort);
   assert.strictEqual(socket.remotePort, common.PORT);
   socket.end();
@@ -11,7 +11,7 @@ var server = net.createServer(function(socket) {
     server.close();
   });
 }).listen(0).on('listening', function() {
-  var client = net.connect({
+  const client = net.connect({
     host: '127.0.0.1',
     port: this.address().port,
     localPort: common.PORT,
diff --git a/test/parallel/test-net-pause-resume-connecting.js b/test/parallel/test-net-pause-resume-connecting.js
index 6f3e78a94a..f441252fed 100644
--- a/test/parallel/test-net-pause-resume-connecting.js
+++ b/test/parallel/test-net-pause-resume-connecting.js
@@ -9,7 +9,7 @@ let conn;
 
 
 // Server
-var server = net.createServer(function(conn) {
+const server = net.createServer(function(conn) {
   connections++;
   conn.end('This was the year he fell to pieces.');
 
diff --git a/test/parallel/test-net-persistent-keepalive.js b/test/parallel/test-net-persistent-keepalive.js
index febdc68851..6338b5932d 100644
--- a/test/parallel/test-net-persistent-keepalive.js
+++ b/test/parallel/test-net-persistent-keepalive.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var serverConnection;
-var clientConnection;
-var echoServer = net.createServer(function(connection) {
+let serverConnection;
+let clientConnection;
+const echoServer = net.createServer(function(connection) {
   serverConnection = connection;
   setTimeout(function() {
     // make sure both connections are still open
@@ -27,7 +27,7 @@ echoServer.on('listening', function() {
   clientConnection = new net.Socket();
   // send a keepalive packet after 1000 ms
   // and make sure it persists
-  var s = clientConnection.setKeepAlive(true, 400);
+  const s = clientConnection.setKeepAlive(true, 400);
   assert.ok(s instanceof net.Socket);
   clientConnection.connect(this.address().port);
   clientConnection.setTimeout(0);
diff --git a/test/parallel/test-net-persistent-nodelay.js b/test/parallel/test-net-persistent-nodelay.js
index bd34d30326..3f94424b0e 100644
--- a/test/parallel/test-net-persistent-nodelay.js
+++ b/test/parallel/test-net-persistent-nodelay.js
@@ -2,17 +2,17 @@
 require('../common');
 const assert = require('assert');
 const net = require('net');
-var TCPWrap = process.binding('tcp_wrap').TCP;
+const TCPWrap = process.binding('tcp_wrap').TCP;
 
-var echoServer = net.createServer(function(connection) {
+const echoServer = net.createServer(function(connection) {
   connection.end();
 });
 echoServer.listen(0);
 
-var callCount = 0;
+let callCount = 0;
 
-var Socket = net.Socket;
-var setNoDelay = TCPWrap.prototype.setNoDelay;
+const Socket = net.Socket;
+const setNoDelay = TCPWrap.prototype.setNoDelay;
 
 TCPWrap.prototype.setNoDelay = function(enable) {
   setNoDelay.call(this, enable);
@@ -20,11 +20,11 @@ TCPWrap.prototype.setNoDelay = function(enable) {
 };
 
 echoServer.on('listening', function() {
-  var sock1 = new Socket();
+  const sock1 = new Socket();
   // setNoDelay before the handle is created
   // there is probably a better way to test this
 
-  var s = sock1.setNoDelay();
+  const s = sock1.setNoDelay();
   assert.ok(s instanceof net.Socket);
   sock1.connect(this.address().port);
   sock1.on('end', function() {
diff --git a/test/parallel/test-net-persistent-ref-unref.js b/test/parallel/test-net-persistent-ref-unref.js
index 9034a16f9a..1d3cbc6ffa 100644
--- a/test/parallel/test-net-persistent-ref-unref.js
+++ b/test/parallel/test-net-persistent-ref-unref.js
@@ -2,16 +2,16 @@
 require('../common');
 const assert = require('assert');
 const net = require('net');
-var TCPWrap = process.binding('tcp_wrap').TCP;
+const TCPWrap = process.binding('tcp_wrap').TCP;
 
-var echoServer = net.createServer(function(conn) {
+const echoServer = net.createServer(function(conn) {
   conn.end();
 });
 
-var ref = TCPWrap.prototype.ref;
-var unref = TCPWrap.prototype.unref;
+const ref = TCPWrap.prototype.ref;
+const unref = TCPWrap.prototype.unref;
 
-var refCount = 0;
+let refCount = 0;
 
 TCPWrap.prototype.ref = function() {
   ref.call(this);
@@ -28,7 +28,7 @@ TCPWrap.prototype.unref = function() {
 echoServer.listen(0);
 
 echoServer.on('listening', function() {
-  var sock = new net.Socket();
+  const sock = new net.Socket();
   sock.unref();
   sock.ref();
   sock.connect(this.address().port);
diff --git a/test/parallel/test-net-pingpong.js b/test/parallel/test-net-pingpong.js
index b49b3a7877..9280991f39 100644
--- a/test/parallel/test-net-pingpong.js
+++ b/test/parallel/test-net-pingpong.js
@@ -5,9 +5,9 @@ const net = require('net');
 
 function pingPongTest(port, host) {
   const N = 1000;
-  var count = 0;
-  var sentPongs = 0;
-  var sent_final_ping = false;
+  let count = 0;
+  let sentPongs = 0;
+  let sent_final_ping = false;
 
   const server = net.createServer(
     { allowHalfOpen: true },
diff --git a/test/parallel/test-net-pipe-connect-errors.js b/test/parallel/test-net-pipe-connect-errors.js
index 3e00befc34..2c1332efa1 100644
--- a/test/parallel/test-net-pipe-connect-errors.js
+++ b/test/parallel/test-net-pipe-connect-errors.js
@@ -8,7 +8,7 @@ const assert = require('assert');
 // Test if ENOTSOCK is fired when trying to connect to a file which is not
 // a socket.
 
-var emptyTxt;
+let emptyTxt;
 
 if (common.isWindows) {
   // on Win, common.PIPE will be a named pipe, so we use an existing empty
@@ -34,7 +34,7 @@ if (common.isWindows) {
   fs.writeFileSync(emptyTxt, '');
 }
 
-var notSocketClient = net.createConnection(emptyTxt, function() {
+const notSocketClient = net.createConnection(emptyTxt, function() {
   common.fail('connection callback should not run');
 });
 
@@ -45,7 +45,7 @@ notSocketClient.on('error', common.mustCall(function(err) {
 
 
 // Trying to connect to not-existing socket should result in ENOENT error
-var noEntSocketClient = net.createConnection('no-ent-file', function() {
+const noEntSocketClient = net.createConnection('no-ent-file', function() {
   common.fail('connection to non-existent socket, callback should not run');
 });
 
@@ -63,7 +63,7 @@ if (!common.isWindows && process.getuid() !== 0) {
   accessServer.listen(common.PIPE, common.mustCall(function() {
     fs.chmodSync(common.PIPE, 0);
 
-    var accessClient = net.createConnection(common.PIPE, function() {
+    const accessClient = net.createConnection(common.PIPE, function() {
       common.fail('connection should get EACCES, callback should not run');
     });
 
diff --git a/test/parallel/test-net-reconnect.js b/test/parallel/test-net-reconnect.js
index c0c5589b39..47171093ca 100644
--- a/test/parallel/test-net-reconnect.js
+++ b/test/parallel/test-net-reconnect.js
@@ -4,12 +4,12 @@ const assert = require('assert');
 
 const net = require('net');
 
-var N = 50;
-var client_recv_count = 0;
-var client_end_count = 0;
-var disconnect_count = 0;
+const N = 50;
+let client_recv_count = 0;
+let client_end_count = 0;
+let disconnect_count = 0;
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   console.error('SERVER: got socket connection');
   socket.resume();
 
@@ -29,7 +29,7 @@ var server = net.createServer(function(socket) {
 
 server.listen(0, function() {
   console.log('SERVER listening');
-  var client = net.createConnection(this.address().port);
+  const client = net.createConnection(this.address().port);
 
   client.setEncoding('UTF8');
 
diff --git a/test/parallel/test-net-remote-address-port.js b/test/parallel/test-net-remote-address-port.js
index 9cdf4ff7f8..a9d02631be 100644
--- a/test/parallel/test-net-remote-address-port.js
+++ b/test/parallel/test-net-remote-address-port.js
@@ -4,15 +4,15 @@ const assert = require('assert');
 
 const net = require('net');
 
-var conns_closed = 0;
+let conns_closed = 0;
 
-var remoteAddrCandidates = [ common.localhostIPv4 ];
+const remoteAddrCandidates = [ common.localhostIPv4 ];
 if (common.hasIPv6) remoteAddrCandidates.push('::ffff:127.0.0.1');
 
-var remoteFamilyCandidates = ['IPv4'];
+const remoteFamilyCandidates = ['IPv4'];
 if (common.hasIPv6) remoteFamilyCandidates.push('IPv6');
 
-var server = net.createServer(common.mustCall(function(socket) {
+const server = net.createServer(common.mustCall(function(socket) {
   assert.ok(remoteAddrCandidates.includes(socket.remoteAddress));
   assert.ok(remoteFamilyCandidates.includes(socket.remoteFamily));
   assert.ok(socket.remotePort);
@@ -28,8 +28,8 @@ var server = net.createServer(common.mustCall(function(socket) {
 }, 2));
 
 server.listen(0, 'localhost', function() {
-  var client = net.createConnection(this.address().port, 'localhost');
-  var client2 = net.createConnection(this.address().port);
+  const client = net.createConnection(this.address().port, 'localhost');
+  const client2 = net.createConnection(this.address().port);
   client.on('connect', function() {
     assert.ok(remoteAddrCandidates.includes(client.remoteAddress));
     assert.ok(remoteFamilyCandidates.includes(client.remoteFamily));
diff --git a/test/parallel/test-net-server-bind.js b/test/parallel/test-net-server-bind.js
index 623962e3ab..e4dc642164 100644
--- a/test/parallel/test-net-server-bind.js
+++ b/test/parallel/test-net-server-bind.js
@@ -6,8 +6,8 @@ const net = require('net');
 
 // With only a callback, server should get a port assigned by the OS
 
-var address0;
-var server0 = net.createServer(function(socket) { });
+let address0;
+const server0 = net.createServer(function(socket) { });
 
 server0.listen(function() {
   address0 = server0.address();
@@ -18,9 +18,9 @@ server0.listen(function() {
 
 // No callback to listen(), assume we can bind in 100 ms
 
-var address1;
-var connectionKey1;
-var server1 = net.createServer(function(socket) { });
+let address1;
+let connectionKey1;
+const server1 = net.createServer(function(socket) { });
 
 server1.listen(common.PORT);
 
@@ -34,8 +34,8 @@ setTimeout(function() {
 
 // Callback to listen()
 
-var address2;
-var server2 = net.createServer(function(socket) { });
+let address2;
+const server2 = net.createServer(function(socket) { });
 
 server2.listen(common.PORT + 1, function() {
   address2 = server2.address();
@@ -46,8 +46,8 @@ server2.listen(common.PORT + 1, function() {
 
 // Backlog argument
 
-var address3;
-var server3 = net.createServer(function(socket) { });
+let address3;
+const server3 = net.createServer(function(socket) { });
 
 server3.listen(common.PORT + 2, '0.0.0.0', 127, function() {
   address3 = server3.address();
@@ -58,8 +58,8 @@ server3.listen(common.PORT + 2, '0.0.0.0', 127, function() {
 
 // Backlog argument without host argument
 
-var address4;
-var server4 = net.createServer(function(socket) { });
+let address4;
+const server4 = net.createServer(function(socket) { });
 
 server4.listen(common.PORT + 3, 127, function() {
   address4 = server4.address();
@@ -72,7 +72,7 @@ process.on('exit', function() {
   assert.ok(address0.port > 100);
   assert.equal(common.PORT, address1.port);
 
-  var expectedConnectionKey1;
+  let expectedConnectionKey1;
 
   if (address1.family === 'IPv6')
     expectedConnectionKey1 = '6::::' + address1.port;
diff --git a/test/parallel/test-net-server-close.js b/test/parallel/test-net-server-close.js
index 9f920a6a69..d52cccaacf 100644
--- a/test/parallel/test-net-server-close.js
+++ b/test/parallel/test-net-server-close.js
@@ -3,8 +3,8 @@ require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var events = [];
-var sockets = [];
+const events = [];
+const sockets = [];
 
 process.on('exit', function() {
   assert.equal(server.connections, 0);
@@ -15,7 +15,7 @@ process.on('exit', function() {
   assert.equal(events.join(' ').match(/client/g).length, 2);
 });
 
-var server = net.createServer(function(c) {
+const server = net.createServer(function(c) {
   c.on('close', function() {
     events.push('client');
   });
diff --git a/test/parallel/test-net-server-connections.js b/test/parallel/test-net-server-connections.js
index ce3087713c..4e41d29a50 100644
--- a/test/parallel/test-net-server-connections.js
+++ b/test/parallel/test-net-server-connections.js
@@ -7,6 +7,6 @@ const net = require('net');
 // test that server.connections property is no longer enumerable now that it
 // has been marked as deprecated
 
-var server = new net.Server();
+const server = new net.Server();
 
 assert.equal(Object.keys(server).indexOf('connections'), -1);
diff --git a/test/parallel/test-net-server-listen-remove-callback.js b/test/parallel/test-net-server-listen-remove-callback.js
index 4d2c12102c..a678922683 100644
--- a/test/parallel/test-net-server-listen-remove-callback.js
+++ b/test/parallel/test-net-server-listen-remove-callback.js
@@ -4,10 +4,10 @@ const assert = require('assert');
 const net = require('net');
 
 // Server should only fire listen callback once
-var server = net.createServer();
+const server = net.createServer();
 
 server.on('close', function() {
-  var listeners = server.listeners('listening');
+  const listeners = server.listeners('listening');
   console.log('Closed, listeners:', listeners.length);
   assert.equal(0, listeners.length);
 });
diff --git a/test/parallel/test-net-server-max-connections-close-makes-more-available.js b/test/parallel/test-net-server-max-connections-close-makes-more-available.js
index 14a27da8f8..c7fe7c74a7 100644
--- a/test/parallel/test-net-server-max-connections-close-makes-more-available.js
+++ b/test/parallel/test-net-server-max-connections-close-makes-more-available.js
@@ -13,16 +13,16 @@ const net = require('net');
 // Connection 2 should be accepted.
 // Connection 3 should be rejected.
 
-var connections = [];
-var received = [];
-var sent = [];
+const connections = [];
+const received = [];
+const sent = [];
 
-var createConnection = function(index) {
+const createConnection = function(index) {
   console.error('creating connection ' + index);
 
   return new Promise(function(resolve, reject) {
-    var connection = net.createConnection(server.address().port, function() {
-      var msg = '' + index;
+    const connection = net.createConnection(server.address().port, function() {
+      const msg = '' + index;
       console.error('sending message: ' + msg);
       this.write(msg);
       sent.push(msg);
@@ -47,7 +47,7 @@ var createConnection = function(index) {
   });
 };
 
-var closeConnection = function(index) {
+const closeConnection = function(index) {
   console.error('closing connection ' + index);
   return new Promise(function(resolve, reject) {
     connections[index].on('end', function() {
@@ -57,7 +57,7 @@ var closeConnection = function(index) {
   });
 };
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.on('data', function(data) {
     console.error('received message: ' + data);
     received.push('' + data);
diff --git a/test/parallel/test-net-server-max-connections.js b/test/parallel/test-net-server-max-connections.js
index 99d3b345f4..f560a7dafc 100644
--- a/test/parallel/test-net-server-max-connections.js
+++ b/test/parallel/test-net-server-max-connections.js
@@ -9,7 +9,7 @@ const net = require('net');
 // and the last 10 connections are rejected.
 
 const N = 20;
-var closes = 0;
+let closes = 0;
 const waits = [];
 
 const server = net.createServer(common.mustCall(function(connection) {
@@ -26,7 +26,7 @@ server.maxConnections = N / 2;
 
 function makeConnection(index) {
   const c = net.createConnection(server.address().port);
-  var gotData = false;
+  let gotData = false;
 
   c.on('connect', function() {
     if (index + 1 < N) {
@@ -45,7 +45,7 @@ function makeConnection(index) {
       }
 
       if (closes === N / 2) {
-        var cb;
+        let cb;
         console.error('calling wait callback.');
         while (cb = waits.shift()) {
           cb();
diff --git a/test/parallel/test-net-server-pause-on-connect.js b/test/parallel/test-net-server-pause-on-connect.js
index 798175737b..ee5ac4e280 100644
--- a/test/parallel/test-net-server-pause-on-connect.js
+++ b/test/parallel/test-net-server-pause-on-connect.js
@@ -3,8 +3,8 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 const msg = 'test';
-var stopped = true;
-var server1Sock;
+let stopped = true;
+let server1Sock;
 
 
 const server1ConnHandler = function(socket) {
diff --git a/test/parallel/test-net-server-unref-persistent.js b/test/parallel/test-net-server-unref-persistent.js
index e75a4d8305..beb91832ea 100644
--- a/test/parallel/test-net-server-unref-persistent.js
+++ b/test/parallel/test-net-server-unref-persistent.js
@@ -1,7 +1,7 @@
 'use strict';
 const common = require('../common');
 const net = require('net');
-var server = net.createServer();
+const server = net.createServer();
 
 // unref before listening
 server.unref();
diff --git a/test/parallel/test-net-server-unref.js b/test/parallel/test-net-server-unref.js
index bf8a9ef43a..60df9fc700 100644
--- a/test/parallel/test-net-server-unref.js
+++ b/test/parallel/test-net-server-unref.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const net = require('net');
 
-var s = net.createServer();
+const s = net.createServer();
 s.listen(0);
 s.unref();
 
diff --git a/test/parallel/test-net-socket-destroy-twice.js b/test/parallel/test-net-socket-destroy-twice.js
index be82d9f2a6..998ad05a1d 100644
--- a/test/parallel/test-net-socket-destroy-twice.js
+++ b/test/parallel/test-net-socket-destroy-twice.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const net = require('net');
 
-var conn = net.createConnection(common.PORT);
+const conn = net.createConnection(common.PORT);
 
 conn.on('error', common.mustCall(function() {
   conn.destroy();
diff --git a/test/parallel/test-net-socket-local-address.js b/test/parallel/test-net-socket-local-address.js
index 82f174dc2c..e888667eab 100644
--- a/test/parallel/test-net-socket-local-address.js
+++ b/test/parallel/test-net-socket-local-address.js
@@ -9,9 +9,9 @@ if (common.inFreeBSDJail) {
   return;
 }
 
-var conns = 0;
-var clientLocalPorts = [];
-var serverRemotePorts = [];
+let conns = 0;
+const clientLocalPorts = [];
+const serverRemotePorts = [];
 const client = new net.Socket();
 const server = net.createServer((socket) => {
   serverRemotePorts.push(socket.remotePort);
diff --git a/test/parallel/test-net-socket-timeout-unref.js b/test/parallel/test-net-socket-timeout-unref.js
index fb4f6a658e..1789ec7a1c 100644
--- a/test/parallel/test-net-socket-timeout-unref.js
+++ b/test/parallel/test-net-socket-timeout-unref.js
@@ -12,7 +12,7 @@ const server = net.createServer(function(c) {
 server.listen(0);
 server.unref();
 
-var connections = 0;
+let connections = 0;
 const sockets = [];
 const delays = [8, 5, 3, 6, 2, 4];
 
diff --git a/test/parallel/test-net-stream.js b/test/parallel/test-net-stream.js
index 12fcc25b0a..9d091bd66f 100644
--- a/test/parallel/test-net-stream.js
+++ b/test/parallel/test-net-stream.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var s = new net.Stream();
+const s = new net.Stream();
 
 // test that destroy called on a stream with a server only ever decrements the
 // server connection count once
@@ -18,11 +18,11 @@ assert.equal(9, s.server.connections);
 s.destroy();
 assert.equal(9, s.server.connections);
 
-var SIZE = 2E6;
-var N = 10;
-var buf = Buffer.alloc(SIZE, 'a');
+const SIZE = 2E6;
+const N = 10;
+const buf = Buffer.alloc(SIZE, 'a');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.setNoDelay();
 
   socket.on('error', function(err) {
@@ -31,13 +31,13 @@ var server = net.createServer(function(socket) {
     server.close();
   });
 
-  for (var i = 0; i < N; ++i) {
+  for (let i = 0; i < N; ++i) {
     socket.write(buf, function() { });
   }
   socket.end();
 
 }).listen(0, function() {
-  var conn = net.connect(this.address().port);
+  const conn = net.connect(this.address().port);
   conn.on('data', function(buf) {
     conn.pause();
     setTimeout(function() {
diff --git a/test/parallel/test-net-write-after-close.js b/test/parallel/test-net-write-after-close.js
index d8213a9b86..0a1dcd98b2 100644
--- a/test/parallel/test-net-write-after-close.js
+++ b/test/parallel/test-net-write-after-close.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const net = require('net');
 
-var server = net.createServer(common.mustCall(function(socket) {
+const server = net.createServer(common.mustCall(function(socket) {
   socket.resume();
 
   socket.on('error', common.mustCall(function(error) {
@@ -17,7 +17,7 @@ var server = net.createServer(common.mustCall(function(socket) {
 }));
 
 server.listen(0, function() {
-  var client = net.connect(this.address().port, function() {
+  const client = net.connect(this.address().port, function() {
     client.end();
   });
 });
diff --git a/test/parallel/test-net-write-connect-write.js b/test/parallel/test-net-write-connect-write.js
index 15a30b826e..ee20ede49f 100644
--- a/test/parallel/test-net-write-connect-write.js
+++ b/test/parallel/test-net-write-connect-write.js
@@ -3,11 +3,11 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.pipe(socket);
 }).listen(0, common.mustCall(function() {
-  var conn = net.connect(this.address().port);
-  var received = '';
+  const conn = net.connect(this.address().port);
+  let received = '';
 
   conn.setEncoding('utf8');
   conn.write('before');
diff --git a/test/parallel/test-net-write-slow.js b/test/parallel/test-net-write-slow.js
index 7e183068f1..c1b2c60e2e 100644
--- a/test/parallel/test-net-write-slow.js
+++ b/test/parallel/test-net-write-slow.js
@@ -3,20 +3,20 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var SIZE = 2E5;
-var N = 10;
-var flushed = 0;
-var received = 0;
-var buf = Buffer.alloc(SIZE, 'a');
+const SIZE = 2E5;
+const N = 10;
+let flushed = 0;
+let received = 0;
+const buf = Buffer.alloc(SIZE, 'a');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.setNoDelay();
   socket.setTimeout(9999);
   socket.on('timeout', function() {
     common.fail(`flushed: ${flushed}, received: ${received}/${SIZE * N}`);
   });
 
-  for (var i = 0; i < N; ++i) {
+  for (let i = 0; i < N; ++i) {
     socket.write(buf, function() {
       ++flushed;
       if (flushed === N) {
@@ -27,7 +27,7 @@ var server = net.createServer(function(socket) {
   socket.end();
 
 }).listen(0, common.mustCall(function() {
-  var conn = net.connect(this.address().port);
+  const conn = net.connect(this.address().port);
   conn.on('data', function(buf) {
     received += buf.length;
     conn.pause();
diff --git a/test/parallel/test-next-tick-domain.js b/test/parallel/test-next-tick-domain.js
index 634e311da7..6edb7e3050 100644
--- a/test/parallel/test-next-tick-domain.js
+++ b/test/parallel/test-next-tick-domain.js
@@ -2,7 +2,7 @@
 require('../common');
 const assert = require('assert');
 
-var origNextTick = process.nextTick;
+const origNextTick = process.nextTick;
 
 require('domain');
 
diff --git a/test/parallel/test-next-tick-intentional-starvation.js b/test/parallel/test-next-tick-intentional-starvation.js
index 3da32b8ffa..ee829c94ab 100644
--- a/test/parallel/test-next-tick-intentional-starvation.js
+++ b/test/parallel/test-next-tick-intentional-starvation.js
@@ -9,14 +9,14 @@ const assert = require('assert');
 // WARNING: unsafe!
 process.maxTickDepth = Infinity;
 
-var ran = false;
-var starved = false;
-var start = +new Date();
-var timerRan = false;
+let ran = false;
+let starved = false;
+const start = +new Date();
+let timerRan = false;
 
 function spin() {
   ran = true;
-  var now = +new Date();
+  const now = +new Date();
   if (now - start > 100) {
     console.log('The timer is starving, just as we planned.');
     starved = true;
diff --git a/test/parallel/test-next-tick-ordering.js b/test/parallel/test-next-tick-ordering.js
index 316fdb9f9a..2f13147d8e 100644
--- a/test/parallel/test-next-tick-ordering.js
+++ b/test/parallel/test-next-tick-ordering.js
@@ -1,10 +1,10 @@
 'use strict';
 require('../common');
 const assert = require('assert');
-var i;
+let i;
 
-var N = 30;
-var done = [];
+const N = 30;
+const done = [];
 
 function get_printer(timeout) {
   return function() {
diff --git a/test/parallel/test-next-tick-ordering2.js b/test/parallel/test-next-tick-ordering2.js
index 32c5bd8e8e..7cfec867f3 100644
--- a/test/parallel/test-next-tick-ordering2.js
+++ b/test/parallel/test-next-tick-ordering2.js
@@ -2,7 +2,7 @@
 require('../common');
 const assert = require('assert');
 
-var order = [];
+const order = [];
 process.nextTick(function() {
   setTimeout(function() {
     order.push('setTimeout');
diff --git a/test/parallel/test-next-tick.js b/test/parallel/test-next-tick.js
index 702d2308ae..3001ad0e56 100644
--- a/test/parallel/test-next-tick.js
+++ b/test/parallel/test-next-tick.js
@@ -14,7 +14,7 @@ setTimeout(common.mustCall(function() {
 
 process.nextTick(common.mustCall(function() {}));
 
-var obj = {};
+const obj = {};
 
 process.nextTick(function(a, b) {
   assert.equal(a, 42);
diff --git a/test/parallel/test-no-enter-tickcallback.js b/test/parallel/test-no-enter-tickcallback.js
index e066286289..aab22c5f3e 100644
--- a/test/parallel/test-no-enter-tickcallback.js
+++ b/test/parallel/test-no-enter-tickcallback.js
@@ -2,8 +2,8 @@
 
 const common = require('../common');
 const assert = require('assert');
-var allsGood = false;
-var cntr = 0;
+let allsGood = false;
+let cntr = 0;
 
 process.on('exit', () => {
   assert.ok(cntr > 0, '_tickDomainCallback was never called');
diff --git a/test/parallel/test-path-makelong.js b/test/parallel/test-path-makelong.js
index 9d3c3852cd..293d6284f1 100644
--- a/test/parallel/test-path-makelong.js
+++ b/test/parallel/test-path-makelong.js
@@ -4,8 +4,8 @@ const assert = require('assert');
 const path = require('path');
 
 if (common.isWindows) {
-  var file = path.join(common.fixturesDir, 'a.js');
-  var resolvedFile = path.resolve(file);
+  const file = path.join(common.fixturesDir, 'a.js');
+  const resolvedFile = path.resolve(file);
 
   assert.equal('\\\\?\\' + resolvedFile, path._makeLong(file));
   assert.equal('\\\\?\\' + resolvedFile, path._makeLong('\\\\?\\' + file));
diff --git a/test/parallel/test-path-parse-format.js b/test/parallel/test-path-parse-format.js
index 2db4c27dce..f112bc0e40 100644
--- a/test/parallel/test-path-parse-format.js
+++ b/test/parallel/test-path-parse-format.js
@@ -175,7 +175,7 @@ function checkErrors(path) {
 
 function checkParseFormat(path, paths) {
   paths.forEach(function(element) {
-    var output = path.parse(element);
+    const output = path.parse(element);
     assert.strictEqual(typeof output.root, 'string');
     assert.strictEqual(typeof output.dir, 'string');
     assert.strictEqual(typeof output.base, 'string');
diff --git a/test/parallel/test-path.js b/test/parallel/test-path.js
index 0a12b5ce92..3843088ebd 100644
--- a/test/parallel/test-path.js
+++ b/test/parallel/test-path.js
@@ -450,9 +450,9 @@ if (common.isWindows) {
   // See https://github.com/nodejs/node/issues/7215
   const currentDriveLetter = path.parse(process.cwd()).root.substring(0, 2);
   const resolveFixture = path.join(common.fixturesDir, 'path-resolve.js');
-  var spawnResult = child.spawnSync(
+  const spawnResult = child.spawnSync(
     process.argv[0], [resolveFixture, currentDriveLetter]);
-  var resolvedPath = spawnResult.stdout.toString().trim();
+  const resolvedPath = spawnResult.stdout.toString().trim();
   assert.equal(resolvedPath.toLowerCase(), process.cwd().toLowerCase());
 }
 
diff --git a/test/parallel/test-pipe-address.js b/test/parallel/test-pipe-address.js
index d4bb0ffbc5..49f6802d3f 100644
--- a/test/parallel/test-pipe-address.js
+++ b/test/parallel/test-pipe-address.js
@@ -2,7 +2,7 @@
 const common = require('../common');
 const assert = require('assert');
 const net = require('net');
-var server = net.createServer(common.fail);
+const server = net.createServer(common.fail);
 
 common.refreshTmpDir();
 
diff --git a/test/parallel/test-pipe-return-val.js b/test/parallel/test-pipe-return-val.js
index a5f2ff6dfa..5e6e83dbc7 100644
--- a/test/parallel/test-pipe-return-val.js
+++ b/test/parallel/test-pipe-return-val.js
@@ -5,8 +5,8 @@ require('../common');
 const Stream = require('stream').Stream;
 const assert = require('assert');
 
-var sourceStream = new Stream();
-var destStream = new Stream();
-var result = sourceStream.pipe(destStream);
+const sourceStream = new Stream();
+const destStream = new Stream();
+const result = sourceStream.pipe(destStream);
 
 assert.strictEqual(result, destStream);
diff --git a/test/parallel/test-pipe-stream.js b/test/parallel/test-pipe-stream.js
index 8392c80110..5e3774fc99 100644
--- a/test/parallel/test-pipe-stream.js
+++ b/test/parallel/test-pipe-stream.js
@@ -6,8 +6,8 @@ const net = require('net');
 common.refreshTmpDir();
 
 function test(clazz, cb) {
-  var have_ping = false;
-  var have_pong = false;
+  let have_ping = false;
+  let have_pong = false;
 
   function check() {
     assert.ok(have_ping);
@@ -15,7 +15,7 @@ function test(clazz, cb) {
   }
 
   function ping() {
-    var conn = new clazz();
+    const conn = new clazz();
 
     conn.on('error', function(err) {
       throw err;
@@ -48,11 +48,11 @@ function test(clazz, cb) {
     });
   }
 
-  var timeout = setTimeout(function() {
+  const timeout = setTimeout(function() {
     server.close();
   }, 2000);
 
-  var server = net.Server();
+  const server = net.Server();
   server.listen(common.PIPE, ping);
   server.on('connection', pong);
   server.on('close', function() {
diff --git a/test/parallel/test-pipe-unref.js b/test/parallel/test-pipe-unref.js
index f926dab3e5..34b5ef651b 100644
--- a/test/parallel/test-pipe-unref.js
+++ b/test/parallel/test-pipe-unref.js
@@ -4,7 +4,7 @@ const net = require('net');
 
 common.refreshTmpDir();
 
-var s = net.Server();
+const s = net.Server();
 s.listen(common.PIPE);
 s.unref();
 
diff --git a/test/parallel/test-preload.js b/test/parallel/test-preload.js
index 3413a2a6dc..444c3a0a6d 100644
--- a/test/parallel/test-preload.js
+++ b/test/parallel/test-preload.js
@@ -14,7 +14,7 @@ if (common.isSunOS) {
 const nodeBinary = process.argv[0];
 
 const preloadOption = function(preloads) {
-  var option = '';
+  let option = '';
   preloads.forEach(function(preload, index) {
     option += '-r ' + preload + ' ';
   });
@@ -75,7 +75,7 @@ const stdinProc = childProcess.spawn(
   {stdio: 'pipe'}
 );
 stdinProc.stdin.end("console.log('hello');");
-var stdinStdout = '';
+let stdinStdout = '';
 stdinProc.stdout.on('data', function(d) {
   stdinStdout += d;
 });
@@ -91,7 +91,7 @@ const replProc = childProcess.spawn(
   {stdio: 'pipe'}
 );
 replProc.stdin.end('.exit\n');
-var replStdout = '';
+let replStdout = '';
 replProc.stdout.on('data', function(d) {
   replStdout += d;
 });
diff --git a/test/parallel/test-process-argv-0.js b/test/parallel/test-process-argv-0.js
index ad078d02b4..d6b4bfe232 100644
--- a/test/parallel/test-process-argv-0.js
+++ b/test/parallel/test-process-argv-0.js
@@ -5,11 +5,11 @@ const assert = require('assert');
 const spawn = require('child_process').spawn;
 
 if (process.argv[2] !== 'child') {
-  var child = spawn(process.execPath, [__filename, 'child'], {
+  const child = spawn(process.execPath, [__filename, 'child'], {
     cwd: path.dirname(process.execPath)
   });
 
-  var childArgv0 = '';
+  let childArgv0 = '';
   child.stdout.on('data', function(chunk) {
     childArgv0 += chunk;
   });
diff --git a/test/parallel/test-process-config.js b/test/parallel/test-process-config.js
index e692a7bf19..a1767464fd 100644
--- a/test/parallel/test-process-config.js
+++ b/test/parallel/test-process-config.js
@@ -10,8 +10,8 @@ assert(process.hasOwnProperty('config'));
 // ensure that `process.config` is an Object
 assert.strictEqual(Object(process.config), process.config);
 
-var configPath = path.resolve(__dirname, '..', '..', 'config.gypi');
-var config = fs.readFileSync(configPath, 'utf8');
+const configPath = path.resolve(__dirname, '..', '..', 'config.gypi');
+let config = fs.readFileSync(configPath, 'utf8');
 
 // clean up comment at the first line
 config = config.split('\n').slice(1).join('\n').replace(/'/g, '"');
diff --git a/test/parallel/test-process-exec-argv.js b/test/parallel/test-process-exec-argv.js
index c7de82adae..8125850b42 100644
--- a/test/parallel/test-process-exec-argv.js
+++ b/test/parallel/test-process-exec-argv.js
@@ -6,10 +6,10 @@ const spawn = require('child_process').spawn;
 if (process.argv[2] === 'child') {
   process.stdout.write(JSON.stringify(process.execArgv));
 } else {
-  var execArgv = ['--stack-size=256'];
-  var args = [__filename, 'child', 'arg0'];
-  var child = spawn(process.execPath, execArgv.concat(args));
-  var out = '';
+  const execArgv = ['--stack-size=256'];
+  const args = [__filename, 'child', 'arg0'];
+  const child = spawn(process.execPath, execArgv.concat(args));
+  let out = '';
 
   child.stdout.on('data', function(chunk) {
     out += chunk;
diff --git a/test/parallel/test-process-exit-code.js b/test/parallel/test-process-exit-code.js
index b93ae443a7..337502ffcb 100644
--- a/test/parallel/test-process-exit-code.js
+++ b/test/parallel/test-process-exit-code.js
@@ -71,9 +71,9 @@ function parent() {
 
 function test(arg, exit) {
   const spawn = require('child_process').spawn;
-  var node = process.execPath;
-  var f = __filename;
-  var option = { stdio: [ 0, 1, 'ignore' ] };
+  const node = process.execPath;
+  const f = __filename;
+  const option = { stdio: [ 0, 1, 'ignore' ] };
   spawn(node, [f, arg], option).on('exit', function(code) {
     assert.equal(code, exit, 'wrong exit for ' +
                  arg + '\nexpected:' + exit +
diff --git a/test/parallel/test-process-exit-recursive.js b/test/parallel/test-process-exit-recursive.js
index 252b282b04..1634aad154 100644
--- a/test/parallel/test-process-exit-recursive.js
+++ b/test/parallel/test-process-exit-recursive.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 
 // recursively calling .exit() should not overflow the call stack
-var nexits = 0;
+let nexits = 0;
 
 process.on('exit', function(code) {
   assert.equal(nexits++, 0);
diff --git a/test/parallel/test-process-exit.js b/test/parallel/test-process-exit.js
index 9d48ea9eac..f7df7737ed 100644
--- a/test/parallel/test-process-exit.js
+++ b/test/parallel/test-process-exit.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 
 // calling .exit() from within "exit" should not overflow the call stack
-var nexits = 0;
+let nexits = 0;
 
 process.on('exit', function(code) {
   assert.equal(nexits++, 0);
diff --git a/test/parallel/test-process-getactivehandles.js b/test/parallel/test-process-getactivehandles.js
index a9222c984b..8ceb2bcd2a 100644
--- a/test/parallel/test-process-getactivehandles.js
+++ b/test/parallel/test-process-getactivehandles.js
@@ -6,7 +6,7 @@ const net = require('net');
 const NUM = 8;
 const connections = [];
 const clients = [];
-var clients_counter = 0;
+let clients_counter = 0;
 
 const server = net.createServer(function listener(c) {
   connections.push(c);
diff --git a/test/parallel/test-process-getgroups.js b/test/parallel/test-process-getgroups.js
index 9e08892b62..e92f048539 100644
--- a/test/parallel/test-process-getgroups.js
+++ b/test/parallel/test-process-getgroups.js
@@ -9,12 +9,12 @@ if (common.isOSX) {
 }
 
 if (typeof process.getgroups === 'function') {
-  var groups = process.getgroups();
+  const groups = process.getgroups();
   assert(Array.isArray(groups));
   assert(groups.length > 0);
   exec('id -G', function(err, stdout) {
     assert.ifError(err);
-    var real_groups = stdout.match(/\d+/g).map(Number);
+    const real_groups = stdout.match(/\d+/g).map(Number);
     assert.equal(groups.length, real_groups.length);
     check(groups, real_groups);
     check(real_groups, groups);
@@ -22,5 +22,5 @@ if (typeof process.getgroups === 'function') {
 }
 
 function check(a, b) {
-  for (var i = 0; i < a.length; ++i) assert.notStrictEqual(b.indexOf(a[i]), -1);
+  for (let i = 0; i < a.length; ++i) assert.notStrictEqual(b.indexOf(a[i]), -1);
 }
diff --git a/test/parallel/test-process-kill-null.js b/test/parallel/test-process-kill-null.js
index 406b294362..6c0ee8fa04 100644
--- a/test/parallel/test-process-kill-null.js
+++ b/test/parallel/test-process-kill-null.js
@@ -3,8 +3,8 @@ require('../common');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-var cat = spawn('cat');
-var called;
+const cat = spawn('cat');
+let called;
 
 assert.ok(process.kill(cat.pid, 0));
 
diff --git a/test/parallel/test-process-kill-pid.js b/test/parallel/test-process-kill-pid.js
index 9b4a5fa47a..2c6ef37fc1 100644
--- a/test/parallel/test-process-kill-pid.js
+++ b/test/parallel/test-process-kill-pid.js
@@ -34,9 +34,9 @@ assert.throws(function() { process.kill(1, 'test'); }, Error);
 // that we don't kill our process group, or try to actually send ANY signals on
 // windows, which doesn't support them.
 function kill(tryPid, trySig, expectPid, expectSig) {
-  var getPid;
-  var getSig;
-  var origKill = process._kill;
+  let getPid;
+  let getSig;
+  const origKill = process._kill;
   process._kill = function(pid, sig) {
     getPid = pid;
     getSig = sig;
diff --git a/test/parallel/test-process-next-tick.js b/test/parallel/test-process-next-tick.js
index 3435114452..e67ebae0a2 100644
--- a/test/parallel/test-process-next-tick.js
+++ b/test/parallel/test-process-next-tick.js
@@ -1,12 +1,12 @@
 'use strict';
 const common = require('../common');
-var N = 2;
+const N = 2;
 
 function cb() {
   throw new Error();
 }
 
-for (var i = 0; i < N; ++i) {
+for (let i = 0; i < N; ++i) {
   process.nextTick(common.mustCall(cb));
 }
 
diff --git a/test/parallel/test-process-raw-debug.js b/test/parallel/test-process-raw-debug.js
index 41774cb87a..74c99b1f8f 100644
--- a/test/parallel/test-process-raw-debug.js
+++ b/test/parallel/test-process-raw-debug.js
@@ -14,9 +14,9 @@ switch (process.argv[2]) {
 
 function parent() {
   const spawn = require('child_process').spawn;
-  var child = spawn(process.execPath, [__filename, 'child']);
+  const child = spawn(process.execPath, [__filename, 'child']);
 
-  var output = '';
+  let output = '';
 
   child.stderr.on('data', function(c) {
     output += c;
@@ -42,7 +42,7 @@ function child() {
     throw new Error('No ticking!');
   };
 
-  var stderr = process.stderr;
+  const stderr = process.stderr;
   stderr.write = function() {
     throw new Error('No writing to stderr!');
   };
diff --git a/test/parallel/test-process-remove-all-signal-listeners.js b/test/parallel/test-process-remove-all-signal-listeners.js
index 131d57cdfc..76a96f7683 100644
--- a/test/parallel/test-process-remove-all-signal-listeners.js
+++ b/test/parallel/test-process-remove-all-signal-listeners.js
@@ -14,7 +14,7 @@ if (process.argv[2] !== '--do-test') {
   // We are the master, fork a child so we can verify it exits with correct
   // status.
   process.env.DOTEST = 'y';
-  var child = spawn(process.execPath, [__filename, '--do-test']);
+  const child = spawn(process.execPath, [__filename, '--do-test']);
 
   child.once('exit', common.mustCall(function(code, signal) {
     assert.equal(signal, 'SIGINT');
diff --git a/test/parallel/test-process-versions.js b/test/parallel/test-process-versions.js
index 181e3ecef2..8532b259e0 100644
--- a/test/parallel/test-process-versions.js
+++ b/test/parallel/test-process-versions.js
@@ -2,8 +2,8 @@
 const common = require('../common');
 const assert = require('assert');
 
-var expected_keys = ['ares', 'http_parser', 'modules', 'node',
-                     'uv', 'v8', 'zlib'];
+const expected_keys = ['ares', 'http_parser', 'modules', 'node',
+                       'uv', 'v8', 'zlib'];
 
 if (common.hasCrypto) {
   expected_keys.push('openssl');
diff --git a/test/parallel/test-process-wrap.js b/test/parallel/test-process-wrap.js
index 08e1b152ed..326854e983 100644
--- a/test/parallel/test-process-wrap.js
+++ b/test/parallel/test-process-wrap.js
@@ -1,14 +1,14 @@
 'use strict';
 require('../common');
 const assert = require('assert');
-var Process = process.binding('process_wrap').Process;
-var Pipe = process.binding('pipe_wrap').Pipe;
-var pipe = new Pipe();
-var p = new Process();
+const Process = process.binding('process_wrap').Process;
+const Pipe = process.binding('pipe_wrap').Pipe;
+const pipe = new Pipe();
+const p = new Process();
 
-var processExited = false;
-var gotPipeEOF = false;
-var gotPipeData = false;
+let processExited = false;
+let gotPipeEOF = false;
+let gotPipeData = false;
 
 p.onexit = function(exitCode, signal) {
   console.log('exit');
@@ -45,7 +45,7 @@ p.spawn({
 // 'this' safety
 // https://github.com/joyent/node/issues/6690
 assert.throws(function() {
-  var notp = { spawn: p.spawn };
+  const notp = { spawn: p.spawn };
   notp.spawn({
     file: process.execPath,
     args: [process.execPath, '-v'],
diff --git a/test/parallel/test-promises-unhandled-rejections.js b/test/parallel/test-promises-unhandled-rejections.js
index e275dac238..27cb94bf70 100644
--- a/test/parallel/test-promises-unhandled-rejections.js
+++ b/test/parallel/test-promises-unhandled-rejections.js
@@ -3,15 +3,15 @@ const common = require('../common');
 const assert = require('assert');
 const domain = require('domain');
 
-var asyncTest = (function() {
-  var asyncTestsEnabled = false;
-  var asyncTestLastCheck;
-  var asyncTestQueue = [];
-  var asyncTestHandle;
-  var currentTest = null;
+const asyncTest = (function() {
+  let asyncTestsEnabled = false;
+  let asyncTestLastCheck;
+  const asyncTestQueue = [];
+  let asyncTestHandle;
+  let currentTest = null;
 
   function fail(error) {
-    var stack = currentTest ?
+    const stack = currentTest ?
       error.stack + '\nFrom previous event:\n' + currentTest.stack :
       error.stack;
 
@@ -23,7 +23,7 @@ var asyncTest = (function() {
   }
 
   function nextAsyncTest() {
-    var called = false;
+    let called = false;
     function done(err) {
       if (called) return fail(new Error('done called twice'));
       called = true;
@@ -33,7 +33,7 @@ var asyncTest = (function() {
     }
 
     if (asyncTestQueue.length) {
-      var test = asyncTestQueue.shift();
+      const test = asyncTestQueue.shift();
       currentTest = test;
       test.action(done);
     } else {
@@ -42,7 +42,7 @@ var asyncTest = (function() {
   }
 
   return function asyncTest(description, fn) {
-    var stack = new Error().stack.split('\n').slice(1).join('\n');
+    const stack = new Error().stack.split('\n').slice(1).join('\n');
     asyncTestQueue.push({
       action: fn,
       stack: stack,
@@ -53,7 +53,7 @@ var asyncTest = (function() {
       asyncTestLastCheck = Date.now();
       process.on('uncaughtException', fail);
       asyncTestHandle = setInterval(function() {
-        var now = Date.now();
+        const now = Date.now();
         if (now - asyncTestLastCheck > 10000) {
           return fail(new Error('Async test timeout exceeded'));
         }
@@ -65,7 +65,7 @@ var asyncTest = (function() {
 })();
 
 function setupException(fn) {
-  var listeners = process.listeners('uncaughtException');
+  const listeners = process.listeners('uncaughtException');
   process.removeAllListeners('uncaughtException');
   process.on('uncaughtException', fn);
   return function clean() {
@@ -108,7 +108,7 @@ function onUnhandledFail(done) {
 
 asyncTest('synchronously rejected promise should trigger' +
           ' unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
   });
@@ -117,7 +117,7 @@ asyncTest('synchronously rejected promise should trigger' +
 
 asyncTest('synchronously rejected promise should trigger' +
           ' unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
   });
@@ -128,7 +128,7 @@ asyncTest('synchronously rejected promise should trigger' +
 
 asyncTest('Promise rejected after setImmediate should trigger' +
           ' unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
   });
@@ -141,7 +141,7 @@ asyncTest('Promise rejected after setImmediate should trigger' +
 
 asyncTest('Promise rejected after setTimeout(,1) should trigger' +
           ' unhandled rejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
   });
@@ -154,12 +154,12 @@ asyncTest('Promise rejected after setTimeout(,1) should trigger' +
 
 asyncTest('Catching a promise rejection after setImmediate is not' +
           ' soon enough to stop unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
   });
-  var _reject;
-  var promise = new Promise(function(_, reject) {
+  let _reject;
+  const promise = new Promise(function(_, reject) {
     _reject = reject;
   });
   _reject(e);
@@ -170,13 +170,13 @@ asyncTest('Catching a promise rejection after setImmediate is not' +
 
 asyncTest('When re-throwing new errors in a promise catch, only the' +
           ' re-thrown error should hit unhandledRejection', function(done) {
-  var e = new Error();
-  var e2 = new Error();
+  const e = new Error();
+  const e2 = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e2, reason);
     assert.strictEqual(promise2, promise);
   });
-  var promise2 = Promise.reject(e).then(common.fail, function(reason) {
+  const promise2 = Promise.reject(e).then(common.fail, function(reason) {
     assert.strictEqual(e, reason);
     throw e2;
   });
@@ -184,7 +184,7 @@ asyncTest('When re-throwing new errors in a promise catch, only the' +
 
 asyncTest('Test params of unhandledRejection for a synchronously-rejected' +
           'promise', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
     assert.strictEqual(promise, promise);
@@ -195,13 +195,13 @@ asyncTest('Test params of unhandledRejection for a synchronously-rejected' +
 asyncTest('When re-throwing new errors in a promise catch, only the ' +
           're-thrown error should hit unhandledRejection: original promise' +
           ' rejected async with setTimeout(,1)', function(done) {
-  var e = new Error();
-  var e2 = new Error();
+  const e = new Error();
+  const e2 = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e2, reason);
     assert.strictEqual(promise2, promise);
   });
-  var promise2 = new Promise(function(_, reject) {
+  const promise2 = new Promise(function(_, reject) {
     setTimeout(function() {
       reject(e);
     }, 1);
@@ -214,13 +214,13 @@ asyncTest('When re-throwing new errors in a promise catch, only the ' +
 asyncTest('When re-throwing new errors in a promise catch, only the re-thrown' +
           ' error should hit unhandledRejection: promise catch attached a' +
           ' process.nextTick after rejection', function(done) {
-  var e = new Error();
-  var e2 = new Error();
+  const e = new Error();
+  const e2 = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e2, reason);
     assert.strictEqual(promise2, promise);
   });
-  var promise = new Promise(function(_, reject) {
+  const promise = new Promise(function(_, reject) {
     setTimeout(function() {
       reject(e);
       process.nextTick(function() {
@@ -231,14 +231,14 @@ asyncTest('When re-throwing new errors in a promise catch, only the re-thrown' +
       });
     }, 1);
   });
-  var promise2;
+  let promise2;
 });
 
 asyncTest(
   'unhandledRejection should not be triggered if a promise catch is' +
   ' attached synchronously upon the promise\'s creation',
   function(done) {
-    var e = new Error();
+    const e = new Error();
     onUnhandledFail(done);
     Promise.reject(e).then(common.fail, function() {});
   }
@@ -248,7 +248,7 @@ asyncTest(
   'unhandledRejection should not be triggered if a promise catch is' +
   ' attached synchronously upon the promise\'s creation',
   function(done) {
-    var e = new Error();
+    const e = new Error();
     onUnhandledFail(done);
     new Promise(function(_, reject) {
       reject(e);
@@ -258,9 +258,9 @@ asyncTest(
 
 asyncTest('Attaching a promise catch in a process.nextTick is soon enough to' +
           ' prevent unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
-  var promise = Promise.reject(e);
+  const promise = Promise.reject(e);
   process.nextTick(function() {
     promise.then(common.fail, function() {});
   });
@@ -268,9 +268,9 @@ asyncTest('Attaching a promise catch in a process.nextTick is soon enough to' +
 
 asyncTest('Attaching a promise catch in a process.nextTick is soon enough to' +
           ' prevent unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
-  var promise = new Promise(function(_, reject) {
+  const promise = new Promise(function(_, reject) {
     reject(e);
   });
   process.nextTick(function() {
@@ -297,7 +297,7 @@ asyncTest('While inside setImmediate, catching a rejected promise derived ' +
 asyncTest('catching a promise which is asynchronously rejected (via' +
           'resolution to an asynchronously-rejected promise) prevents' +
           ' unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
   Promise.resolve().then(function() {
     return new Promise(function(_, reject) {
@@ -312,7 +312,7 @@ asyncTest('catching a promise which is asynchronously rejected (via' +
 
 asyncTest('Catching a rejected promise derived from throwing in a' +
           ' fulfillment handler prevents unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
   Promise.resolve().then(function() {
     throw e;
@@ -324,7 +324,7 @@ asyncTest('Catching a rejected promise derived from throwing in a' +
 asyncTest('Catching a rejected promise derived from returning a' +
           ' synchronously-rejected promise in a fulfillment handler' +
           ' prevents unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
   Promise.resolve().then(function() {
     return Promise.reject(e);
@@ -336,13 +336,12 @@ asyncTest('Catching a rejected promise derived from returning a' +
 asyncTest('A rejected promise derived from returning an' +
           ' asynchronously-rejected promise in a fulfillment handler' +
           ' does trigger unhandledRejection', function(done) {
-  var e = new Error();
-  var _promise;
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
     assert.strictEqual(_promise, promise);
   });
-  _promise = Promise.resolve().then(function() {
+  const _promise = Promise.resolve().then(function() {
     return new Promise(function(_, reject) {
       setTimeout(function() {
         reject(e);
@@ -353,13 +352,12 @@ asyncTest('A rejected promise derived from returning an' +
 
 asyncTest('A rejected promise derived from throwing in a fulfillment handler' +
           ' does trigger unhandledRejection', function(done) {
-  var e = new Error();
-  var _promise;
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
     assert.strictEqual(_promise, promise);
   });
-  _promise = Promise.resolve().then(function() {
+  const _promise = Promise.resolve().then(function() {
     throw e;
   });
 });
@@ -368,13 +366,12 @@ asyncTest(
   'A rejected promise derived from returning a synchronously-rejected' +
   ' promise in a fulfillment handler does trigger unhandledRejection',
   function(done) {
-    var e = new Error();
-    var _promise;
+    const e = new Error();
     onUnhandledSucceed(done, function(reason, promise) {
       assert.strictEqual(e, reason);
       assert.strictEqual(_promise, promise);
     });
-    _promise = Promise.resolve().then(function() {
+    const _promise = Promise.resolve().then(function() {
       return Promise.reject(e);
     });
   }
@@ -383,7 +380,7 @@ asyncTest(
 // Combinations with Promise.all
 asyncTest('Catching the Promise.all() of a collection that includes a' +
           'rejected promise prevents unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
   Promise.all([Promise.reject(e)]).then(common.fail, function() {});
 });
@@ -392,9 +389,9 @@ asyncTest(
   'Catching the Promise.all() of a collection that includes a ' +
   'nextTick-async rejected promise prevents unhandledRejection',
   function(done) {
-    var e = new Error();
+    const e = new Error();
     onUnhandledFail(done);
-    var p = new Promise(function(_, reject) {
+    let p = new Promise(function(_, reject) {
       process.nextTick(function() {
         reject(e);
       });
@@ -409,19 +406,19 @@ asyncTest(
 asyncTest('Failing to catch the Promise.all() of a collection that includes' +
           ' a rejected promise triggers unhandledRejection for the returned' +
           ' promise, not the passed promise', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
     assert.strictEqual(p, promise);
   });
-  var p = Promise.all([Promise.reject(e)]);
+  const p = Promise.all([Promise.reject(e)]);
 });
 
 asyncTest('Waiting setTimeout(, 10) to catch a promise causes an' +
           ' unhandledRejection + rejectionHandled pair', function(done) {
   clean();
-  var unhandledPromises = [];
-  var e = new Error();
+  const unhandledPromises = [];
+  const e = new Error();
   process.on('unhandledRejection', function(reason, promise) {
     assert.strictEqual(e, reason);
     unhandledPromises.push(promise);
@@ -433,7 +430,7 @@ asyncTest('Waiting setTimeout(, 10) to catch a promise causes an' +
     done();
   });
 
-  var thePromise = new Promise(function() {
+  const thePromise = new Promise(function() {
     throw e;
   });
   setTimeout(function() {
@@ -446,11 +443,11 @@ asyncTest('Waiting setTimeout(, 10) to catch a promise causes an' +
 asyncTest('Waiting for some combination of process.nextTick + promise' +
           ' microtasks to attach a catch handler is still soon enough to' +
           ' prevent unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
 
 
-  var a = Promise.reject(e);
+  const a = Promise.reject(e);
   process.nextTick(function() {
     Promise.resolve().then(function() {
       process.nextTick(function() {
@@ -465,11 +462,11 @@ asyncTest('Waiting for some combination of process.nextTick + promise' +
 asyncTest('Waiting for some combination of process.nextTick + promise' +
           ' microtasks to attach a catch handler is still soon enough to ' +
           'prevent unhandledRejection: inside setImmediate', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
 
   setImmediate(function() {
-    var a = Promise.reject(e);
+    const a = Promise.reject(e);
     process.nextTick(function() {
       Promise.resolve().then(function() {
         process.nextTick(function() {
@@ -485,11 +482,11 @@ asyncTest('Waiting for some combination of process.nextTick + promise' +
 asyncTest('Waiting for some combination of process.nextTick + promise ' +
           'microtasks to attach a catch handler is still soon enough to ' +
           'prevent unhandledRejection: inside setTimeout', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
 
   setTimeout(function() {
-    var a = Promise.reject(e);
+    const a = Promise.reject(e);
     process.nextTick(function() {
       Promise.resolve().then(function() {
         process.nextTick(function() {
@@ -505,11 +502,11 @@ asyncTest('Waiting for some combination of process.nextTick + promise ' +
 asyncTest('Waiting for some combination of promise microtasks + ' +
           'process.nextTick to attach a catch handler is still soon enough' +
           ' to prevent unhandledRejection', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
 
 
-  var a = Promise.reject(e);
+  const a = Promise.reject(e);
   Promise.resolve().then(function() {
     process.nextTick(function() {
       Promise.resolve().then(function() {
@@ -526,11 +523,11 @@ asyncTest(
   ' process.nextTick to attach a catch handler is still soon enough' +
   ' to prevent unhandledRejection: inside setImmediate',
   function(done) {
-    var e = new Error();
+    const e = new Error();
     onUnhandledFail(done);
 
     setImmediate(function() {
-      var a = Promise.reject(e);
+      const a = Promise.reject(e);
       Promise.resolve().then(function() {
         process.nextTick(function() {
           Promise.resolve().then(function() {
@@ -547,11 +544,11 @@ asyncTest(
 asyncTest('Waiting for some combination of promise microtasks +' +
           ' process.nextTick to attach a catch handler is still soon enough' +
           ' to prevent unhandledRejection: inside setTimeout', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledFail(done);
 
   setTimeout(function() {
-    var a = Promise.reject(e);
+    const a = Promise.reject(e);
     Promise.resolve().then(function() {
       process.nextTick(function() {
         Promise.resolve().then(function() {
@@ -567,12 +564,12 @@ asyncTest('Waiting for some combination of promise microtasks +' +
 asyncTest('setImmediate + promise microtasks is too late to attach a catch' +
           ' handler; unhandledRejection will be triggered in that case.' +
           ' (setImmediate before promise creation/rejection)', function(done) {
-  var e = new Error();
+  const e = new Error();
   onUnhandledSucceed(done, function(reason, promise) {
     assert.strictEqual(e, reason);
     assert.strictEqual(p, promise);
   });
-  var p = Promise.reject(e);
+  const p = Promise.reject(e);
   setImmediate(function() {
     Promise.resolve().then(function() {
       p.catch(function() {});
@@ -598,7 +595,7 @@ asyncTest('setImmediate + promise microtasks is too late to attach a catch' +
       });
     });
   });
-  var p = Promise.reject();
+  const p = Promise.reject();
 });
 
 asyncTest('setImmediate + promise microtasks is too late to attach a catch' +
@@ -608,7 +605,7 @@ asyncTest('setImmediate + promise microtasks is too late to attach a catch' +
     assert.strictEqual(undefined, reason);
     assert.strictEqual(p, promise);
   });
-  var p = Promise.reject();
+  const p = Promise.reject();
   setImmediate(function() {
     Promise.resolve().then(function() {
       Promise.resolve().then(function() {
@@ -626,14 +623,14 @@ asyncTest(
   'Promise unhandledRejection handler does not interfere with domain' +
   ' error handlers being given exceptions thrown from nextTick.',
   function(done) {
-    var d = domain.create();
-    var domainReceivedError;
+    const d = domain.create();
+    let domainReceivedError;
     d.on('error', function(e) {
       domainReceivedError = e;
     });
     d.run(function() {
-      var e = new Error('error');
-      var domainError = new Error('domain error');
+      const e = new Error('error');
+      const domainError = new Error('domain error');
       onUnhandledSucceed(done, function(reason, promise) {
         assert.strictEqual(reason, e);
         assert.strictEqual(domainReceivedError, domainError);
@@ -650,9 +647,9 @@ asyncTest(
 asyncTest('nextTick is immediately scheduled when called inside an event' +
           ' handler', function(done) {
   clean();
-  var e = new Error('error');
+  const e = new Error('error');
   process.on('unhandledRejection', function(reason, promise) {
-    var order = [];
+    const order = [];
     process.nextTick(function() {
       order.push(1);
     });
@@ -669,9 +666,9 @@ asyncTest('Throwing an error inside a rejectionHandled handler goes to' +
           ' unhandledException, and does not cause .catch() to throw an' +
           'exception', function(done) {
   clean();
-  var e = new Error();
-  var e2 = new Error();
-  var tearDownException = setupException(function(err) {
+  const e = new Error();
+  const e2 = new Error();
+  const tearDownException = setupException(function(err) {
     assert.equal(e2, err);
     tearDownException();
     done();
@@ -679,7 +676,7 @@ asyncTest('Throwing an error inside a rejectionHandled handler goes to' +
   process.on('rejectionHandled', function() {
     throw e2;
   });
-  var p = Promise.reject(e);
+  const p = Promise.reject(e);
   setTimeout(function() {
     try {
       p.catch(function() {});
diff --git a/test/parallel/test-promises-warning-on-unhandled-rejection.js b/test/parallel/test-promises-warning-on-unhandled-rejection.js
index b50a5cae41..10f95162a0 100644
--- a/test/parallel/test-promises-warning-on-unhandled-rejection.js
+++ b/test/parallel/test-promises-warning-on-unhandled-rejection.js
@@ -7,7 +7,7 @@
 const common = require('../common');
 const assert = require('assert');
 
-var b = 0;
+let b = 0;
 
 process.on('warning', common.mustCall((warning) => {
   switch (b++) {
diff --git a/test/parallel/test-querystring-maxKeys-non-finite.js b/test/parallel/test-querystring-maxKeys-non-finite.js
index aab7c45f1b..8aa47f7de0 100644
--- a/test/parallel/test-querystring-maxKeys-non-finite.js
+++ b/test/parallel/test-querystring-maxKeys-non-finite.js
@@ -13,7 +13,7 @@ https://github.com/expressjs/body-parser/
 blob/ed25264fb494cf0c8bc992b8257092cd4f694d5e/test/urlencoded.js#L636-L651
 */
 function createManyParams(count) {
-  var str = '';
+  let str = '';
 
   if (count === 0) {
     return str;
@@ -21,8 +21,8 @@ function createManyParams(count) {
 
   str += '0=0';
 
-  for (var i = 1; i < count; i++) {
-    var n = i.toString(36);
+  for (let i = 1; i < count; i++) {
+    const n = i.toString(36);
     str += '&' + n + '=' + n;
   }
 
diff --git a/test/parallel/test-querystring.js b/test/parallel/test-querystring.js
index 7192ac6a1b..8cc589944b 100644
--- a/test/parallel/test-querystring.js
+++ b/test/parallel/test-querystring.js
@@ -15,7 +15,7 @@ function createWithNoPrototype(properties) {
 // folding block, commented to pass gjslint
 // {{{
 // [ wonkyQS, canonicalQS, obj ]
-var qsTestCases = [
+const qsTestCases = [
   ['__proto__=1',
    '__proto__=1',
    createWithNoPrototype([{key: '__proto__', value: '1'}])],
@@ -56,7 +56,7 @@ var qsTestCases = [
 ];
 
 // [ wonkyQS, canonicalQS, obj ]
-var qsColonTestCases = [
+const qsColonTestCases = [
   ['foo:bar', 'foo:bar', {'foo': 'bar'}],
   ['foo:bar;foo:quux', 'foo:bar;foo:quux', {'foo': ['bar', 'quux']}],
   ['foo:1&bar:2;baz:quux',
@@ -67,9 +67,9 @@ var qsColonTestCases = [
 ];
 
 // [wonkyObj, qs, canonicalObj]
-var extendedFunction = function() {};
+const extendedFunction = function() {};
 extendedFunction.prototype = {a: 'b'};
-var qsWeirdObjects = [
+const qsWeirdObjects = [
   [{regexp: /./g}, 'regexp=', {'regexp': ''}],
   [{regexp: new RegExp('.', 'g')}, 'regexp=', {'regexp': ''}],
   [{fn: function() {}}, 'fn=', {'fn': ''}],
@@ -88,9 +88,9 @@ var qsWeirdObjects = [
 // }}}
 
 const vm = require('vm');
-var foreignObject = vm.runInNewContext('({"foo": ["bar", "baz"]})');
+const foreignObject = vm.runInNewContext('({"foo": ["bar", "baz"]})');
 
-var qsNoMungeTestCases = [
+const qsNoMungeTestCases = [
   ['', {}],
   ['foo=bar&foo=baz', {'foo': ['bar', 'baz']}],
   ['foo=bar&foo=baz', foreignObject],
@@ -232,7 +232,7 @@ assert.equal(
 function testUnlimitedKeys() {
   const query = {};
 
-  for (var i = 0; i < 2000; i++) query[i] = i;
+  for (let i = 0; i < 2000; i++) query[i] = i;
 
   const url = qs.stringify(query);
 
@@ -243,7 +243,7 @@ function testUnlimitedKeys() {
 testUnlimitedKeys();
 
 
-var b = qs.unescapeBuffer('%d3%f2Ug%1f6v%24%5e%98%cb' +
+const b = qs.unescapeBuffer('%d3%f2Ug%1f6v%24%5e%98%cb' +
                           '%0d%ac%a2%2f%9d%eb%d8%a2%e6');
 // <Buffer d3 f2 55 67 1f 36 76 24 5e 98 cb 0d ac a2 2f 9d eb d8 a2 e6>
 assert.equal(0xd3, b[0]);
@@ -280,13 +280,13 @@ check(qs.parse('a=a&b=b&c=c', null, null, { decodeURIComponent: demoDecode }),
 function demoEncode(str) {
   return str[0];
 }
-var obj = { aa: 'aa', bb: 'bb', cc: 'cc' };
+const obj = { aa: 'aa', bb: 'bb', cc: 'cc' };
 assert.equal(
   qs.stringify(obj, null, null, { encodeURIComponent: demoEncode }),
   'a=a&b=b&c=c');
 
 // test overriding .unescape
-var prevUnescape = qs.unescape;
+const prevUnescape = qs.unescape;
 qs.unescape = function(str) {
   return str.replace(/o/g, '_');
 };
diff --git a/test/parallel/test-readdouble.js b/test/parallel/test-readdouble.js
index a6fc91c351..728a2b4605 100644
--- a/test/parallel/test-readdouble.js
+++ b/test/parallel/test-readdouble.js
@@ -9,7 +9,7 @@ const assert = require('assert');
  * Test (64 bit) double
  */
 function test(clazz) {
-  var buffer = new clazz(8);
+  const buffer = new clazz(8);
 
   buffer[0] = 0x55;
   buffer[1] = 0x55;
diff --git a/test/parallel/test-readfloat.js b/test/parallel/test-readfloat.js
index 533b7caa8f..437e477867 100644
--- a/test/parallel/test-readfloat.js
+++ b/test/parallel/test-readfloat.js
@@ -9,7 +9,7 @@ const assert = require('assert');
  * Test (32 bit) float
  */
 function test(clazz) {
-  var buffer = new clazz(4);
+  const buffer = new clazz(4);
 
   buffer[0] = 0;
   buffer[1] = 0;
diff --git a/test/parallel/test-readint.js b/test/parallel/test-readint.js
index c068ddcb6c..086dd4e3aa 100644
--- a/test/parallel/test-readint.js
+++ b/test/parallel/test-readint.js
@@ -9,7 +9,7 @@ const assert = require('assert');
  * Test 8 bit signed integers
  */
 function test8(clazz) {
-  var data = new clazz(4);
+  const data = new clazz(4);
 
   data[0] = 0x23;
   assert.equal(0x23, data.readInt8(0));
@@ -29,7 +29,7 @@ function test8(clazz) {
 
 
 function test16(clazz) {
-  var buffer = new clazz(6);
+  const buffer = new clazz(6);
 
   buffer[0] = 0x16;
   buffer[1] = 0x79;
@@ -62,7 +62,7 @@ function test16(clazz) {
 
 
 function test32(clazz) {
-  var buffer = new clazz(6);
+  const buffer = new clazz(6);
 
   buffer[0] = 0x43;
   buffer[1] = 0x53;
diff --git a/test/parallel/test-readline-interface.js b/test/parallel/test-readline-interface.js
index 509995bc9a..e8fc92ad1c 100644
--- a/test/parallel/test-readline-interface.js
+++ b/test/parallel/test-readline-interface.js
@@ -19,8 +19,8 @@ FakeInput.prototype.write = function() {};
 FakeInput.prototype.end = function() {};
 
 function isWarned(emitter) {
-  for (var name in emitter) {
-    var listeners = emitter[name];
+  for (const name in emitter) {
+    const listeners = emitter[name];
     if (listeners.warned) return true;
   }
   return false;
@@ -55,9 +55,9 @@ function isWarned(emitter) {
 }
 
 [ true, false ].forEach(function(terminal) {
-  var fi;
-  var rli;
-  var called;
+  let fi;
+  let rli;
+  let called;
 
   // disable history
   fi = new FakeInput();
@@ -128,8 +128,8 @@ function isWarned(emitter) {
   // sending multiple newlines at once
   fi = new FakeInput();
   rli = new readline.Interface({ input: fi, output: fi, terminal: terminal });
-  var expectedLines = ['foo', 'bar', 'baz'];
-  var callCount = 0;
+  let expectedLines = ['foo', 'bar', 'baz'];
+  let callCount = 0;
   rli.on('line', function(line) {
     assert.equal(line, expectedLines[callCount]);
     callCount++;
@@ -267,7 +267,7 @@ function isWarned(emitter) {
 
   // \t does not become part of the input when there is a completer function
   fi = new FakeInput();
-  var completer = function(line) {
+  const completer = function(line) {
     return [[], line];
   };
   rli = new readline.Interface({
@@ -282,7 +282,7 @@ function isWarned(emitter) {
     assert.strictEqual(called, false);
     called = true;
   });
-  for (var character of '\tfo\to\t') {
+  for (const character of '\tfo\to\t') {
     fi.emit('data', character);
   }
   fi.emit('data', '\n');
@@ -306,7 +306,7 @@ function isWarned(emitter) {
   });
 
   // sending a multi-byte utf8 char over multiple writes
-  var buf = Buffer.from('☮', 'utf8');
+  const buf = Buffer.from('☮', 'utf8');
   fi = new FakeInput();
   rli = new readline.Interface({ input: fi, output: fi, terminal: terminal });
   callCount = 0;
@@ -326,7 +326,7 @@ function isWarned(emitter) {
   // check that nothing fails if 'keypress' event throws.
   fi = new FakeInput();
   rli = new readline.Interface({ input: fi, output: fi, terminal: true });
-  var keys = [];
+  const keys = [];
   fi.on('keypress', function(key) {
     keys.push(key);
     if (key === 'X') {
@@ -360,7 +360,7 @@ function isWarned(emitter) {
     rli.question(expectedLines[0], function() {
       rli.close();
     });
-    var cursorPos = rli._getCursorPos();
+    let cursorPos = rli._getCursorPos();
     assert.equal(cursorPos.rows, 0);
     assert.equal(cursorPos.cols, expectedLines[0].length);
     rli.close();
@@ -421,8 +421,8 @@ function isWarned(emitter) {
   assert.deepStrictEqual(fi.listeners(terminal ? 'keypress' : 'data'), []);
 
   // check EventEmitter memory leak
-  for (var i = 0; i < 12; i++) {
-    var rl = readline.createInterface({
+  for (let i = 0; i < 12; i++) {
+    const rl = readline.createInterface({
       input: process.stdin,
       output: process.stdout
     });
diff --git a/test/parallel/test-readline-keys.js b/test/parallel/test-readline-keys.js
index eae4ba55b7..2581276556 100644
--- a/test/parallel/test-readline-keys.js
+++ b/test/parallel/test-readline-keys.js
@@ -13,11 +13,11 @@ function FakeInput() {
 inherits(FakeInput, PassThrough);
 
 
-var fi = new FakeInput();
-var fo = new FakeInput();
+const fi = new FakeInput();
+const fo = new FakeInput();
 new Interface({ input: fi, output: fo, terminal: true });
 
-var keys = [];
+let keys = [];
 fi.on('keypress', function(s, k) {
   keys.push(k);
 });
diff --git a/test/parallel/test-readline-set-raw-mode.js b/test/parallel/test-readline-set-raw-mode.js
index e9e0ff4b4d..78c42f5f5f 100644
--- a/test/parallel/test-readline-set-raw-mode.js
+++ b/test/parallel/test-readline-set-raw-mode.js
@@ -4,11 +4,11 @@ const assert = require('assert');
 const readline = require('readline');
 const Stream = require('stream');
 
-var stream = new Stream();
-var expectedRawMode = true;
-var rawModeCalled = false;
-var resumeCalled = false;
-var pauseCalled = false;
+const stream = new Stream();
+let expectedRawMode = true;
+let rawModeCalled = false;
+let resumeCalled = false;
+let pauseCalled = false;
 
 stream.setRawMode = function(mode) {
   rawModeCalled = true;
@@ -23,7 +23,7 @@ stream.pause = function() {
 
 // when the "readline" starts in "terminal" mode,
 // then setRawMode(true) should be called
-var rli = readline.createInterface({
+const rli = readline.createInterface({
   input: stream,
   output: stream,
   terminal: true
diff --git a/test/parallel/test-readline-undefined-columns.js b/test/parallel/test-readline-undefined-columns.js
index 5f8a1c02d3..14895ac637 100644
--- a/test/parallel/test-readline-undefined-columns.js
+++ b/test/parallel/test-readline-undefined-columns.js
@@ -20,7 +20,7 @@ readline.createInterface({
   }
 });
 
-var output = '';
+let output = '';
 
 oStream.on('data', function(data) {
   output += data;
diff --git a/test/parallel/test-readuint.js b/test/parallel/test-readuint.js
index 9fcaf45603..0d1f79b12e 100644
--- a/test/parallel/test-readuint.js
+++ b/test/parallel/test-readuint.js
@@ -14,7 +14,7 @@ const assert = require('assert');
  *  - Correctly interpreting values that are beyond the signed range as unsigned
  */
 function test8(clazz) {
-  var data = new clazz(4);
+  const data = new clazz(4);
 
   data[0] = 23;
   data[1] = 23;
@@ -39,7 +39,7 @@ function test8(clazz) {
  *  - Correctly interpreting values that are beyond the signed range as unsigned
  */
 function test16(clazz) {
-  var data = new clazz(4);
+  const data = new clazz(4);
 
   data[0] = 0;
   data[1] = 0x23;
@@ -67,7 +67,7 @@ function test16(clazz) {
  *  - Correctly interpreting values that are beyond the signed range as unsigned
  */
 function test32(clazz) {
-  var data = new clazz(8);
+  const data = new clazz(8);
 
   data[0] = 0x32;
   data[1] = 0x65;
diff --git a/test/parallel/test-regress-GH-1531.js b/test/parallel/test-regress-GH-1531.js
index 0ca1186d01..5189e50929 100644
--- a/test/parallel/test-regress-GH-1531.js
+++ b/test/parallel/test-regress-GH-1531.js
@@ -9,12 +9,12 @@ const https = require('https');
 
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var server = https.createServer(options, function(req, res) {
+const server = https.createServer(options, function(req, res) {
   res.writeHead(200);
   res.end('hello world\n');
 });
diff --git a/test/parallel/test-regress-GH-1899.js b/test/parallel/test-regress-GH-1899.js
index 1c98b30a8d..838391acd2 100644
--- a/test/parallel/test-regress-GH-1899.js
+++ b/test/parallel/test-regress-GH-1899.js
@@ -4,10 +4,10 @@ const path = require('path');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-var child = spawn(process.argv[0], [
+const child = spawn(process.argv[0], [
   path.join(common.fixturesDir, 'GH-1899-output.js')
 ]);
-var output = '';
+let output = '';
 
 child.stdout.on('data', function(data) {
   output += data;
diff --git a/test/parallel/test-regress-GH-2245.js b/test/parallel/test-regress-GH-2245.js
index 129b1428d3..c5b0ae4982 100644
--- a/test/parallel/test-regress-GH-2245.js
+++ b/test/parallel/test-regress-GH-2245.js
@@ -10,7 +10,7 @@ their environment when evaluated. When run in 0.10 `test()` fails with a
 
 function test() {
 
-  var code = [
+  const code = [
     'var foo = {m: 1};',
     '',
     'function bar() {',
diff --git a/test/parallel/test-regress-GH-3542.js b/test/parallel/test-regress-GH-3542.js
index 8db34e7215..cc0285f747 100644
--- a/test/parallel/test-regress-GH-3542.js
+++ b/test/parallel/test-regress-GH-3542.js
@@ -11,7 +11,7 @@ if (!common.isWindows) {
 }
 
 function test(p) {
-  var result = fs.realpathSync(p);
+  const result = fs.realpathSync(p);
   assert.strictEqual(result.toLowerCase(), path.resolve(p).toLowerCase());
 
   fs.realpath(p, common.mustCall(function(err, result) {
diff --git a/test/parallel/test-regress-GH-3739.js b/test/parallel/test-regress-GH-3739.js
index 11caddae08..b845a2b7f1 100644
--- a/test/parallel/test-regress-GH-3739.js
+++ b/test/parallel/test-regress-GH-3739.js
@@ -5,13 +5,13 @@ const assert = require('assert');
 const fs = require('fs');
 const path = require('path');
 
-var dir = path.resolve(common.tmpDir);
+let dir = path.resolve(common.tmpDir);
 
 // Make sure that the tmp directory is clean
 common.refreshTmpDir();
 
 // Make a long path.
-for (var i = 0; i < 50; i++) {
+for (let i = 0; i < 50; i++) {
   dir = dir + '/1234567890';
   try {
     fs.mkdirSync(dir, '0777');
diff --git a/test/parallel/test-regress-GH-4948.js b/test/parallel/test-regress-GH-4948.js
index 69f4e47324..196c973cd0 100644
--- a/test/parallel/test-regress-GH-4948.js
+++ b/test/parallel/test-regress-GH-4948.js
@@ -4,8 +4,8 @@
 require('../common');
 const http = require('http');
 
-var reqCount = 0;
-var server = http.createServer(function(serverReq, serverRes) {
+let reqCount = 0;
+const server = http.createServer(function(serverReq, serverRes) {
   if (reqCount) {
     serverRes.end();
     server.close();
@@ -16,8 +16,8 @@ var server = http.createServer(function(serverReq, serverRes) {
 
   // normally the use case would be to call an external site
   // does not require connecting locally or to itself to fail
-  var r = http.request({hostname: 'localhost',
-                        port: this.address().port}, function(res) {
+  const r = http.request({hostname: 'localhost',
+                          port: this.address().port}, function(res) {
     // required, just needs to be in the client response somewhere
     serverRes.end();
 
@@ -33,7 +33,7 @@ var server = http.createServer(function(serverReq, serverRes) {
   // simulate a client request that closes early
   const net = require('net');
 
-  var sock = new net.Socket();
+  const sock = new net.Socket();
   sock.connect(this.address().port, 'localhost');
 
   sock.on('connect', function() {
diff --git a/test/parallel/test-regress-GH-5927.js b/test/parallel/test-regress-GH-5927.js
index 779a2c2c64..7f55356a15 100644
--- a/test/parallel/test-regress-GH-5927.js
+++ b/test/parallel/test-regress-GH-5927.js
@@ -3,18 +3,18 @@ require('../common');
 const assert = require('assert');
 const readline = require('readline');
 
-var rl = readline.createInterface(process.stdin, process.stdout);
+const rl = readline.createInterface(process.stdin, process.stdout);
 rl.resume();
 
-var hasPaused = false;
+let hasPaused = false;
 
-var origPause = rl.pause;
+const origPause = rl.pause;
 rl.pause = function() {
   hasPaused = true;
   origPause.apply(this, arguments);
 };
 
-var origSetRawMode = rl._setRawMode;
+const origSetRawMode = rl._setRawMode;
 rl._setRawMode = function(mode) {
   assert.ok(hasPaused);
   origSetRawMode.apply(this, arguments);
diff --git a/test/parallel/test-regress-GH-746.js b/test/parallel/test-regress-GH-746.js
index a5e0d9d447..9a6e0e8980 100644
--- a/test/parallel/test-regress-GH-746.js
+++ b/test/parallel/test-regress-GH-746.js
@@ -7,7 +7,7 @@ const net = require('net');
 
 process.stdin.destroy();
 
-var server = net.createServer(common.mustCall(function(socket) {
+const server = net.createServer(common.mustCall(function(socket) {
   console.log('accepted');
   socket.end();
   server.close();
diff --git a/test/parallel/test-regress-GH-7511.js b/test/parallel/test-regress-GH-7511.js
index 97e2fdb811..a7ce8311d7 100644
--- a/test/parallel/test-regress-GH-7511.js
+++ b/test/parallel/test-regress-GH-7511.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const vm = require('vm');
 
 assert.doesNotThrow(function() {
-  var context = vm.createContext({ process: process });
-  var result = vm.runInContext('process.env["PATH"]', context);
+  const context = vm.createContext({ process: process });
+  const result = vm.runInContext('process.env["PATH"]', context);
   assert.notStrictEqual(undefined, result);
 });
diff --git a/test/parallel/test-regress-GH-819.js b/test/parallel/test-regress-GH-819.js
index bc2201b614..719d439825 100644
--- a/test/parallel/test-regress-GH-819.js
+++ b/test/parallel/test-regress-GH-819.js
@@ -3,5 +3,5 @@ require('../common');
 const net = require('net');
 
 // Connect to something that we need to DNS resolve
-var c = net.createConnection(80, 'google.com');
+const c = net.createConnection(80, 'google.com');
 c.destroy();
diff --git a/test/parallel/test-regress-GH-node-9326.js b/test/parallel/test-regress-GH-node-9326.js
index 3dbbe2958b..78565e3f59 100644
--- a/test/parallel/test-regress-GH-node-9326.js
+++ b/test/parallel/test-regress-GH-node-9326.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const child_process = require('child_process');
 
 // NOTE: Was crashing on FreeBSD
-var cp = child_process.spawn(process.execPath, [
+const cp = child_process.spawn(process.execPath, [
   '-e',
   'process.kill(process.pid, "SIGINT")'
 ]);
diff --git a/test/parallel/test-repl-.save.load.js b/test/parallel/test-repl-.save.load.js
index 98417e0d06..00d5f6791d 100644
--- a/test/parallel/test-repl-.save.load.js
+++ b/test/parallel/test-repl-.save.load.js
@@ -8,17 +8,17 @@ common.refreshTmpDir();
 
 const repl = require('repl');
 
-var works = [['inner.one'], 'inner.o'];
+const works = [['inner.one'], 'inner.o'];
 
 const putIn = new common.ArrayStream();
-var testMe = repl.start('', putIn);
+const testMe = repl.start('', putIn);
 
 
-var testFile = [
+const testFile = [
   'var top = function() {',
   'var inner = {one:1};'
 ];
-var saveFileName = join(common.tmpDir, 'test.save.js');
+const saveFileName = join(common.tmpDir, 'test.save.js');
 
 // input some data
 putIn.run(testFile);
@@ -69,7 +69,7 @@ testMe.complete('inner.o', function(error, data) {
 // clear the REPL
 putIn.run(['.clear']);
 
-var loadFile = join(common.tmpDir, 'file.does.not.exist');
+let loadFile = join(common.tmpDir, 'file.does.not.exist');
 
 // should not break
 putIn.write = function(data) {
diff --git a/test/parallel/test-repl-autolibs.js b/test/parallel/test-repl-autolibs.js
index 3cc2ba4aa5..e0554831cd 100644
--- a/test/parallel/test-repl-autolibs.js
+++ b/test/parallel/test-repl-autolibs.js
@@ -13,7 +13,7 @@ repl.start('', putIn, null, true);
 test1();
 
 function test1() {
-  var gotWrite = false;
+  let gotWrite = false;
   putIn.write = function(data) {
     gotWrite = true;
     if (data.length) {
@@ -31,7 +31,7 @@ function test1() {
 }
 
 function test2() {
-  var gotWrite = false;
+  let gotWrite = false;
   putIn.write = function(data) {
     gotWrite = true;
     if (data.length) {
@@ -41,7 +41,7 @@ function test2() {
       assert.equal(val, global.url);
     }
   };
-  var val = {};
+  const val = {};
   global.url = val;
   assert(!gotWrite);
   putIn.run(['url']);
diff --git a/test/parallel/test-repl-console.js b/test/parallel/test-repl-console.js
index 98cb958cac..f37dde0080 100644
--- a/test/parallel/test-repl-console.js
+++ b/test/parallel/test-repl-console.js
@@ -6,7 +6,7 @@ const repl = require('repl');
 // Create a dummy stream that does nothing
 const stream = new common.ArrayStream();
 
-var r = repl.start({
+const r = repl.start({
   input: stream,
   output: stream,
   useGlobal: false
diff --git a/test/parallel/test-repl-definecommand.js b/test/parallel/test-repl-definecommand.js
index 6c0c75bca8..07732d534e 100644
--- a/test/parallel/test-repl-definecommand.js
+++ b/test/parallel/test-repl-definecommand.js
@@ -6,7 +6,7 @@ const stream = require('stream');
 const assert = require('assert');
 const repl = require('repl');
 
-var output = '';
+let output = '';
 const inputStream = new stream.PassThrough();
 const outputStream = new stream.PassThrough();
 outputStream.on('data', function(d) {
diff --git a/test/parallel/test-repl-end-emits-exit.js b/test/parallel/test-repl-end-emits-exit.js
index 7f2c4b9eb2..661f9f0ad0 100644
--- a/test/parallel/test-repl-end-emits-exit.js
+++ b/test/parallel/test-repl-end-emits-exit.js
@@ -9,7 +9,7 @@ let regularExit = 0;
 const stream = new common.ArrayStream();
 
 function testTerminalMode() {
-  var r1 = repl.start({
+  const r1 = repl.start({
     input: stream,
     output: stream,
     terminal: true
@@ -28,7 +28,7 @@ function testTerminalMode() {
 }
 
 function testRegularMode() {
-  var r2 = repl.start({
+  const r2 = repl.start({
     input: stream,
     output: stream,
     terminal: false
diff --git a/test/parallel/test-repl-harmony.js b/test/parallel/test-repl-harmony.js
index b3e25e62ab..743bbe4fab 100644
--- a/test/parallel/test-repl-harmony.js
+++ b/test/parallel/test-repl-harmony.js
@@ -3,11 +3,11 @@ require('../common');
 const assert = require('assert');
 
 const spawn = require('child_process').spawn;
-var args = ['-i'];
-var child = spawn(process.execPath, args);
+const args = ['-i'];
+const child = spawn(process.execPath, args);
 
-var input = '(function(){"use strict"; const y=1;y=2})()\n';
-var expectOut = /^> TypeError: Assignment to constant variable.\n/;
+const input = '(function(){"use strict"; const y=1;y=2})()\n';
+const expectOut = /^> TypeError: Assignment to constant variable.\n/;
 
 child.stderr.setEncoding('utf8');
 child.stderr.on('data', function(c) {
@@ -15,7 +15,7 @@ child.stderr.on('data', function(c) {
 });
 
 child.stdout.setEncoding('utf8');
-var out = '';
+let out = '';
 child.stdout.on('data', function(c) {
   out += c;
 });
diff --git a/test/parallel/test-repl-history-perm.js b/test/parallel/test-repl-history-perm.js
index 16732a4429..25d2aa63ff 100644
--- a/test/parallel/test-repl-history-perm.js
+++ b/test/parallel/test-repl-history-perm.js
@@ -18,7 +18,7 @@ const Duplex = require('stream').Duplex;
 // Invoking the REPL should create a repl history file at the specified path
 // and mode 600.
 
-var stream = new Duplex();
+const stream = new Duplex();
 stream.pause = stream.resume = function() {};
 // ends immediately
 stream._read = function() {
diff --git a/test/parallel/test-repl-null.js b/test/parallel/test-repl-null.js
index 337e194447..66d09b28f2 100644
--- a/test/parallel/test-repl-null.js
+++ b/test/parallel/test-repl-null.js
@@ -3,7 +3,7 @@ require('../common');
 const repl = require('repl');
 const assert = require('assert');
 
-var replserver = new repl.REPLServer();
+const replserver = new repl.REPLServer();
 
 replserver._inTemplateLiteral = true;
 
diff --git a/test/parallel/test-repl-options.js b/test/parallel/test-repl-options.js
index 70244802dd..79994c66d1 100644
--- a/test/parallel/test-repl-options.js
+++ b/test/parallel/test-repl-options.js
@@ -9,7 +9,7 @@ common.globalCheck = false;
 const stream = new common.ArrayStream();
 
 // 1, mostly defaults
-var r1 = repl.start({
+const r1 = repl.start({
   input: stream,
   output: stream,
   terminal: true
@@ -37,7 +37,7 @@ assert.equal(r1.useColors, r1.rli.terminal);
 // 2
 function writer() {}
 function evaler() {}
-var r2 = repl.start({
+const r2 = repl.start({
   input: stream,
   output: stream,
   terminal: false,
@@ -67,7 +67,7 @@ assert.equal(r2.rli.output, r2.outputStream);
 assert.equal(r2.rli.terminal, false);
 
 // testing out "magic" replMode
-var r3 = repl.start({
+const r3 = repl.start({
   input: stream,
   output: stream,
   writer: writer,
diff --git a/test/parallel/test-repl-persistent-history.js b/test/parallel/test-repl-persistent-history.js
index 25fdef81a8..2b0ceac530 100644
--- a/test/parallel/test-repl-persistent-history.js
+++ b/test/parallel/test-repl-persistent-history.js
@@ -178,7 +178,7 @@ const tests = [
 const numtests = tests.length;
 
 
-var testsNotRan = tests.length;
+let testsNotRan = tests.length;
 
 process.on('beforeExit', () =>
   assert.strictEqual(testsNotRan, 0)
diff --git a/test/parallel/test-repl-require.js b/test/parallel/test-repl-require.js
index 9dc3b51de7..25aaac47a4 100644
--- a/test/parallel/test-repl-require.js
+++ b/test/parallel/test-repl-require.js
@@ -18,7 +18,7 @@ const host = common.localhostIPv4;
 const port = 0;
 const options = { host, port };
 
-var answer = '';
+let answer = '';
 server.listen(options, function() {
   options.port = this.address().port;
   const conn = net.connect(options);
diff --git a/test/parallel/test-repl-setprompt.js b/test/parallel/test-repl-setprompt.js
index 2708a3e6af..8dcc4fe944 100644
--- a/test/parallel/test-repl-setprompt.js
+++ b/test/parallel/test-repl-setprompt.js
@@ -4,18 +4,18 @@ const assert = require('assert');
 const spawn = require('child_process').spawn;
 const os = require('os');
 
-var args = [
+const args = [
   '-e',
   'var e = new (require("repl")).REPLServer("foo.. "); e.context.e = e;',
 ];
 
-var p = 'bar.. ';
+const p = 'bar.. ';
 
-var child = spawn(process.execPath, args);
+const child = spawn(process.execPath, args);
 
 child.stdout.setEncoding('utf8');
 
-var data = '';
+let data = '';
 child.stdout.on('data', function(d) { data += d; });
 
 child.stdin.end(`e.setPrompt("${p}");${os.EOL}`);
@@ -23,6 +23,6 @@ child.stdin.end(`e.setPrompt("${p}");${os.EOL}`);
 child.on('close', function(code, signal) {
   assert.strictEqual(code, 0);
   assert.ok(!signal);
-  var lines = data.split(/\n/);
+  const lines = data.split(/\n/);
   assert.strictEqual(lines.pop(), p);
 });
diff --git a/test/parallel/test-repl-syntax-error-handling.js b/test/parallel/test-repl-syntax-error-handling.js
index 2728ffdfe9..6a65d53a02 100644
--- a/test/parallel/test-repl-syntax-error-handling.js
+++ b/test/parallel/test-repl-syntax-error-handling.js
@@ -13,7 +13,7 @@ switch (process.argv[2]) {
 
 function parent() {
   const spawn = require('child_process').spawn;
-  var child = spawn(process.execPath, [__filename, 'child']);
+  const child = spawn(process.execPath, [__filename, 'child']);
 
   child.stderr.setEncoding('utf8');
   child.stderr.on('data', function(c) {
@@ -22,7 +22,7 @@ function parent() {
   });
 
   child.stdout.setEncoding('utf8');
-  var out = '';
+  let out = '';
   child.stdout.on('data', function(c) {
     out += c;
   });
@@ -39,10 +39,11 @@ function parent() {
 
 function child() {
   const vm = require('vm');
+  let caught;
   try {
     vm.runInThisContext('haf!@##&$!@$*!@', { displayErrors: false });
   } catch (er) {
-    var caught = true;
+    caught = true;
   }
   assert(caught);
   vm.runInThisContext('console.log(10)', { displayErrors: false });
diff --git a/test/parallel/test-repl-tab-complete.js b/test/parallel/test-repl-tab-complete.js
index 6df0ad7fa9..835634b0a4 100644
--- a/test/parallel/test-repl-tab-complete.js
+++ b/test/parallel/test-repl-tab-complete.js
@@ -17,9 +17,9 @@ function getNoResultsFunction() {
   });
 }
 
-var works = [['inner.one'], 'inner.o'];
+const works = [['inner.one'], 'inner.o'];
 const putIn = new common.ArrayStream();
-var testMe = repl.start('', putIn);
+const testMe = repl.start('', putIn);
 
 // Some errors are passed to the domain, but do not callback
 testMe._domain.on('error', function(err) {
@@ -165,7 +165,7 @@ testMe.complete('str.len', common.mustCall(function(error, data) {
 putIn.run(['.clear']);
 
 // tab completion should not break on spaces
-var spaceTimeout = setTimeout(function() {
+const spaceTimeout = setTimeout(function() {
   throw new Error('timeout');
 }, 1000);
 
diff --git a/test/parallel/test-repl-tab.js b/test/parallel/test-repl-tab.js
index b41f43c667..7baa7d0aac 100644
--- a/test/parallel/test-repl-tab.js
+++ b/test/parallel/test-repl-tab.js
@@ -5,8 +5,9 @@ const repl = require('repl');
 const zlib = require('zlib');
 
 // just use builtin stream inherited from Duplex
-var putIn = zlib.createGzip();
-var testMe = repl.start('', putIn, function(cmd, context, filename, callback) {
+const putIn = zlib.createGzip();
+const testMe = repl.start('', putIn, function(cmd, context, filename,
+      callback) {
   callback(null, cmd);
 });
 
diff --git a/test/parallel/test-repl-unexpected-token-recoverable.js b/test/parallel/test-repl-unexpected-token-recoverable.js
index 7ace511d09..0f783d3dca 100644
--- a/test/parallel/test-repl-unexpected-token-recoverable.js
+++ b/test/parallel/test-repl-unexpected-token-recoverable.js
@@ -7,12 +7,12 @@ const assert = require('assert');
 
 const spawn = require('child_process').spawn;
 // use -i to force node into interactive mode, despite stdout not being a TTY
-var args = [ '-i' ];
-var child = spawn(process.execPath, args);
+const args = [ '-i' ];
+const child = spawn(process.execPath, args);
 
-var input = 'var foo = "bar\\\nbaz"';
+const input = 'var foo = "bar\\\nbaz"';
 // Match '...' as well since it marks a multi-line statement
-var expectOut = /^> ... undefined\n/;
+const expectOut = /^> ... undefined\n/;
 
 child.stderr.setEncoding('utf8');
 child.stderr.on('data', function(c) {
@@ -20,7 +20,7 @@ child.stderr.on('data', function(c) {
 });
 
 child.stdout.setEncoding('utf8');
-var out = '';
+let out = '';
 child.stdout.on('data', function(c) {
   out += c;
 });
diff --git a/test/parallel/test-sigint-infinite-loop.js b/test/parallel/test-sigint-infinite-loop.js
index aafcd9e41d..30eb98ecb8 100644
--- a/test/parallel/test-sigint-infinite-loop.js
+++ b/test/parallel/test-sigint-infinite-loop.js
@@ -8,9 +8,9 @@ const spawn = require('child_process').spawn;
 
 console.log('start');
 
-var c = spawn(process.execPath, ['-e', 'while(true) { console.log("hi"); }']);
+const c = spawn(process.execPath, ['-e', 'while(true) { console.log("hi"); }']);
 
-var sentKill = false;
+let sentKill = false;
 
 c.stdout.on('data', function(s) {
   // Prevent race condition:
diff --git a/test/parallel/test-signal-handler.js b/test/parallel/test-signal-handler.js
index 079c72a9e5..b796ebb11d 100644
--- a/test/parallel/test-signal-handler.js
+++ b/test/parallel/test-signal-handler.js
@@ -18,7 +18,7 @@ process.on('SIGUSR1', common.mustCall(function() {
   }, 5);
 }));
 
-var i = 0;
+let i = 0;
 setInterval(function() {
   console.log('running process...' + ++i);
 
diff --git a/test/parallel/test-signal-safety.js b/test/parallel/test-signal-safety.js
index eb29554b8b..34722b712f 100644
--- a/test/parallel/test-signal-safety.js
+++ b/test/parallel/test-signal-safety.js
@@ -1,12 +1,12 @@
 'use strict';
 require('../common');
 const assert = require('assert');
-var Signal = process.binding('signal_wrap').Signal;
+const Signal = process.binding('signal_wrap').Signal;
 
 // Test Signal `this` safety
 // https://github.com/joyent/node/issues/6690
 assert.throws(function() {
-  var s = new Signal();
-  var nots = { start: s.start };
+  const s = new Signal();
+  const nots = { start: s.start };
   nots.start(9);
 }, TypeError);
diff --git a/test/parallel/test-socket-write-after-fin-error.js b/test/parallel/test-socket-write-after-fin-error.js
index 0d280115a4..f21b52d519 100644
--- a/test/parallel/test-socket-write-after-fin-error.js
+++ b/test/parallel/test-socket-write-after-fin-error.js
@@ -8,13 +8,13 @@ const assert = require('assert');
 // message is too vague, and doesn't actually tell you what happens.
 
 const net = require('net');
-var serverData = '';
-var gotServerEnd = false;
-var clientData = '';
-var gotClientEnd = false;
-var gotServerError = false;
+let serverData = '';
+let gotServerEnd = false;
+let clientData = '';
+let gotClientEnd = false;
+let gotServerError = false;
 
-var server = net.createServer(function(sock) {
+const server = net.createServer(function(sock) {
   sock.setEncoding('utf8');
   sock.on('error', function(er) {
     console.error(er.code + ': ' + er.message);
@@ -32,7 +32,7 @@ var server = net.createServer(function(sock) {
   server.close();
 });
 server.listen(0, function() {
-  var sock = net.connect(this.address().port);
+  const sock = net.connect(this.address().port);
   sock.setEncoding('utf8');
   sock.on('data', function(c) {
     clientData += c;
diff --git a/test/parallel/test-socket-write-after-fin.js b/test/parallel/test-socket-write-after-fin.js
index 17847f9733..c8ff568717 100644
--- a/test/parallel/test-socket-write-after-fin.js
+++ b/test/parallel/test-socket-write-after-fin.js
@@ -4,10 +4,10 @@ const assert = require('assert');
 const net = require('net');
 const expected = 'hello1hello2hello3\nTHUNDERMUSCLE!';
 
-var server = net.createServer({
+const server = net.createServer({
   allowHalfOpen: true
 }, common.mustCall(function(sock) {
-  var serverData = '';
+  let serverData = '';
 
   sock.setEncoding('utf8');
   sock.on('data', function(c) {
@@ -20,8 +20,8 @@ var server = net.createServer({
   }));
 }));
 server.listen(0, common.mustCall(function() {
-  var sock = net.connect(this.address().port);
-  var clientData = '';
+  const sock = net.connect(this.address().port);
+  let clientData = '';
 
   sock.setEncoding('utf8');
   sock.on('data', function(c) {
diff --git a/test/parallel/test-stdin-pipe-resume.js b/test/parallel/test-stdin-pipe-resume.js
index 663649134f..06cce3edfe 100644
--- a/test/parallel/test-stdin-pipe-resume.js
+++ b/test/parallel/test-stdin-pipe-resume.js
@@ -7,13 +7,13 @@ if (process.argv[2] === 'child') {
   process.stdin.pipe(process.stdout);
 } else {
   const spawn = require('child_process').spawn;
-  var buffers = [];
-  var child = spawn(process.execPath, [__filename, 'child']);
+  const buffers = [];
+  const child = spawn(process.execPath, [__filename, 'child']);
   child.stdout.on('data', function(c) {
     buffers.push(c);
   });
   child.stdout.on('close', function() {
-    var b = Buffer.concat(buffers).toString();
+    const b = Buffer.concat(buffers).toString();
     assert.equal(b, 'Hello, world\n');
     console.log('ok');
   });
diff --git a/test/parallel/test-stdin-script-child.js b/test/parallel/test-stdin-script-child.js
index 091c5cb29a..a8a161686e 100644
--- a/test/parallel/test-stdin-script-child.js
+++ b/test/parallel/test-stdin-script-child.js
@@ -9,7 +9,7 @@ const child = spawn(process.execPath, [], {
   })
 });
 const wanted = child.pid + '\n';
-var found = '';
+let found = '';
 
 child.stdout.setEncoding('utf8');
 child.stdout.on('data', function(c) {
diff --git a/test/parallel/test-stdout-cannot-be-closed-child-process-pipe.js b/test/parallel/test-stdout-cannot-be-closed-child-process-pipe.js
index bfc7a1ddf4..aa1939ebe2 100644
--- a/test/parallel/test-stdout-cannot-be-closed-child-process-pipe.js
+++ b/test/parallel/test-stdout-cannot-be-closed-child-process-pipe.js
@@ -9,9 +9,9 @@ else
 
 function parent() {
   const spawn = require('child_process').spawn;
-  var child = spawn(process.execPath, [__filename, 'child']);
-  var out = '';
-  var err = '';
+  const child = spawn(process.execPath, [__filename, 'child']);
+  let out = '';
+  let err = '';
 
   child.stdout.setEncoding('utf8');
   child.stderr.setEncoding('utf8');
diff --git a/test/parallel/test-stdout-stderr-reading.js b/test/parallel/test-stdout-stderr-reading.js
index b61edf194f..6aa9613013 100644
--- a/test/parallel/test-stdout-stderr-reading.js
+++ b/test/parallel/test-stdout-stderr-reading.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 
 // verify that stdout is never read from.
 const net = require('net');
-var read = net.Socket.prototype.read;
+const read = net.Socket.prototype.read;
 
 net.Socket.prototype.read = function() {
   if (this.fd === 1)
@@ -21,10 +21,10 @@ else
 
 function parent() {
   const spawn = require('child_process').spawn;
-  var node = process.execPath;
+  const node = process.execPath;
 
-  var c1 = spawn(node, [__filename, 'child']);
-  var c1out = '';
+  const c1 = spawn(node, [__filename, 'child']);
+  let c1out = '';
   c1.stdout.setEncoding('utf8');
   c1.stdout.on('data', function(chunk) {
     c1out += chunk;
@@ -40,8 +40,8 @@ function parent() {
     console.log('ok');
   }));
 
-  var c2 = spawn(node, ['-e', 'console.log("ok")']);
-  var c2out = '';
+  const c2 = spawn(node, ['-e', 'console.log("ok")']);
+  let c2out = '';
   c2.stdout.setEncoding('utf8');
   c2.stdout.on('data', function(chunk) {
     c2out += chunk;
diff --git a/test/parallel/test-stdout-to-file.js b/test/parallel/test-stdout-to-file.js
index 8009316021..914fe5673e 100644
--- a/test/parallel/test-stdout-to-file.js
+++ b/test/parallel/test-stdout-to-file.js
@@ -13,13 +13,13 @@ const tmpFile = path.join(common.tmpDir, 'stdout.txt');
 common.refreshTmpDir();
 
 function test(size, useBuffer, cb) {
-  var cmd = '"' + process.argv[0] + '"' +
-            ' ' +
-            '"' + (useBuffer ? scriptBuffer : scriptString) + '"' +
-            ' ' +
-            size +
-            ' > ' +
-            '"' + tmpFile + '"';
+  const cmd = '"' + process.argv[0] + '"' +
+              ' ' +
+              '"' + (useBuffer ? scriptBuffer : scriptString) + '"' +
+              ' ' +
+              size +
+              ' > ' +
+              '"' + tmpFile + '"';
 
   try {
     fs.unlinkSync(tmpFile);
@@ -31,7 +31,7 @@ function test(size, useBuffer, cb) {
     assert.ifError(err);
     console.log('done!');
 
-    var stat = fs.statSync(tmpFile);
+    const stat = fs.statSync(tmpFile);
 
     console.log(`${tmpFile} has ${stat.size} bytes`);
 
diff --git a/test/parallel/test-stream-base-no-abort.js b/test/parallel/test-stream-base-no-abort.js
index 329948836a..6b1cc67ee7 100644
--- a/test/parallel/test-stream-base-no-abort.js
+++ b/test/parallel/test-stream-base-no-abort.js
@@ -14,7 +14,7 @@ const fs = require('fs');
 const net = require('net');
 const tls = require('tls');
 const providers = Object.keys(async_wrap.Providers);
-var flags = 0;
+let flags = 0;
 
 // Make sure all asserts have run at least once.
 process.on('exit', () => assert.equal(flags, 0b111));
diff --git a/test/parallel/test-stream-big-push.js b/test/parallel/test-stream-big-push.js
index 27feef35ef..9083820d42 100644
--- a/test/parallel/test-stream-big-push.js
+++ b/test/parallel/test-stream-big-push.js
@@ -18,7 +18,7 @@ function _read() {
     }, 1);
     reads++;
   } else if (reads === 1) {
-    var ret = r.push(str);
+    const ret = r.push(str);
     assert.strictEqual(ret, false);
     reads++;
   } else {
@@ -32,10 +32,10 @@ r.on('end', common.mustCall(function() {}));
 
 // push some data in to start.
 // we've never gotten any read event at this point.
-var ret = r.push(str);
+const ret = r.push(str);
 // should be false.  > hwm
 assert(!ret);
-var chunk = r.read();
+let chunk = r.read();
 assert.strictEqual(chunk, str);
 chunk = r.read();
 assert.strictEqual(chunk, null);
diff --git a/test/parallel/test-stream-pipe-event.js b/test/parallel/test-stream-pipe-event.js
index 77cd5d26f9..1b5447eaa2 100644
--- a/test/parallel/test-stream-pipe-event.js
+++ b/test/parallel/test-stream-pipe-event.js
@@ -16,14 +16,14 @@ function Readable() {
 }
 util.inherits(Readable, stream.Stream);
 
-var passed = false;
+let passed = false;
 
-var w = new Writable();
+const w = new Writable();
 w.on('pipe', function(src) {
   passed = true;
 });
 
-var r = new Readable();
+const r = new Readable();
 r.pipe(w);
 
 assert.ok(passed);
diff --git a/test/parallel/test-stream-push-order.js b/test/parallel/test-stream-push-order.js
index 4be2ca4a9f..5fc1f5e89c 100644
--- a/test/parallel/test-stream-push-order.js
+++ b/test/parallel/test-stream-push-order.js
@@ -3,19 +3,19 @@ require('../common');
 const Readable = require('stream').Readable;
 const assert = require('assert');
 
-var s = new Readable({
+const s = new Readable({
   highWaterMark: 20,
   encoding: 'ascii'
 });
 
-var list = ['1', '2', '3', '4', '5', '6'];
+const list = ['1', '2', '3', '4', '5', '6'];
 
 s._read = function(n) {
-  var one = list.shift();
+  const one = list.shift();
   if (!one) {
     s.push(null);
   } else {
-    var two = list.shift();
+    const two = list.shift();
     s.push(one);
     s.push(two);
   }
diff --git a/test/parallel/test-stream-push-strings.js b/test/parallel/test-stream-push-strings.js
index 85f5a6bf27..7bce493bbd 100644
--- a/test/parallel/test-stream-push-strings.js
+++ b/test/parallel/test-stream-push-strings.js
@@ -30,15 +30,15 @@ MyStream.prototype._read = function(n) {
   }
 };
 
-var ms = new MyStream();
-var results = [];
+const ms = new MyStream();
+const results = [];
 ms.on('readable', function() {
-  var chunk;
+  let chunk;
   while (null !== (chunk = ms.read()))
     results.push(chunk + '');
 });
 
-var expect = [ 'first chunksecond to last chunk', 'last chunk' ];
+const expect = [ 'first chunksecond to last chunk', 'last chunk' ];
 process.on('exit', function() {
   assert.equal(ms._chunks, -1);
   assert.deepStrictEqual(results, expect);
diff --git a/test/parallel/test-stream-readable-constructor-set-methods.js b/test/parallel/test-stream-readable-constructor-set-methods.js
index 86fab5ae54..a5d7e48bfb 100644
--- a/test/parallel/test-stream-readable-constructor-set-methods.js
+++ b/test/parallel/test-stream-readable-constructor-set-methods.js
@@ -4,13 +4,13 @@ const assert = require('assert');
 
 const Readable = require('stream').Readable;
 
-var _readCalled = false;
+let _readCalled = false;
 function _read(n) {
   _readCalled = true;
   this.push(null);
 }
 
-var r = new Readable({ read: _read });
+const r = new Readable({ read: _read });
 r.resume();
 
 process.on('exit', function() {
diff --git a/test/parallel/test-stream-readable-flow-recursion.js b/test/parallel/test-stream-readable-flow-recursion.js
index 48615ed0a9..b1e421a71f 100644
--- a/test/parallel/test-stream-readable-flow-recursion.js
+++ b/test/parallel/test-stream-readable-flow-recursion.js
@@ -12,9 +12,9 @@ const Readable = require('stream').Readable;
 // throw an error if we trigger a nextTick warning.
 process.throwDeprecation = true;
 
-var stream = new Readable({ highWaterMark: 2 });
-var reads = 0;
-var total = 5000;
+const stream = new Readable({ highWaterMark: 2 });
+let reads = 0;
+let total = 5000;
 stream._read = function(size) {
   reads++;
   size = Math.min(size, total);
@@ -25,11 +25,11 @@ stream._read = function(size) {
     stream.push(Buffer.allocUnsafe(size));
 };
 
-var depth = 0;
+let depth = 0;
 
 function flow(stream, size, callback) {
   depth += 1;
-  var chunk = stream.read(size);
+  const chunk = stream.read(size);
 
   if (!chunk)
     stream.once('readable', flow.bind(null, stream, size, callback));
diff --git a/test/parallel/test-stream-transform-constructor-set-methods.js b/test/parallel/test-stream-transform-constructor-set-methods.js
index c57a633c55..7be673c517 100644
--- a/test/parallel/test-stream-transform-constructor-set-methods.js
+++ b/test/parallel/test-stream-transform-constructor-set-methods.js
@@ -4,19 +4,19 @@ const assert = require('assert');
 
 const Transform = require('stream').Transform;
 
-var _transformCalled = false;
+let _transformCalled = false;
 function _transform(d, e, n) {
   _transformCalled = true;
   n();
 }
 
-var _flushCalled = false;
+let _flushCalled = false;
 function _flush(n) {
   _flushCalled = true;
   n();
 }
 
-var t = new Transform({
+const t = new Transform({
   transform: _transform,
   flush: _flush
 });
diff --git a/test/parallel/test-stream-transform-flush-data.js b/test/parallel/test-stream-transform-flush-data.js
index dede64ba10..6d6aedd3a7 100644
--- a/test/parallel/test-stream-transform-flush-data.js
+++ b/test/parallel/test-stream-transform-flush-data.js
@@ -16,7 +16,7 @@ function _flush(n) {
   n(null, expected);
 }
 
-var t = new Transform({
+const t = new Transform({
   transform: _transform,
   flush: _flush
 });
diff --git a/test/parallel/test-stream-transform-split-objectmode.js b/test/parallel/test-stream-transform-split-objectmode.js
index 10fae6573c..52185e1871 100644
--- a/test/parallel/test-stream-transform-split-objectmode.js
+++ b/test/parallel/test-stream-transform-split-objectmode.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 
 const Transform = require('stream').Transform;
 
-var parser = new Transform({ readableObjectMode: true });
+const parser = new Transform({ readableObjectMode: true });
 
 assert(parser._readableState.objectMode);
 assert(!parser._writableState.objectMode);
@@ -15,7 +15,7 @@ parser._transform = function(chunk, enc, callback) {
   callback(null, { val: chunk[0] });
 };
 
-var parsed;
+let parsed;
 
 parser.on('data', function(obj) {
   parsed = obj;
@@ -28,7 +28,7 @@ process.on('exit', function() {
 });
 
 
-var serializer = new Transform({ writableObjectMode: true });
+const serializer = new Transform({ writableObjectMode: true });
 
 assert(!serializer._readableState.objectMode);
 assert(serializer._writableState.objectMode);
@@ -39,7 +39,7 @@ serializer._transform = function(obj, _, callback) {
   callback(null, Buffer.from([obj.val]));
 };
 
-var serialized;
+let serialized;
 
 serializer.on('data', function(chunk) {
   serialized = chunk;
diff --git a/test/parallel/test-stream-unshift-empty-chunk.js b/test/parallel/test-stream-unshift-empty-chunk.js
index 196a36e7eb..d555fd7cae 100644
--- a/test/parallel/test-stream-unshift-empty-chunk.js
+++ b/test/parallel/test-stream-unshift-empty-chunk.js
@@ -6,9 +6,9 @@ const assert = require('assert');
 // stream.unshift('') does not set state.reading=false.
 const Readable = require('stream').Readable;
 
-var r = new Readable();
-var nChunks = 10;
-var chunk = Buffer.alloc(10, 'x');
+const r = new Readable();
+let nChunks = 10;
+const chunk = Buffer.alloc(10, 'x');
 
 r._read = function(n) {
   setImmediate(function() {
@@ -16,10 +16,10 @@ r._read = function(n) {
   });
 };
 
-var readAll = false;
-var seen = [];
+let readAll = false;
+const seen = [];
 r.on('readable', function() {
-  var chunk;
+  let chunk;
   while (chunk = r.read()) {
     seen.push(chunk.toString());
     // simulate only reading a certain amount of the data,
@@ -27,13 +27,13 @@ r.on('readable', function() {
     // stream, like a parser might do.  We just fill it with
     // 'y' so that it's easy to see which bits were touched,
     // and which were not.
-    var putBack = Buffer.alloc(readAll ? 0 : 5, 'y');
+    const putBack = Buffer.alloc(readAll ? 0 : 5, 'y');
     readAll = !readAll;
     r.unshift(putBack);
   }
 });
 
-var expect =
+const expect =
   [ 'xxxxxxxxxx',
     'yyyyy',
     'xxxxxxxxxx',
diff --git a/test/parallel/test-stream-wrap.js b/test/parallel/test-stream-wrap.js
index 69e4dee89d..dbcb58a98c 100644
--- a/test/parallel/test-stream-wrap.js
+++ b/test/parallel/test-stream-wrap.js
@@ -7,16 +7,16 @@ const Duplex = require('stream').Duplex;
 const ShutdownWrap = process.binding('stream_wrap').ShutdownWrap;
 
 function testShutdown(callback) {
-  var stream = new Duplex({
+  const stream = new Duplex({
     read: function() {
     },
     write: function() {
     }
   });
 
-  var wrap = new StreamWrap(stream);
+  const wrap = new StreamWrap(stream);
 
-  var req = new ShutdownWrap();
+  const req = new ShutdownWrap();
   req.oncomplete = function(code) {
     assert(code < 0);
     callback();
diff --git a/test/parallel/test-stream-writable-change-default-encoding.js b/test/parallel/test-stream-writable-change-default-encoding.js
index 9d527fa9c5..9eac52bc1a 100644
--- a/test/parallel/test-stream-writable-change-default-encoding.js
+++ b/test/parallel/test-stream-writable-change-default-encoding.js
@@ -18,7 +18,7 @@ MyWritable.prototype._write = function(chunk, encoding, callback) {
 };
 
 (function defaultCondingIsUtf8() {
-  var m = new MyWritable(function(isBuffer, type, enc) {
+  const m = new MyWritable(function(isBuffer, type, enc) {
     assert.equal(enc, 'utf8');
   }, { decodeStrings: false });
   m.write('foo');
@@ -26,7 +26,7 @@ MyWritable.prototype._write = function(chunk, encoding, callback) {
 }());
 
 (function changeDefaultEncodingToAscii() {
-  var m = new MyWritable(function(isBuffer, type, enc) {
+  const m = new MyWritable(function(isBuffer, type, enc) {
     assert.equal(enc, 'ascii');
   }, { decodeStrings: false });
   m.setDefaultEncoding('ascii');
@@ -35,7 +35,7 @@ MyWritable.prototype._write = function(chunk, encoding, callback) {
 }());
 
 assert.throws(function changeDefaultEncodingToInvalidValue() {
-  var m = new MyWritable(function(isBuffer, type, enc) {
+  const m = new MyWritable(function(isBuffer, type, enc) {
   }, { decodeStrings: false });
   m.setDefaultEncoding({});
   m.write('bar');
@@ -43,7 +43,7 @@ assert.throws(function changeDefaultEncodingToInvalidValue() {
 }, TypeError);
 
 (function checkVairableCaseEncoding() {
-  var m = new MyWritable(function(isBuffer, type, enc) {
+  const m = new MyWritable(function(isBuffer, type, enc) {
     assert.equal(enc, 'ascii');
   }, { decodeStrings: false });
   m.setDefaultEncoding('AsCii');
diff --git a/test/parallel/test-stream-writable-constructor-set-methods.js b/test/parallel/test-stream-writable-constructor-set-methods.js
index 40f624f293..9b344487f6 100644
--- a/test/parallel/test-stream-writable-constructor-set-methods.js
+++ b/test/parallel/test-stream-writable-constructor-set-methods.js
@@ -4,22 +4,22 @@ const assert = require('assert');
 
 const Writable = require('stream').Writable;
 
-var _writeCalled = false;
+let _writeCalled = false;
 function _write(d, e, n) {
   _writeCalled = true;
 }
 
-var w = new Writable({ write: _write });
+const w = new Writable({ write: _write });
 w.end(Buffer.from('blerg'));
 
-var _writevCalled = false;
-var dLength = 0;
+let _writevCalled = false;
+let dLength = 0;
 function _writev(d, n) {
   dLength = d.length;
   _writevCalled = true;
 }
 
-var w2 = new Writable({ writev: _writev });
+const w2 = new Writable({ writev: _writev });
 w2.cork();
 
 w2.write(Buffer.from('blerg'));
diff --git a/test/parallel/test-stream-writable-null.js b/test/parallel/test-stream-writable-null.js
index 2cc396bd91..438f250a17 100644
--- a/test/parallel/test-stream-writable-null.js
+++ b/test/parallel/test-stream-writable-null.js
@@ -17,11 +17,11 @@ MyWritable.prototype._write = function(chunk, encoding, callback) {
 };
 
 assert.throws(() => {
-  var m = new MyWritable({objectMode: true});
+  const m = new MyWritable({objectMode: true});
   m.write(null, (err) => assert.ok(err));
 }, TypeError, 'May not write null values to stream');
 assert.doesNotThrow(() => {
-  var m = new MyWritable({objectMode: true}).on('error', (e) => {
+  const m = new MyWritable({objectMode: true}).on('error', (e) => {
     assert.ok(e);
   });
   m.write(null, (err) => {
@@ -30,11 +30,11 @@ assert.doesNotThrow(() => {
 });
 
 assert.throws(() => {
-  var m = new MyWritable();
+  const m = new MyWritable();
   m.write(false, (err) => assert.ok(err));
 }, TypeError, 'Invalid non-string/buffer chunk');
 assert.doesNotThrow(() => {
-  var m = new MyWritable().on('error', (e) => {
+  const m = new MyWritable().on('error', (e) => {
     assert.ok(e);
   });
   m.write(false, (err) => {
@@ -43,11 +43,11 @@ assert.doesNotThrow(() => {
 });
 
 assert.doesNotThrow(() => {
-  var m = new MyWritable({objectMode: true});
+  const m = new MyWritable({objectMode: true});
   m.write(false, (err) => assert.ifError(err));
 });
 assert.doesNotThrow(() => {
-  var m = new MyWritable({objectMode: true}).on('error', (e) => {
+  const m = new MyWritable({objectMode: true}).on('error', (e) => {
     assert.ifError(e || new Error('should not get here'));
   });
   m.write(false, (err) => {
diff --git a/test/parallel/test-stream-writev.js b/test/parallel/test-stream-writev.js
index fc3b099857..4e05d2ba8e 100644
--- a/test/parallel/test-stream-writev.js
+++ b/test/parallel/test-stream-writev.js
@@ -4,10 +4,10 @@ const assert = require('assert');
 
 const stream = require('stream');
 
-var queue = [];
-for (var decode = 0; decode < 2; decode++) {
-  for (var uncork = 0; uncork < 2; uncork++) {
-    for (var multi = 0; multi < 2; multi++) {
+const queue = [];
+for (let decode = 0; decode < 2; decode++) {
+  for (let uncork = 0; uncork < 2; uncork++) {
+    for (let multi = 0; multi < 2; multi++) {
       queue.push([!!decode, !!uncork, !!multi]);
     }
   }
@@ -16,7 +16,7 @@ for (var decode = 0; decode < 2; decode++) {
 run();
 
 function run() {
-  var t = queue.pop();
+  const t = queue.pop();
   if (t)
     test(t[0], t[1], t[2], run);
   else
@@ -25,11 +25,11 @@ function run() {
 
 function test(decode, uncork, multi, next) {
   console.log('# decode=%j uncork=%j multi=%j', decode, uncork, multi);
-  var counter = 0;
-  var expectCount = 0;
+  let counter = 0;
+  let expectCount = 0;
   function cnt(msg) {
     expectCount++;
-    var expect = expectCount;
+    const expect = expectCount;
     return function(er) {
       assert.ifError(er);
       counter++;
@@ -37,12 +37,12 @@ function test(decode, uncork, multi, next) {
     };
   }
 
-  var w = new stream.Writable({ decodeStrings: decode });
+  const w = new stream.Writable({ decodeStrings: decode });
   w._write = function(chunk, e, cb) {
     assert(false, 'Should not call _write');
   };
 
-  var expectChunks = decode ? [
+  const expectChunks = decode ? [
     { encoding: 'buffer',
       chunk: [104, 101, 108, 108, 111, 44, 32] },
     { encoding: 'buffer',
@@ -61,7 +61,7 @@ function test(decode, uncork, multi, next) {
     { encoding: 'hex', chunk: 'facebea7deadbeefdecafbad' }
   ];
 
-  var actualChunks;
+  let actualChunks;
   w._writev = function(chunks, cb) {
     actualChunks = chunks.map(function(chunk) {
       return {
diff --git a/test/parallel/test-stream2-base64-single-char-read-end.js b/test/parallel/test-stream2-base64-single-char-read-end.js
index 4107533424..da63644de0 100644
--- a/test/parallel/test-stream2-base64-single-char-read-end.js
+++ b/test/parallel/test-stream2-base64-single-char-read-end.js
@@ -4,11 +4,10 @@ const R = require('_stream_readable');
 const W = require('_stream_writable');
 const assert = require('assert');
 
-var src = new R({encoding: 'base64'});
-var dst = new W();
-var hasRead = false;
-var accum = [];
-var timeout;
+const src = new R({encoding: 'base64'});
+const dst = new W();
+let hasRead = false;
+const accum = [];
 
 src._read = function(n) {
   if (!hasRead) {
@@ -32,6 +31,6 @@ src.on('end', function() {
 
 src.pipe(dst);
 
-timeout = setTimeout(function() {
+const timeout = setTimeout(function() {
   common.fail('timed out waiting for _write');
 }, 100);
diff --git a/test/parallel/test-stream2-compatibility.js b/test/parallel/test-stream2-compatibility.js
index d4bf32a199..f7c9807913 100644
--- a/test/parallel/test-stream2-compatibility.js
+++ b/test/parallel/test-stream2-compatibility.js
@@ -6,7 +6,7 @@ const assert = require('assert');
 
 const util = require('util');
 
-var ondataCalled = 0;
+let ondataCalled = 0;
 
 function TestReader() {
   R.apply(this);
@@ -24,7 +24,7 @@ TestReader.prototype._read = function(n) {
   this._buffer = Buffer.alloc(0);
 };
 
-var reader = new TestReader();
+const reader = new TestReader();
 setImmediate(function() {
   assert.equal(ondataCalled, 1);
   console.log('ok');
@@ -43,7 +43,7 @@ TestWriter.prototype._write = function(chunk, enc, cb) {
   cb();
 };
 
-var writer = new TestWriter();
+const writer = new TestWriter();
 
 process.on('exit', function() {
   assert.strictEqual(reader.readable, false);
diff --git a/test/parallel/test-stream2-decode-partial.js b/test/parallel/test-stream2-decode-partial.js
index b58e192b9c..b43101dbc8 100644
--- a/test/parallel/test-stream2-decode-partial.js
+++ b/test/parallel/test-stream2-decode-partial.js
@@ -3,7 +3,7 @@ require('../common');
 const Readable = require('_stream_readable');
 const assert = require('assert');
 
-var buf = '';
+let buf = '';
 const euro = new Buffer([0xE2, 0x82, 0xAC]);
 const cent = new Buffer([0xC2, 0xA2]);
 const source = Buffer.concat([euro, cent]);
diff --git a/test/parallel/test-stream2-finish-pipe.js b/test/parallel/test-stream2-finish-pipe.js
index ee9ee20bd5..87edf5a5fa 100644
--- a/test/parallel/test-stream2-finish-pipe.js
+++ b/test/parallel/test-stream2-finish-pipe.js
@@ -3,12 +3,12 @@ require('../common');
 const stream = require('stream');
 const Buffer = require('buffer').Buffer;
 
-var r = new stream.Readable();
+const r = new stream.Readable();
 r._read = function(size) {
   r.push(Buffer.allocUnsafe(size));
 };
 
-var w = new stream.Writable();
+const w = new stream.Writable();
 w._write = function(data, encoding, cb) {
   cb(null);
 };
diff --git a/test/parallel/test-stream2-httpclient-response-end.js b/test/parallel/test-stream2-httpclient-response-end.js
index c4570e8b02..6d3da191b0 100644
--- a/test/parallel/test-stream2-httpclient-response-end.js
+++ b/test/parallel/test-stream2-httpclient-response-end.js
@@ -2,13 +2,13 @@
 const common = require('../common');
 const assert = require('assert');
 const http = require('http');
-var msg = 'Hello';
-var server = http.createServer(function(req, res) {
+const msg = 'Hello';
+const server = http.createServer(function(req, res) {
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.end(msg);
 }).listen(0, function() {
   http.get({port: this.address().port}, function(res) {
-    var data = '';
+    let data = '';
     res.on('readable', common.mustCall(function() {
       console.log('readable event');
       data += res.read();
diff --git a/test/parallel/test-stream2-large-read-stall.js b/test/parallel/test-stream2-large-read-stall.js
index 60e4a2873a..88fef4f24b 100644
--- a/test/parallel/test-stream2-large-read-stall.js
+++ b/test/parallel/test-stream2-large-read-stall.js
@@ -5,24 +5,25 @@ const assert = require('assert');
 // If everything aligns so that you do a read(n) of exactly the
 // remaining buffer, then make sure that 'end' still emits.
 
-var READSIZE = 100;
-var PUSHSIZE = 20;
-var PUSHCOUNT = 1000;
-var HWM = 50;
+const READSIZE = 100;
+const PUSHSIZE = 20;
+const PUSHCOUNT = 1000;
+const HWM = 50;
 
 const Readable = require('stream').Readable;
-var r = new Readable({
+const r = new Readable({
   highWaterMark: HWM
 });
-var rs = r._readableState;
+const rs = r._readableState;
 
 r._read = push;
 
 r.on('readable', function() {
   console.error('>> readable');
+  let ret;
   do {
     console.error('  > read(%d)', READSIZE);
-    var ret = r.read(READSIZE);
+    ret = r.read(READSIZE);
     console.error('  < %j (%d remain)', ret && ret.length, rs.length);
   } while (ret && ret.length === READSIZE);
 
@@ -34,7 +35,7 @@ r.on('readable', function() {
 
 r.on('end', common.mustCall(function() {}));
 
-var pushes = 0;
+let pushes = 0;
 function push() {
   if (pushes > PUSHCOUNT)
     return;
diff --git a/test/parallel/test-stream2-objects.js b/test/parallel/test-stream2-objects.js
index 6b50ba8436..6c00f1b869 100644
--- a/test/parallel/test-stream2-objects.js
+++ b/test/parallel/test-stream2-objects.js
@@ -5,8 +5,8 @@ const Writable = require('_stream_writable');
 const assert = require('assert');
 
 // tiny node-tap lookalike.
-var tests = [];
-var count = 0;
+const tests = [];
+let count = 0;
 
 function test(name, fn) {
   count++;
@@ -14,12 +14,12 @@ function test(name, fn) {
 }
 
 function run() {
-  var next = tests.shift();
+  const next = tests.shift();
   if (!next)
     return console.error('ok');
 
-  var name = next[0];
-  var fn = next[1];
+  const name = next[0];
+  const fn = next[1];
   console.log('# %s', name);
   fn({
     same: assert.deepStrictEqual,
@@ -39,8 +39,8 @@ process.on('exit', function() {
 process.nextTick(run);
 
 function toArray(callback) {
-  var stream = new Writable({ objectMode: true });
-  var list = [];
+  const stream = new Writable({ objectMode: true });
+  const list = [];
   stream.write = function(chunk) {
     list.push(chunk);
   };
@@ -53,7 +53,7 @@ function toArray(callback) {
 }
 
 function fromArray(list) {
-  var r = new Readable({ objectMode: true });
+  const r = new Readable({ objectMode: true });
   r._read = noop;
   list.forEach(function(chunk) {
     r.push(chunk);
@@ -66,11 +66,11 @@ function fromArray(list) {
 function noop() {}
 
 test('can read objects from stream', function(t) {
-  var r = fromArray([{ one: '1'}, { two: '2' }]);
+  const r = fromArray([{ one: '1'}, { two: '2' }]);
 
-  var v1 = r.read();
-  var v2 = r.read();
-  var v3 = r.read();
+  const v1 = r.read();
+  const v2 = r.read();
+  const v3 = r.read();
 
   assert.deepStrictEqual(v1, { one: '1' });
   assert.deepStrictEqual(v2, { two: '2' });
@@ -80,7 +80,7 @@ test('can read objects from stream', function(t) {
 });
 
 test('can pipe objects into stream', function(t) {
-  var r = fromArray([{ one: '1'}, { two: '2' }]);
+  const r = fromArray([{ one: '1'}, { two: '2' }]);
 
   r.pipe(toArray(function(list) {
     assert.deepStrictEqual(list, [
@@ -93,9 +93,9 @@ test('can pipe objects into stream', function(t) {
 });
 
 test('read(n) is ignored', function(t) {
-  var r = fromArray([{ one: '1'}, { two: '2' }]);
+  const r = fromArray([{ one: '1'}, { two: '2' }]);
 
-  var value = r.read(2);
+  const value = r.read(2);
 
   assert.deepStrictEqual(value, { one: '1' });
 
@@ -103,10 +103,10 @@ test('read(n) is ignored', function(t) {
 });
 
 test('can read objects from _read (sync)', function(t) {
-  var r = new Readable({ objectMode: true });
-  var list = [{ one: '1'}, { two: '2' }];
+  const r = new Readable({ objectMode: true });
+  const list = [{ one: '1'}, { two: '2' }];
   r._read = function(n) {
-    var item = list.shift();
+    const item = list.shift();
     r.push(item || null);
   };
 
@@ -121,10 +121,10 @@ test('can read objects from _read (sync)', function(t) {
 });
 
 test('can read objects from _read (async)', function(t) {
-  var r = new Readable({ objectMode: true });
-  var list = [{ one: '1'}, { two: '2' }];
+  const r = new Readable({ objectMode: true });
+  const list = [{ one: '1'}, { two: '2' }];
   r._read = function(n) {
-    var item = list.shift();
+    const item = list.shift();
     process.nextTick(function() {
       r.push(item || null);
     });
@@ -141,11 +141,11 @@ test('can read objects from _read (async)', function(t) {
 });
 
 test('can read strings as objects', function(t) {
-  var r = new Readable({
+  const r = new Readable({
     objectMode: true
   });
   r._read = noop;
-  var list = ['one', 'two', 'three'];
+  const list = ['one', 'two', 'three'];
   list.forEach(function(str) {
     r.push(str);
   });
@@ -159,7 +159,7 @@ test('can read strings as objects', function(t) {
 });
 
 test('read(0) for object streams', function(t) {
-  var r = new Readable({
+  const r = new Readable({
     objectMode: true
   });
   r._read = noop;
@@ -175,7 +175,7 @@ test('read(0) for object streams', function(t) {
 });
 
 test('falsey values', function(t) {
-  var r = new Readable({
+  const r = new Readable({
     objectMode: true
   });
   r._read = noop;
@@ -193,12 +193,12 @@ test('falsey values', function(t) {
 });
 
 test('high watermark _read', function(t) {
-  var r = new Readable({
+  const r = new Readable({
     highWaterMark: 6,
     objectMode: true
   });
-  var calls = 0;
-  var list = ['1', '2', '3', '4', '5', '6', '7', '8'];
+  let calls = 0;
+  const list = ['1', '2', '3', '4', '5', '6', '7', '8'];
 
   r._read = function(n) {
     calls++;
@@ -208,15 +208,15 @@ test('high watermark _read', function(t) {
     r.push(c);
   });
 
-  var v = r.read();
+  const v = r.read();
 
   assert.strictEqual(calls, 0);
   assert.strictEqual(v, '1');
 
-  var v2 = r.read();
+  const v2 = r.read();
   assert.strictEqual(v2, '2');
 
-  var v3 = r.read();
+  const v3 = r.read();
   assert.strictEqual(v3, '3');
 
   assert.strictEqual(calls, 1);
@@ -225,13 +225,13 @@ test('high watermark _read', function(t) {
 });
 
 test('high watermark push', function(t) {
-  var r = new Readable({
+  const r = new Readable({
     highWaterMark: 6,
     objectMode: true
   });
   r._read = function(n) {};
-  for (var i = 0; i < 6; i++) {
-    var bool = r.push(i);
+  for (let i = 0; i < 6; i++) {
+    const bool = r.push(i);
     assert.strictEqual(bool, i !== 5);
   }
 
@@ -239,7 +239,7 @@ test('high watermark push', function(t) {
 });
 
 test('can write objects to stream', function(t) {
-  var w = new Writable({ objectMode: true });
+  const w = new Writable({ objectMode: true });
 
   w._write = function(chunk, encoding, cb) {
     assert.deepStrictEqual(chunk, { foo: 'bar' });
@@ -255,8 +255,8 @@ test('can write objects to stream', function(t) {
 });
 
 test('can write multiple objects to stream', function(t) {
-  var w = new Writable({ objectMode: true });
-  var list = [];
+  const w = new Writable({ objectMode: true });
+  const list = [];
 
   w._write = function(chunk, encoding, cb) {
     list.push(chunk);
@@ -278,10 +278,10 @@ test('can write multiple objects to stream', function(t) {
 });
 
 test('can write strings as objects', function(t) {
-  var w = new Writable({
+  const w = new Writable({
     objectMode: true
   });
-  var list = [];
+  const list = [];
 
   w._write = function(chunk, encoding, cb) {
     list.push(chunk);
@@ -303,10 +303,10 @@ test('can write strings as objects', function(t) {
 });
 
 test('buffers finish until cb is called', function(t) {
-  var w = new Writable({
+  const w = new Writable({
     objectMode: true
   });
-  var called = false;
+  let called = false;
 
   w._write = function(chunk, encoding, cb) {
     assert.strictEqual(chunk, 'foo');
diff --git a/test/parallel/test-stream2-pipe-error-once-listener.js b/test/parallel/test-stream2-pipe-error-once-listener.js
index a2ddb7f5d6..908ac1bdb8 100644
--- a/test/parallel/test-stream2-pipe-error-once-listener.js
+++ b/test/parallel/test-stream2-pipe-error-once-listener.js
@@ -5,7 +5,7 @@ const util = require('util');
 const stream = require('stream');
 
 
-var Read = function() {
+const Read = function() {
   stream.Readable.call(this);
 };
 util.inherits(Read, stream.Readable);
@@ -16,7 +16,7 @@ Read.prototype._read = function(size) {
 };
 
 
-var Write = function() {
+const Write = function() {
   stream.Writable.call(this);
 };
 util.inherits(Write, stream.Writable);
@@ -26,8 +26,8 @@ Write.prototype._write = function(buffer, encoding, cb) {
   this.emit('alldone');
 };
 
-var read = new Read();
-var write = new Write();
+const read = new Read();
+const write = new Write();
 
 write.once('error', function(err) {});
 write.once('alldone', function(err) {
diff --git a/test/parallel/test-stream2-push.js b/test/parallel/test-stream2-push.js
index 26f2752907..95a98450a3 100644
--- a/test/parallel/test-stream2-push.js
+++ b/test/parallel/test-stream2-push.js
@@ -14,20 +14,20 @@ const stream = new Readable({
   encoding: 'utf8'
 });
 
-var source = new EE();
+const source = new EE();
 
 stream._read = function() {
   console.error('stream._read');
   readStart();
 };
 
-var ended = false;
+let ended = false;
 stream.on('end', function() {
   ended = true;
 });
 
 source.on('data', function(chunk) {
-  var ret = stream.push(chunk);
+  const ret = stream.push(chunk);
   console.error('data', stream._readableState.length);
   if (!ret)
     readStop();
@@ -37,7 +37,7 @@ source.on('end', function() {
   stream.push(null);
 });
 
-var reading = false;
+let reading = false;
 
 function readStart() {
   console.error('readStart');
@@ -48,19 +48,19 @@ function readStop() {
   console.error('readStop');
   reading = false;
   process.nextTick(function() {
-    var r = stream.read();
+    const r = stream.read();
     if (r !== null)
       writer.write(r);
   });
 }
 
-var writer = new Writable({
+const writer = new Writable({
   decodeStrings: false
 });
 
-var written = [];
+const written = [];
 
-var expectWritten =
+const expectWritten =
   [ 'asdfgasdfgasdfgasdfg',
     'asdfgasdfgasdfgasdfg',
     'asdfgasdfgasdfgasdfg',
@@ -79,9 +79,9 @@ writer.on('finish', finish);
 
 // now emit some chunks.
 
-var chunk = 'asdfg';
+const chunk = 'asdfg';
 
-var set = 0;
+let set = 0;
 readStart();
 data();
 function data() {
diff --git a/test/parallel/test-stream2-read-sync-stack.js b/test/parallel/test-stream2-read-sync-stack.js
index 1ba31a4b31..d4e11af3b8 100644
--- a/test/parallel/test-stream2-read-sync-stack.js
+++ b/test/parallel/test-stream2-read-sync-stack.js
@@ -1,16 +1,16 @@
 'use strict';
 const common = require('../common');
 const Readable = require('stream').Readable;
-var r = new Readable();
-var N = 256 * 1024;
+const r = new Readable();
+const N = 256 * 1024;
 
 // Go ahead and allow the pathological case for this test.
 // Yes, it's an infinite loop, that's the point.
 process.maxTickDepth = N + 2;
 
-var reads = 0;
+let reads = 0;
 r._read = function(n) {
-  var chunk = reads++ === N ? null : Buffer.allocUnsafe(1);
+  const chunk = reads++ === N ? null : Buffer.allocUnsafe(1);
   r.push(chunk);
 };
 
diff --git a/test/parallel/test-stream2-readable-empty-buffer-no-eof.js b/test/parallel/test-stream2-readable-empty-buffer-no-eof.js
index ccbf087df0..71d6f1e723 100644
--- a/test/parallel/test-stream2-readable-empty-buffer-no-eof.js
+++ b/test/parallel/test-stream2-readable-empty-buffer-no-eof.js
@@ -69,8 +69,8 @@ function test1() {
 }
 
 function test2() {
-  var r = new Readable({ encoding: 'base64' });
-  var reads = 5;
+  const r = new Readable({ encoding: 'base64' });
+  let reads = 5;
   r._read = function(n) {
     if (!reads--)
       return r.push(null); // EOF
@@ -78,9 +78,9 @@ function test2() {
       return r.push(Buffer.from('x'));
   };
 
-  var results = [];
+  const results = [];
   function flow() {
-    var chunk;
+    let chunk;
     while (null !== (chunk = r.read()))
       results.push(chunk + '');
   }
diff --git a/test/parallel/test-stream2-readable-from-list.js b/test/parallel/test-stream2-readable-from-list.js
index e38200e9d6..f3f39507e8 100644
--- a/test/parallel/test-stream2-readable-from-list.js
+++ b/test/parallel/test-stream2-readable-from-list.js
@@ -6,8 +6,8 @@ const fromList = require('_stream_readable')._fromList;
 const BufferList = require('internal/streams/BufferList');
 
 // tiny node-tap lookalike.
-var tests = [];
-var count = 0;
+const tests = [];
+let count = 0;
 
 function test(name, fn) {
   count++;
@@ -15,12 +15,12 @@ function test(name, fn) {
 }
 
 function run() {
-  var next = tests.shift();
+  const next = tests.shift();
   if (!next)
     return console.error('ok');
 
-  var name = next[0];
-  var fn = next[1];
+  const name = next[0];
+  const fn = next[1];
   console.log('# %s', name);
   fn({
     same: assert.deepStrictEqual,
@@ -34,7 +34,7 @@ function run() {
 
 function bufferListFromArray(arr) {
   const bl = new BufferList();
-  for (var i = 0; i < arr.length; ++i)
+  for (let i = 0; i < arr.length; ++i)
     bl.push(arr[i]);
   return bl;
 }
@@ -48,14 +48,14 @@ process.nextTick(run);
 
 
 test('buffers', function(t) {
-  var list = [ Buffer.from('foog'),
+  let list = [ Buffer.from('foog'),
                Buffer.from('bark'),
                Buffer.from('bazy'),
                Buffer.from('kuel') ];
   list = bufferListFromArray(list);
 
   // read more than the first element.
-  var ret = fromList(6, { buffer: list, length: 16 });
+  let ret = fromList(6, { buffer: list, length: 16 });
   t.equal(ret.toString(), 'foogba');
 
   // read exactly the first element.
@@ -77,14 +77,14 @@ test('buffers', function(t) {
 });
 
 test('strings', function(t) {
-  var list = [ 'foog',
+  let list = [ 'foog',
                'bark',
                'bazy',
                'kuel' ];
   list = bufferListFromArray(list);
 
   // read more than the first element.
-  var ret = fromList(6, { buffer: list, length: 16, decoder: true });
+  let ret = fromList(6, { buffer: list, length: 16, decoder: true });
   t.equal(ret, 'foogba');
 
   // read exactly the first element.
diff --git a/test/parallel/test-stream2-readable-legacy-drain.js b/test/parallel/test-stream2-readable-legacy-drain.js
index 1b8a4ac1ff..d6db6ba232 100644
--- a/test/parallel/test-stream2-readable-legacy-drain.js
+++ b/test/parallel/test-stream2-readable-legacy-drain.js
@@ -3,20 +3,20 @@ const common = require('../common');
 const assert = require('assert');
 
 const Stream = require('stream');
-var Readable = Stream.Readable;
+const Readable = Stream.Readable;
 
-var r = new Readable();
-var N = 256;
-var reads = 0;
+const r = new Readable();
+const N = 256;
+let reads = 0;
 r._read = function(n) {
   return r.push(++reads === N ? null : Buffer.allocUnsafe(1));
 };
 
 r.on('end', common.mustCall(function() {}));
 
-var w = new Stream();
+const w = new Stream();
 w.writable = true;
-var buffered = 0;
+let buffered = 0;
 w.write = function(c) {
   buffered += c.length;
   process.nextTick(drain);
diff --git a/test/parallel/test-stream2-readable-non-empty-end.js b/test/parallel/test-stream2-readable-non-empty-end.js
index 0a4eb131c3..66fae8274e 100644
--- a/test/parallel/test-stream2-readable-non-empty-end.js
+++ b/test/parallel/test-stream2-readable-non-empty-end.js
@@ -3,17 +3,17 @@ const common = require('../common');
 const assert = require('assert');
 const Readable = require('_stream_readable');
 
-var len = 0;
-var chunks = new Array(10);
-for (var i = 1; i <= 10; i++) {
+let len = 0;
+const chunks = new Array(10);
+for (let i = 1; i <= 10; i++) {
   chunks[i - 1] = Buffer.allocUnsafe(i);
   len += i;
 }
 
-var test = new Readable();
-var n = 0;
+const test = new Readable();
+let n = 0;
 test._read = function(size) {
-  var chunk = chunks[n++];
+  const chunk = chunks[n++];
   setTimeout(function() {
     test.push(chunk === undefined ? null : chunk);
   }, 1);
@@ -24,10 +24,10 @@ function thrower() {
   throw new Error('this should not happen!');
 }
 
-var bytesread = 0;
+let bytesread = 0;
 test.on('readable', function() {
-  var b = len - bytesread - 1;
-  var res = test.read(b);
+  const b = len - bytesread - 1;
+  const res = test.read(b);
   if (res) {
     bytesread += res.length;
     console.error('br=%d len=%d', bytesread, len);
@@ -43,7 +43,7 @@ function next() {
   test.on('end', common.mustCall(function() {}));
 
   // one to get the last byte
-  var r = test.read();
+  let r = test.read();
   assert(r);
   assert.equal(r.length, 1);
   r = test.read();
diff --git a/test/parallel/test-stream2-readable-wrap-empty.js b/test/parallel/test-stream2-readable-wrap-empty.js
index aa48c20bb8..5b2dae3c53 100644
--- a/test/parallel/test-stream2-readable-wrap-empty.js
+++ b/test/parallel/test-stream2-readable-wrap-empty.js
@@ -4,11 +4,11 @@ const common = require('../common');
 const Readable = require('_stream_readable');
 const EE = require('events').EventEmitter;
 
-var oldStream = new EE();
+const oldStream = new EE();
 oldStream.pause = function() {};
 oldStream.resume = function() {};
 
-var newStream = new Readable().wrap(oldStream);
+const newStream = new Readable().wrap(oldStream);
 
 newStream
   .on('readable', function() {})
diff --git a/test/parallel/test-stream2-set-encoding.js b/test/parallel/test-stream2-set-encoding.js
index c7cc7c3c0b..fdbd81e613 100644
--- a/test/parallel/test-stream2-set-encoding.js
+++ b/test/parallel/test-stream2-set-encoding.js
@@ -5,8 +5,8 @@ const R = require('_stream_readable');
 const util = require('util');
 
 // tiny node-tap lookalike.
-var tests = [];
-var count = 0;
+const tests = [];
+let count = 0;
 
 function test(name, fn) {
   count++;
@@ -14,12 +14,12 @@ function test(name, fn) {
 }
 
 function run() {
-  var next = tests.shift();
+  const next = tests.shift();
   if (!next)
     return console.error('ok');
 
-  var name = next[0];
-  var fn = next[1];
+  const name = next[0];
+  const fn = next[1];
   console.log('# %s', name);
   fn({
     same: assert.deepStrictEqual,
@@ -66,7 +66,7 @@ TestReader.prototype._read = function(n) {
     }
 
     this.pos += n;
-    var ret = Buffer.alloc(n, 'a');
+    const ret = Buffer.alloc(n, 'a');
 
     console.log('this.push(ret)', ret);
 
@@ -75,10 +75,10 @@ TestReader.prototype._read = function(n) {
 };
 
 test('setEncoding utf8', function(t) {
-  var tr = new TestReader(100);
+  const tr = new TestReader(100);
   tr.setEncoding('utf8');
-  var out = [];
-  var expect =
+  const out = [];
+  const expect =
     [ 'aaaaaaaaaa',
       'aaaaaaaaaa',
       'aaaaaaaaaa',
@@ -91,7 +91,7 @@ test('setEncoding utf8', function(t) {
       'aaaaaaaaaa' ];
 
   tr.on('readable', function flow() {
-    var chunk;
+    let chunk;
     while (null !== (chunk = tr.read(10)))
       out.push(chunk);
   });
@@ -104,10 +104,10 @@ test('setEncoding utf8', function(t) {
 
 
 test('setEncoding hex', function(t) {
-  var tr = new TestReader(100);
+  const tr = new TestReader(100);
   tr.setEncoding('hex');
-  var out = [];
-  var expect =
+  const out = [];
+  const expect =
     [ '6161616161',
       '6161616161',
       '6161616161',
@@ -130,7 +130,7 @@ test('setEncoding hex', function(t) {
       '6161616161' ];
 
   tr.on('readable', function flow() {
-    var chunk;
+    let chunk;
     while (null !== (chunk = tr.read(10)))
       out.push(chunk);
   });
@@ -142,10 +142,10 @@ test('setEncoding hex', function(t) {
 });
 
 test('setEncoding hex with read(13)', function(t) {
-  var tr = new TestReader(100);
+  const tr = new TestReader(100);
   tr.setEncoding('hex');
-  var out = [];
-  var expect =
+  const out = [];
+  const expect =
     [ '6161616161616',
       '1616161616161',
       '6161616161616',
@@ -165,7 +165,7 @@ test('setEncoding hex with read(13)', function(t) {
 
   tr.on('readable', function flow() {
     console.log('readable once');
-    var chunk;
+    let chunk;
     while (null !== (chunk = tr.read(13)))
       out.push(chunk);
   });
@@ -178,10 +178,10 @@ test('setEncoding hex with read(13)', function(t) {
 });
 
 test('setEncoding base64', function(t) {
-  var tr = new TestReader(100);
+  const tr = new TestReader(100);
   tr.setEncoding('base64');
-  var out = [];
-  var expect =
+  const out = [];
+  const expect =
     [ 'YWFhYWFhYW',
       'FhYWFhYWFh',
       'YWFhYWFhYW',
@@ -198,7 +198,7 @@ test('setEncoding base64', function(t) {
       'FhYQ==' ];
 
   tr.on('readable', function flow() {
-    var chunk;
+    let chunk;
     while (null !== (chunk = tr.read(10)))
       out.push(chunk);
   });
@@ -210,9 +210,9 @@ test('setEncoding base64', function(t) {
 });
 
 test('encoding: utf8', function(t) {
-  var tr = new TestReader(100, { encoding: 'utf8' });
-  var out = [];
-  var expect =
+  const tr = new TestReader(100, { encoding: 'utf8' });
+  const out = [];
+  const expect =
     [ 'aaaaaaaaaa',
       'aaaaaaaaaa',
       'aaaaaaaaaa',
@@ -225,7 +225,7 @@ test('encoding: utf8', function(t) {
       'aaaaaaaaaa' ];
 
   tr.on('readable', function flow() {
-    var chunk;
+    let chunk;
     while (null !== (chunk = tr.read(10)))
       out.push(chunk);
   });
@@ -238,9 +238,9 @@ test('encoding: utf8', function(t) {
 
 
 test('encoding: hex', function(t) {
-  var tr = new TestReader(100, { encoding: 'hex' });
-  var out = [];
-  var expect =
+  const tr = new TestReader(100, { encoding: 'hex' });
+  const out = [];
+  const expect =
     [ '6161616161',
       '6161616161',
       '6161616161',
@@ -263,7 +263,7 @@ test('encoding: hex', function(t) {
       '6161616161' ];
 
   tr.on('readable', function flow() {
-    var chunk;
+    let chunk;
     while (null !== (chunk = tr.read(10)))
       out.push(chunk);
   });
@@ -275,9 +275,9 @@ test('encoding: hex', function(t) {
 });
 
 test('encoding: hex with read(13)', function(t) {
-  var tr = new TestReader(100, { encoding: 'hex' });
-  var out = [];
-  var expect =
+  const tr = new TestReader(100, { encoding: 'hex' });
+  const out = [];
+  const expect =
     [ '6161616161616',
       '1616161616161',
       '6161616161616',
@@ -296,7 +296,7 @@ test('encoding: hex with read(13)', function(t) {
       '16161' ];
 
   tr.on('readable', function flow() {
-    var chunk;
+    let chunk;
     while (null !== (chunk = tr.read(13)))
       out.push(chunk);
   });
@@ -308,9 +308,9 @@ test('encoding: hex with read(13)', function(t) {
 });
 
 test('encoding: base64', function(t) {
-  var tr = new TestReader(100, { encoding: 'base64' });
-  var out = [];
-  var expect =
+  const tr = new TestReader(100, { encoding: 'base64' });
+  const out = [];
+  const expect =
     [ 'YWFhYWFhYW',
       'FhYWFhYWFh',
       'YWFhYWFhYW',
@@ -327,7 +327,7 @@ test('encoding: base64', function(t) {
       'FhYQ==' ];
 
   tr.on('readable', function flow() {
-    var chunk;
+    let chunk;
     while (null !== (chunk = tr.read(10)))
       out.push(chunk);
   });
@@ -339,7 +339,7 @@ test('encoding: base64', function(t) {
 });
 
 test('chainable', function(t) {
-  var tr = new TestReader(100);
+  const tr = new TestReader(100);
   t.equal(tr.setEncoding('utf8'), tr);
   t.end();
 });
diff --git a/test/parallel/test-stream2-transform.js b/test/parallel/test-stream2-transform.js
index 0777ada97f..d66f1f0643 100644
--- a/test/parallel/test-stream2-transform.js
+++ b/test/parallel/test-stream2-transform.js
@@ -5,8 +5,8 @@ const PassThrough = require('_stream_passthrough');
 const Transform = require('_stream_transform');
 
 // tiny node-tap lookalike.
-var tests = [];
-var count = 0;
+const tests = [];
+let count = 0;
 
 function test(name, fn) {
   count++;
@@ -14,12 +14,12 @@ function test(name, fn) {
 }
 
 function run() {
-  var next = tests.shift();
+  const next = tests.shift();
   if (!next)
     return console.error('ok');
 
-  var name = next[0];
-  var fn = next[1];
+  const name = next[0];
+  const fn = next[1];
   console.log('# %s', name);
   fn({
     same: assert.deepStrictEqual,
@@ -42,18 +42,18 @@ process.nextTick(run);
 /////
 
 test('writable side consumption', function(t) {
-  var tx = new Transform({
+  const tx = new Transform({
     highWaterMark: 10
   });
 
-  var transformed = 0;
+  let transformed = 0;
   tx._transform = function(chunk, encoding, cb) {
     transformed += chunk.length;
     tx.push(chunk);
     cb();
   };
 
-  for (var i = 1; i <= 10; i++) {
+  for (let i = 1; i <= 10; i++) {
     tx.write(Buffer.allocUnsafe(i));
   }
   tx.end();
@@ -69,7 +69,7 @@ test('writable side consumption', function(t) {
 });
 
 test('passthrough', function(t) {
-  var pt = new PassThrough();
+  const pt = new PassThrough();
 
   pt.write(Buffer.from('foog'));
   pt.write(Buffer.from('bark'));
@@ -85,7 +85,7 @@ test('passthrough', function(t) {
 });
 
 test('object passthrough', function(t) {
-  var pt = new PassThrough({ objectMode: true });
+  const pt = new PassThrough({ objectMode: true });
 
   pt.write(1);
   pt.write(true);
@@ -115,9 +115,9 @@ test('passthrough constructor', function(t) {
 });
 
 test('simple transform', function(t) {
-  var pt = new Transform();
+  const pt = new Transform();
   pt._transform = function(c, e, cb) {
-    var ret = Buffer.alloc(c.length, 'x');
+    const ret = Buffer.alloc(c.length, 'x');
     pt.push(ret);
     cb();
   };
@@ -136,7 +136,7 @@ test('simple transform', function(t) {
 });
 
 test('simple object transform', function(t) {
-  var pt = new Transform({ objectMode: true });
+  const pt = new Transform({ objectMode: true });
   pt._transform = function(c, e, cb) {
     pt.push(JSON.stringify(c));
     cb();
@@ -162,7 +162,7 @@ test('simple object transform', function(t) {
 });
 
 test('async passthrough', function(t) {
-  var pt = new Transform();
+  const pt = new Transform();
   pt._transform = function(chunk, encoding, cb) {
     setTimeout(function() {
       pt.push(chunk);
@@ -186,7 +186,7 @@ test('async passthrough', function(t) {
 });
 
 test('assymetric transform (expand)', function(t) {
-  var pt = new Transform();
+  const pt = new Transform();
 
   // emit each chunk 2 times.
   pt._transform = function(chunk, encoding, cb) {
@@ -218,7 +218,7 @@ test('assymetric transform (expand)', function(t) {
 });
 
 test('assymetric transform (compress)', function(t) {
-  var pt = new Transform();
+  const pt = new Transform();
 
   // each output is the first char of 3 consecutive chunks,
   // or whatever's left.
@@ -227,7 +227,7 @@ test('assymetric transform (compress)', function(t) {
   pt._transform = function(chunk, encoding, cb) {
     if (!chunk)
       chunk = '';
-    var s = chunk.toString();
+    const s = chunk.toString();
     setTimeout(function() {
       this.state += s.charAt(0);
       if (this.state.length === 3) {
@@ -273,9 +273,9 @@ test('assymetric transform (compress)', function(t) {
 // this tests for a stall when data is written to a full stream
 // that has empty transforms.
 test('complex transform', function(t) {
-  var count = 0;
-  var saved = null;
-  var pt = new Transform({highWaterMark: 3});
+  let count = 0;
+  let saved = null;
+  const pt = new Transform({highWaterMark: 3});
   pt._transform = function(c, e, cb) {
     if (count++ === 1)
       saved = c;
@@ -307,8 +307,8 @@ test('complex transform', function(t) {
 
 
 test('passthrough event emission', function(t) {
-  var pt = new PassThrough();
-  var emits = 0;
+  const pt = new PassThrough();
+  let emits = 0;
   pt.on('readable', function() {
     console.error('>>> emit readable %d', emits);
     emits++;
@@ -353,8 +353,8 @@ test('passthrough event emission', function(t) {
 });
 
 test('passthrough event emission reordered', function(t) {
-  var pt = new PassThrough();
-  var emits = 0;
+  const pt = new PassThrough();
+  let emits = 0;
   pt.on('readable', function() {
     console.error('emit readable', emits);
     emits++;
@@ -395,8 +395,8 @@ test('passthrough event emission reordered', function(t) {
 
 test('passthrough facaded', function(t) {
   console.error('passthrough facaded');
-  var pt = new PassThrough();
-  var datas = [];
+  const pt = new PassThrough();
+  const datas = [];
   pt.on('data', function(chunk) {
     datas.push(chunk.toString());
   });
@@ -423,7 +423,7 @@ test('passthrough facaded', function(t) {
 
 test('object transform (json parse)', function(t) {
   console.error('json parse stream');
-  var jp = new Transform({ objectMode: true });
+  const jp = new Transform({ objectMode: true });
   jp._transform = function(data, encoding, cb) {
     try {
       jp.push(JSON.parse(data));
@@ -435,21 +435,21 @@ test('object transform (json parse)', function(t) {
 
   // anything except null/undefined is fine.
   // those are "magic" in the stream API, because they signal EOF.
-  var objects = [
+  const objects = [
     { foo: 'bar' },
     100,
     'string',
     { nested: { things: [ { foo: 'bar' }, 100, 'string' ] } }
   ];
 
-  var ended = false;
+  let ended = false;
   jp.on('end', function() {
     ended = true;
   });
 
   objects.forEach(function(obj) {
     jp.write(JSON.stringify(obj));
-    var res = jp.read();
+    const res = jp.read();
     t.same(res, obj);
   });
 
@@ -465,7 +465,7 @@ test('object transform (json parse)', function(t) {
 
 test('object transform (json stringify)', function(t) {
   console.error('json parse stream');
-  var js = new Transform({ objectMode: true });
+  const js = new Transform({ objectMode: true });
   js._transform = function(data, encoding, cb) {
     try {
       js.push(JSON.stringify(data));
@@ -477,21 +477,21 @@ test('object transform (json stringify)', function(t) {
 
   // anything except null/undefined is fine.
   // those are "magic" in the stream API, because they signal EOF.
-  var objects = [
+  const objects = [
     { foo: 'bar' },
     100,
     'string',
     { nested: { things: [ { foo: 'bar' }, 100, 'string' ] } }
   ];
 
-  var ended = false;
+  let ended = false;
   js.on('end', function() {
     ended = true;
   });
 
   objects.forEach(function(obj) {
     js.write(obj);
-    var res = js.read();
+    const res = js.read();
     t.equal(res, JSON.stringify(obj));
   });
 
diff --git a/test/parallel/test-stream2-unpipe-leak.js b/test/parallel/test-stream2-unpipe-leak.js
index 2004a8194c..1a1599ca32 100644
--- a/test/parallel/test-stream2-unpipe-leak.js
+++ b/test/parallel/test-stream2-unpipe-leak.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const stream = require('stream');
 
-var chunk = Buffer.from('hallo');
+const chunk = Buffer.from('hallo');
 
 const util = require('util');
 
@@ -16,7 +16,7 @@ TestWriter.prototype._write = function(buffer, encoding, callback) {
   callback(null);
 };
 
-var dest = new TestWriter();
+const dest = new TestWriter();
 
 // Set this high so that we'd trigger a nextTick warning
 // and/or RangeError if we do maybeReadMore wrong.
@@ -29,9 +29,9 @@ TestReader.prototype._read = function(size) {
   this.push(chunk);
 };
 
-var src = new TestReader();
+const src = new TestReader();
 
-for (var i = 0; i < 10; i++) {
+for (let i = 0; i < 10; i++) {
   src.pipe(dest);
   src.unpipe(dest);
 }
diff --git a/test/parallel/test-stream2-writable.js b/test/parallel/test-stream2-writable.js
index 4d98457e7d..c377ce9a51 100644
--- a/test/parallel/test-stream2-writable.js
+++ b/test/parallel/test-stream2-writable.js
@@ -22,14 +22,14 @@ TestWriter.prototype._write = function(chunk, encoding, cb) {
   }.bind(this), Math.floor(Math.random() * 10));
 };
 
-var chunks = new Array(50);
-for (var i = 0; i < chunks.length; i++) {
+const chunks = new Array(50);
+for (let i = 0; i < chunks.length; i++) {
   chunks[i] = new Array(i + 1).join('x');
 }
 
 // tiny node-tap lookalike.
-var tests = [];
-var count = 0;
+const tests = [];
+let count = 0;
 
 function test(name, fn) {
   count++;
@@ -37,12 +37,12 @@ function test(name, fn) {
 }
 
 function run() {
-  var next = tests.shift();
+  const next = tests.shift();
   if (!next)
     return console.error('ok');
 
-  var name = next[0];
-  var fn = next[1];
+  const name = next[0];
+  const fn = next[1];
   console.log('# %s', name);
   fn({
     same: assert.deepStrictEqual,
@@ -62,7 +62,7 @@ process.on('exit', function() {
 process.nextTick(run);
 
 test('write fast', function(t) {
-  var tw = new TestWriter({
+  const tw = new TestWriter({
     highWaterMark: 100
   });
 
@@ -79,7 +79,7 @@ test('write fast', function(t) {
 });
 
 test('write slow', function(t) {
-  var tw = new TestWriter({
+  const tw = new TestWriter({
     highWaterMark: 100
   });
 
@@ -88,7 +88,7 @@ test('write slow', function(t) {
     t.end();
   });
 
-  var i = 0;
+  let i = 0;
   (function W() {
     tw.write(chunks[i++]);
     if (i < chunks.length)
@@ -99,11 +99,11 @@ test('write slow', function(t) {
 });
 
 test('write backpressure', function(t) {
-  var tw = new TestWriter({
+  const tw = new TestWriter({
     highWaterMark: 50
   });
 
-  var drains = 0;
+  let drains = 0;
 
   tw.on('finish', function() {
     t.same(tw.buffer, chunks, 'got chunks in the right order');
@@ -115,10 +115,11 @@ test('write backpressure', function(t) {
     drains++;
   });
 
-  var i = 0;
+  let i = 0;
   (function W() {
+    let ret;
     do {
-      var ret = tw.write(chunks[i++]);
+      ret = tw.write(chunks[i++]);
     } while (ret !== false && i < chunks.length);
 
     if (i < chunks.length) {
@@ -131,11 +132,11 @@ test('write backpressure', function(t) {
 });
 
 test('write bufferize', function(t) {
-  var tw = new TestWriter({
+  const tw = new TestWriter({
     highWaterMark: 100
   });
 
-  var encodings =
+  const encodings =
     [ 'hex',
       'utf8',
       'utf-8',
@@ -154,7 +155,7 @@ test('write bufferize', function(t) {
   });
 
   chunks.forEach(function(chunk, i) {
-    var enc = encodings[i % encodings.length];
+    const enc = encodings[i % encodings.length];
     chunk = Buffer.from(chunk);
     tw.write(chunk.toString(enc), enc);
   });
@@ -162,7 +163,7 @@ test('write bufferize', function(t) {
 });
 
 test('write no bufferize', function(t) {
-  var tw = new TestWriter({
+  const tw = new TestWriter({
     highWaterMark: 100,
     decodeStrings: false
   });
@@ -173,7 +174,7 @@ test('write no bufferize', function(t) {
     return TestWriter.prototype._write.call(this, chunk, encoding, cb);
   };
 
-  var encodings =
+  const encodings =
     [ 'hex',
       'utf8',
       'utf-8',
@@ -192,7 +193,7 @@ test('write no bufferize', function(t) {
   });
 
   chunks.forEach(function(chunk, i) {
-    var enc = encodings[i % encodings.length];
+    const enc = encodings[i % encodings.length];
     chunk = Buffer.from(chunk);
     tw.write(chunk.toString(enc), enc);
   });
@@ -200,7 +201,7 @@ test('write no bufferize', function(t) {
 });
 
 test('write callbacks', function(t) {
-  var callbacks = chunks.map(function(chunk, i) {
+  const callbacks = chunks.map(function(chunk, i) {
     return [i, function() {
       callbacks._called[i] = chunk;
     }];
@@ -210,7 +211,7 @@ test('write callbacks', function(t) {
   }, {});
   callbacks._called = [];
 
-  var tw = new TestWriter({
+  const tw = new TestWriter({
     highWaterMark: 100
   });
 
@@ -229,28 +230,28 @@ test('write callbacks', function(t) {
 });
 
 test('end callback', function(t) {
-  var tw = new TestWriter();
+  const tw = new TestWriter();
   tw.end(function() {
     t.end();
   });
 });
 
 test('end callback with chunk', function(t) {
-  var tw = new TestWriter();
+  const tw = new TestWriter();
   tw.end(Buffer.from('hello world'), function() {
     t.end();
   });
 });
 
 test('end callback with chunk and encoding', function(t) {
-  var tw = new TestWriter();
+  const tw = new TestWriter();
   tw.end('hello world', 'ascii', function() {
     t.end();
   });
 });
 
 test('end callback after .write() call', function(t) {
-  var tw = new TestWriter();
+  const tw = new TestWriter();
   tw.write(Buffer.from('hello world'));
   tw.end(function() {
     t.end();
@@ -258,8 +259,8 @@ test('end callback after .write() call', function(t) {
 });
 
 test('end callback called after write callback', function(t) {
-  var tw = new TestWriter();
-  var writeCalledback = false;
+  const tw = new TestWriter();
+  let writeCalledback = false;
   tw.write(Buffer.from('hello world'), function() {
     writeCalledback = true;
   });
@@ -270,20 +271,20 @@ test('end callback called after write callback', function(t) {
 });
 
 test('encoding should be ignored for buffers', function(t) {
-  var tw = new W();
-  var hex = '018b5e9a8f6236ffe30e31baf80d2cf6eb';
+  const tw = new W();
+  const hex = '018b5e9a8f6236ffe30e31baf80d2cf6eb';
   tw._write = function(chunk) {
     t.equal(chunk.toString('hex'), hex);
     t.end();
   };
-  var buf = Buffer.from(hex, 'hex');
+  const buf = Buffer.from(hex, 'hex');
   tw.write(buf, 'latin1');
 });
 
 test('writables are not pipable', function(t) {
-  var w = new W();
+  const w = new W();
   w._write = function() {};
-  var gotError = false;
+  let gotError = false;
   w.on('error', function() {
     gotError = true;
   });
@@ -293,10 +294,10 @@ test('writables are not pipable', function(t) {
 });
 
 test('duplexes are pipable', function(t) {
-  var d = new D();
+  const d = new D();
   d._read = function() {};
   d._write = function() {};
-  var gotError = false;
+  let gotError = false;
   d.on('error', function() {
     gotError = true;
   });
@@ -306,9 +307,9 @@ test('duplexes are pipable', function(t) {
 });
 
 test('end(chunk) two times is an error', function(t) {
-  var w = new W();
+  const w = new W();
   w._write = function() {};
-  var gotError = false;
+  let gotError = false;
   w.on('error', function(er) {
     gotError = true;
     t.equal(er.message, 'write after end');
@@ -322,8 +323,8 @@ test('end(chunk) two times is an error', function(t) {
 });
 
 test('dont end while writing', function(t) {
-  var w = new W();
-  var wrote = false;
+  const w = new W();
+  let wrote = false;
   w._write = function(chunk, e, cb) {
     assert(!this.writing);
     wrote = true;
@@ -342,8 +343,8 @@ test('dont end while writing', function(t) {
 });
 
 test('finish does not come before write cb', function(t) {
-  var w = new W();
-  var writeCb = false;
+  const w = new W();
+  let writeCb = false;
   w._write = function(chunk, e, cb) {
     setTimeout(function() {
       writeCb = true;
@@ -359,8 +360,8 @@ test('finish does not come before write cb', function(t) {
 });
 
 test('finish does not come before sync _write cb', function(t) {
-  var w = new W();
-  var writeCb = false;
+  const w = new W();
+  let writeCb = false;
   w._write = function(chunk, e, cb) {
     cb();
   };
@@ -375,7 +376,7 @@ test('finish does not come before sync _write cb', function(t) {
 });
 
 test('finish is emitted if last chunk is empty', function(t) {
-  var w = new W();
+  const w = new W();
   w._write = function(chunk, e, cb) {
     process.nextTick(cb);
   };
diff --git a/test/parallel/test-stream3-cork-end.js b/test/parallel/test-stream3-cork-end.js
index a0b611d0da..230227e0b1 100644
--- a/test/parallel/test-stream3-cork-end.js
+++ b/test/parallel/test-stream3-cork-end.js
@@ -12,11 +12,11 @@ const Writable = stream.Writable;
 // node version target: 0.12
 
 const expectedChunks = ['please', 'buffer', 'me', 'kindly'];
-var inputChunks = expectedChunks.slice(0);
-var seenChunks = [];
-var seenEnd = false;
+const inputChunks = expectedChunks.slice(0);
+let seenChunks = [];
+let seenEnd = false;
 
-var w = new Writable();
+const w = new Writable();
 // lets arrange to store the chunks
 w._write = function(chunk, encoding, cb) {
   // stream end event is not seen before the last write
@@ -33,8 +33,8 @@ w.on('finish', () => {
 });
 
 function writeChunks(remainingChunks, callback) {
-  var writeChunk = remainingChunks.shift();
-  var writeState;
+  const writeChunk = remainingChunks.shift();
+  let writeState;
 
   if (writeChunk) {
     setImmediate(() => {
@@ -74,12 +74,12 @@ writeChunks(inputChunks, () => {
   assert.equal(seenChunks.length, 4);
 
   // did the chunks match
-  for (var i = 0, l = expectedChunks.length; i < l; i++) {
-    var seen = seenChunks[i];
+  for (let i = 0, l = expectedChunks.length; i < l; i++) {
+    const seen = seenChunks[i];
     // there was a chunk
     assert.ok(seen);
 
-    var expected = new Buffer(expectedChunks[i]);
+    const expected = new Buffer(expectedChunks[i]);
     // it was what we expected
     assert.ok(seen.equals(expected));
   }
diff --git a/test/parallel/test-stream3-cork-uncork.js b/test/parallel/test-stream3-cork-uncork.js
index cd86e8cf80..821fd22bf8 100644
--- a/test/parallel/test-stream3-cork-uncork.js
+++ b/test/parallel/test-stream3-cork-uncork.js
@@ -12,11 +12,11 @@ const Writable = stream.Writable;
 // node version target: 0.12
 
 const expectedChunks = ['please', 'buffer', 'me', 'kindly'];
-var inputChunks = expectedChunks.slice(0);
-var seenChunks = [];
-var seenEnd = false;
+const inputChunks = expectedChunks.slice(0);
+let seenChunks = [];
+let seenEnd = false;
 
-var w = new Writable();
+const w = new Writable();
 // lets arrange to store the chunks
 w._write = function(chunk, encoding, cb) {
   // default encoding given none was specified
@@ -31,8 +31,8 @@ w.on('finish', () => {
 });
 
 function writeChunks(remainingChunks, callback) {
-  var writeChunk = remainingChunks.shift();
-  var writeState;
+  const writeChunk = remainingChunks.shift();
+  let writeState;
 
   if (writeChunk) {
     setImmediate(() => {
@@ -69,12 +69,12 @@ writeChunks(inputChunks, () => {
   assert.equal(seenChunks.length, 4);
 
   // did the chunks match
-  for (var i = 0, l = expectedChunks.length; i < l; i++) {
-    var seen = seenChunks[i];
+  for (let i = 0, l = expectedChunks.length; i < l; i++) {
+    const seen = seenChunks[i];
     // there was a chunk
     assert.ok(seen);
 
-    var expected = new Buffer(expectedChunks[i]);
+    const expected = new Buffer(expectedChunks[i]);
     // it was what we expected
     assert.ok(seen.equals(expected));
   }
diff --git a/test/parallel/test-stream3-pause-then-read.js b/test/parallel/test-stream3-pause-then-read.js
index 996f91cf79..5a9550bcd4 100644
--- a/test/parallel/test-stream3-pause-then-read.js
+++ b/test/parallel/test-stream3-pause-then-read.js
@@ -3,16 +3,16 @@ require('../common');
 const assert = require('assert');
 
 const stream = require('stream');
-var Readable = stream.Readable;
-var Writable = stream.Writable;
+const Readable = stream.Readable;
+const Writable = stream.Writable;
 
-var totalChunks = 100;
-var chunkSize = 99;
-var expectTotalData = totalChunks * chunkSize;
-var expectEndingData = expectTotalData;
+const totalChunks = 100;
+const chunkSize = 99;
+const expectTotalData = totalChunks * chunkSize;
+let expectEndingData = expectTotalData;
 
-var r = new Readable({ highWaterMark: 1000 });
-var chunks = totalChunks;
+const r = new Readable({ highWaterMark: 1000 });
+let chunks = totalChunks;
 r._read = function(n) {
   if (!(chunks % 2))
     setImmediate(push);
@@ -22,9 +22,9 @@ r._read = function(n) {
     push();
 };
 
-var totalPushed = 0;
+let totalPushed = 0;
 function push() {
-  var chunk = chunks-- > 0 ? Buffer.alloc(chunkSize, 'x') : null;
+  const chunk = chunks-- > 0 ? Buffer.alloc(chunkSize, 'x') : null;
   if (chunk) {
     totalPushed += chunk.length;
   }
@@ -42,7 +42,7 @@ function readn(n, then) {
   console.error('read %d', n);
   expectEndingData -= n;
   (function read() {
-    var c = r.read(n);
+    const c = r.read(n);
     if (!c)
       r.once('readable', read);
     else {
@@ -57,7 +57,7 @@ function readn(n, then) {
 function onData() {
   expectEndingData -= 100;
   console.error('onData');
-  var seen = 0;
+  let seen = 0;
   r.on('data', function od(c) {
     seen += c.length;
     if (seen >= 100) {
@@ -67,7 +67,7 @@ function onData() {
       if (seen > 100) {
         // oh no, seen too much!
         // put the extra back.
-        var diff = seen - 100;
+        const diff = seen - 100;
         r.unshift(c.slice(c.length - diff));
         console.error('seen too much', seen, diff);
       }
@@ -82,8 +82,8 @@ function onData() {
 function pipeLittle() {
   expectEndingData -= 200;
   console.error('pipe a little');
-  var w = new Writable();
-  var written = 0;
+  const w = new Writable();
+  let written = 0;
   w.on('finish', function() {
     assert.equal(written, 200);
     setImmediate(read1234);
@@ -95,7 +95,7 @@ function pipeLittle() {
       w.end();
       cb();
       if (written > 200) {
-        var diff = written - 200;
+        const diff = written - 200;
         written -= diff;
         r.unshift(chunk.slice(chunk.length - diff));
       }
@@ -130,8 +130,8 @@ function resumePause() {
 
 function pipe() {
   console.error('pipe the rest');
-  var w = new Writable();
-  var written = 0;
+  const w = new Writable();
+  let written = 0;
   w._write = function(chunk, encoding, cb) {
     written += chunk.length;
     cb();
diff --git a/test/parallel/test-string-decoder-end.js b/test/parallel/test-string-decoder-end.js
index 9b6696977c..2c187d6014 100644
--- a/test/parallel/test-string-decoder-end.js
+++ b/test/parallel/test-string-decoder-end.js
@@ -6,15 +6,15 @@
 require('../common');
 const assert = require('assert');
 const SD = require('string_decoder').StringDecoder;
-var encodings = ['base64', 'hex', 'utf8', 'utf16le', 'ucs2'];
+const encodings = ['base64', 'hex', 'utf8', 'utf16le', 'ucs2'];
 
-var bufs = [ '☃💩', 'asdf' ].map(function(b) {
+const bufs = [ '☃💩', 'asdf' ].map(function(b) {
   return Buffer.from(b);
 });
 
 // also test just arbitrary bytes from 0-15.
-for (var i = 1; i <= 16; i++) {
-  var bytes = new Array(i).join('.').split('.').map(function(_, j) {
+for (let i = 1; i <= 16; i++) {
+  const bytes = new Array(i).join('.').split('.').map(function(_, j) {
     return j + 0x78;
   });
   bufs.push(Buffer.from(bytes));
@@ -34,21 +34,21 @@ function testBuf(encoding, buf) {
   console.error('# %s', encoding, buf);
 
   // write one byte at a time.
-  var s = new SD(encoding);
-  var res1 = '';
-  for (var i = 0; i < buf.length; i++) {
+  let s = new SD(encoding);
+  let res1 = '';
+  for (let i = 0; i < buf.length; i++) {
     res1 += s.write(buf.slice(i, i + 1));
   }
   res1 += s.end();
 
   // write the whole buffer at once.
-  var res2 = '';
+  let res2 = '';
   s = new SD(encoding);
   res2 += s.write(buf);
   res2 += s.end();
 
   // .toString() on the buffer
-  var res3 = buf.toString(encoding);
+  const res3 = buf.toString(encoding);
 
   console.log('expect=%j', res3);
   console.log('res1=%j', res1);
diff --git a/test/parallel/test-string-decoder.js b/test/parallel/test-string-decoder.js
index 97c0aafef2..77ffbfcc73 100644
--- a/test/parallel/test-string-decoder.js
+++ b/test/parallel/test-string-decoder.js
@@ -5,7 +5,7 @@ const inspect = require('util').inspect;
 const StringDecoder = require('string_decoder').StringDecoder;
 
 // Test default encoding
-var decoder = new StringDecoder();
+let decoder = new StringDecoder();
 assert.strictEqual(decoder.encoding, 'utf8');
 
 process.stdout.write('scanning ');
@@ -110,22 +110,22 @@ assert.strictEqual(decoder.end(), '\ud83d');
 // singleSequence allows for easy debugging of a specific sequence which is
 // useful in case of test failures.
 function test(encoding, input, expected, singleSequence) {
-  var sequences;
+  let sequences;
   if (!singleSequence) {
     sequences = writeSequences(input.length);
   } else {
     sequences = [singleSequence];
   }
   sequences.forEach(function(sequence) {
-    var decoder = new StringDecoder(encoding);
-    var output = '';
+    const decoder = new StringDecoder(encoding);
+    let output = '';
     sequence.forEach(function(write) {
       output += decoder.write(input.slice(write[0], write[1]));
     });
     output += decoder.end();
     process.stdout.write('.');
     if (output !== expected) {
-      var message =
+      const message =
         'Expected "' + unicodeEscape(expected) + '", ' +
         'but got "' + unicodeEscape(output) + '"\n' +
         'input: ' + input.toString('hex').match(/.{2}/g) + '\n' +
@@ -138,8 +138,8 @@ function test(encoding, input, expected, singleSequence) {
 
 // unicodeEscape prints the str contents as unicode escape codes.
 function unicodeEscape(str) {
-  var r = '';
-  for (var i = 0; i < str.length; i++) {
+  let r = '';
+  for (let i = 0; i < str.length; i++) {
     r += '\\u' + str.charCodeAt(i).toString(16);
   }
   return r;
@@ -162,10 +162,10 @@ function writeSequences(length, start, sequence) {
   } else if (start === length) {
     return [sequence];
   }
-  var sequences = [];
-  for (var end = length; end > start; end--) {
-    var subSequence = sequence.concat([[start, end]]);
-    var subSequences = writeSequences(length, end, subSequence, sequences);
+  let sequences = [];
+  for (let end = length; end > start; end--) {
+    const subSequence = sequence.concat([[start, end]]);
+    const subSequences = writeSequences(length, end, subSequence, sequences);
     sequences = sequences.concat(subSequences);
   }
   return sequences;
diff --git a/test/parallel/test-stringbytes-external.js b/test/parallel/test-stringbytes-external.js
index e0d8c4552e..a242b943c9 100644
--- a/test/parallel/test-stringbytes-external.js
+++ b/test/parallel/test-stringbytes-external.js
@@ -2,17 +2,17 @@
 require('../common');
 const assert = require('assert');
 // minimum string size to overflow into external string space
-var EXTERN_APEX = 0xFBEE9;
+const EXTERN_APEX = 0xFBEE9;
 
 // manually controlled string for checking binary output
-var ucs2_control = 'a\u0000';
-var write_str = 'a';
+let ucs2_control = 'a\u0000';
+let write_str = 'a';
 
 
 // first do basic checks
-var b = Buffer.from(write_str, 'ucs2');
+let b = Buffer.from(write_str, 'ucs2');
 // first check latin1
-var c = b.toString('latin1');
+let c = b.toString('latin1');
 assert.equal(b[0], 0x61);
 assert.equal(b[1], 0);
 assert.equal(ucs2_control, c);
@@ -23,7 +23,7 @@ assert.equal(b[1], 0);
 assert.equal(ucs2_control, c);
 
 // now create big strings
-var size = 1 + (1 << 20);
+const size = 1 + (1 << 20);
 write_str = Array(size).join(write_str);
 ucs2_control = Array(size).join(ucs2_control);
 
@@ -36,19 +36,19 @@ for (let i = 0; i < b.length; i += 2) {
 }
 
 // create another string to create an external string
-var b_ucs = b.toString('ucs2');
+const b_ucs = b.toString('ucs2');
 
 // check control against external binary string
-var l_bin = b.toString('latin1');
+const l_bin = b.toString('latin1');
 assert.equal(ucs2_control, l_bin);
 
 // check control against external binary string
-var b_bin = b.toString('binary');
+const b_bin = b.toString('binary');
 assert.equal(ucs2_control, b_bin);
 
 // create buffer copy from external
-var c_bin = Buffer.from(l_bin, 'latin1');
-var c_ucs = Buffer.from(b_ucs, 'ucs2');
+const c_bin = Buffer.from(l_bin, 'latin1');
+const c_ucs = Buffer.from(b_ucs, 'ucs2');
 // make sure they're the same length
 assert.equal(c_bin.length, c_ucs.length);
 // make sure Buffers from externals are the same
@@ -62,9 +62,9 @@ assert.equal(c_ucs.toString('latin1'), ucs2_control);
 
 
 // now let's test BASE64 and HEX ecoding/decoding
-var RADIOS = 2;
-var PRE_HALF_APEX = Math.ceil(EXTERN_APEX / 2) - RADIOS;
-var PRE_3OF4_APEX = Math.ceil((EXTERN_APEX / 4) * 3) - RADIOS;
+const RADIOS = 2;
+const PRE_HALF_APEX = Math.ceil(EXTERN_APEX / 2) - RADIOS;
+const PRE_3OF4_APEX = Math.ceil((EXTERN_APEX / 4) * 3) - RADIOS;
 
 {
   for (let j = 0; j < RADIOS * 2; j += 1) {
diff --git a/test/parallel/test-sync-io-option.js b/test/parallel/test-sync-io-option.js
index fec61ef62f..6166bffc0b 100644
--- a/test/parallel/test-sync-io-option.js
+++ b/test/parallel/test-sync-io-option.js
@@ -12,9 +12,9 @@ if (process.argv[2] === 'child') {
 
 } else {
   (function runTest(flags) {
-    var execArgv = [flags.pop()];
-    var args = [__filename, 'child'];
-    var cntr = 0;
+    const execArgv = [flags.pop()];
+    let args = [__filename, 'child'];
+    let cntr = 0;
     args = execArgv.concat(args);
     if (!args[0]) args.shift();
     execFile(process.execPath, args, function(err, stdout, stderr) {
diff --git a/test/parallel/test-tcp-wrap-connect.js b/test/parallel/test-tcp-wrap-connect.js
index 92114bad39..77f9814db2 100644
--- a/test/parallel/test-tcp-wrap-connect.js
+++ b/test/parallel/test-tcp-wrap-connect.js
@@ -1,15 +1,15 @@
 'use strict';
 require('../common');
 const assert = require('assert');
-var TCP = process.binding('tcp_wrap').TCP;
-var TCPConnectWrap = process.binding('tcp_wrap').TCPConnectWrap;
-var ShutdownWrap = process.binding('stream_wrap').ShutdownWrap;
+const TCP = process.binding('tcp_wrap').TCP;
+const TCPConnectWrap = process.binding('tcp_wrap').TCPConnectWrap;
+const ShutdownWrap = process.binding('stream_wrap').ShutdownWrap;
 
 function makeConnection() {
-  var client = new TCP();
+  const client = new TCP();
 
-  var req = new TCPConnectWrap();
-  var err = client.connect(req, '127.0.0.1', this.address().port);
+  const req = new TCPConnectWrap();
+  const err = client.connect(req, '127.0.0.1', this.address().port);
   assert.strictEqual(err, 0);
 
   req.oncomplete = function(status, client_, req_, readable, writable) {
@@ -19,8 +19,8 @@ function makeConnection() {
     assert.strictEqual(true, readable);
     assert.strictEqual(true, writable);
 
-    var shutdownReq = new ShutdownWrap();
-    var err = client.shutdown(shutdownReq);
+    const shutdownReq = new ShutdownWrap();
+    const err = client.shutdown(shutdownReq);
     assert.strictEqual(err, 0);
 
     shutdownReq.oncomplete = function(status, client_, req_) {
@@ -35,9 +35,9 @@ function makeConnection() {
 
 /////
 
-var connectCount = 0;
-var endCount = 0;
-var shutdownCount = 0;
+let connectCount = 0;
+let endCount = 0;
+let shutdownCount = 0;
 
 const server = require('net').Server(function(s) {
   connectCount++;
diff --git a/test/parallel/test-tcp-wrap.js b/test/parallel/test-tcp-wrap.js
index 0d023ebf8d..6b9dbd90a4 100644
--- a/test/parallel/test-tcp-wrap.js
+++ b/test/parallel/test-tcp-wrap.js
@@ -2,17 +2,17 @@
 require('../common');
 const assert = require('assert');
 
-var TCP = process.binding('tcp_wrap').TCP;
-var uv = process.binding('uv');
+const TCP = process.binding('tcp_wrap').TCP;
+const uv = process.binding('uv');
 
-var handle = new TCP();
+const handle = new TCP();
 
 // Should be able to bind to the port
-var err = handle.bind('0.0.0.0', 0);
+let err = handle.bind('0.0.0.0', 0);
 assert.equal(err, 0);
 
 // Should not be able to bind to the same port again
-var out = {};
+const out = {};
 handle.getsockname(out);
 err = handle.bind('0.0.0.0', out.port);
 assert.equal(err, uv.UV_EINVAL);
diff --git a/test/parallel/test-timers-active.js b/test/parallel/test-timers-active.js
index 8677e43971..62707cf18c 100644
--- a/test/parallel/test-timers-active.js
+++ b/test/parallel/test-timers-active.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const active = require('timers').active;
 
 // active() should create timers for these
-var legitTimers = [
+const legitTimers = [
   { _idleTimeout: 0 },
   { _idleTimeout: 1 }
 ];
@@ -21,7 +21,7 @@ legitTimers.forEach(function(legit) {
 
 
 // active() should not create a timer for these
-var bogusTimers = [
+const bogusTimers = [
   { _idleTimeout: -1 },
   { _idleTimeout: undefined },
 ];
diff --git a/test/parallel/test-timers-clearImmediate.js b/test/parallel/test-timers-clearImmediate.js
index ab65b7bf1c..c49fd7622f 100644
--- a/test/parallel/test-timers-clearImmediate.js
+++ b/test/parallel/test-timers-clearImmediate.js
@@ -3,14 +3,14 @@ require('../common');
 const assert = require('assert');
 
 const N = 3;
-var count = 0;
+let count = 0;
 function next() {
   const immediate = setImmediate(function() {
     clearImmediate(immediate);
     ++count;
   });
 }
-for (var i = 0; i < N; ++i)
+for (let i = 0; i < N; ++i)
   next();
 
 process.on('exit', () => {
diff --git a/test/parallel/test-timers-immediate-queue.js b/test/parallel/test-timers-immediate-queue.js
index f53f18140f..103cd96451 100644
--- a/test/parallel/test-timers-immediate-queue.js
+++ b/test/parallel/test-timers-immediate-queue.js
@@ -24,7 +24,7 @@ function run() {
   setImmediate(run);
 }
 
-for (var i = 0; i < QUEUE; i++)
+for (let i = 0; i < QUEUE; i++)
   setImmediate(run);
 
 process.on('exit', function() {
diff --git a/test/parallel/test-timers-immediate.js b/test/parallel/test-timers-immediate.js
index 9632022917..4afc642a33 100644
--- a/test/parallel/test-timers-immediate.js
+++ b/test/parallel/test-timers-immediate.js
@@ -2,7 +2,6 @@
 const common = require('../common');
 const assert = require('assert');
 
-let immediateB;
 let immediateC;
 let immediateD;
 
@@ -13,7 +12,7 @@ setImmediate(common.mustCall(function() {
   clearImmediate(immediateB);
 }));
 
-immediateB = setImmediate(function() {
+const immediateB = setImmediate(function() {
   common.fail('this immediate should not run');
 });
 
diff --git a/test/parallel/test-timers-nested.js b/test/parallel/test-timers-nested.js
index d5d272ed2e..0ff228abf5 100644
--- a/test/parallel/test-timers-nested.js
+++ b/test/parallel/test-timers-nested.js
@@ -11,7 +11,7 @@ const assert = require('assert');
 const scenarios = [0, 100];
 
 scenarios.forEach(function(delay) {
-  var nestedCalled = false;
+  let nestedCalled = false;
 
   setTimeout(function A() {
     // Create the nested timer with the same delay as the outer timer so that it
diff --git a/test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js b/test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js
index 8670c3c2b6..f42144360b 100644
--- a/test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js
+++ b/test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js
@@ -32,7 +32,7 @@ const server = net.createServer(function onClient(client) {
 });
 
 server.listen(0, common.localhostIPv4, function() {
-  var nbClientsEnded = 0;
+  let nbClientsEnded = 0;
 
   function addEndedClient(client) {
     ++nbClientsEnded;
diff --git a/test/parallel/test-timers-unref-active.js b/test/parallel/test-timers-unref-active.js
index 69027277ea..8b1c89673b 100644
--- a/test/parallel/test-timers-unref-active.js
+++ b/test/parallel/test-timers-unref-active.js
@@ -19,8 +19,8 @@ require('../common');
 const timers = require('timers');
 const assert = require('assert');
 
-var someObject = {};
-var nbTimeouts = 0;
+const someObject = {};
+let nbTimeouts = 0;
 
 /*
  * libuv 0.10.x uses GetTickCount on Windows to implement timers, which uses
diff --git a/test/parallel/test-timers-unref-call.js b/test/parallel/test-timers-unref-call.js
index edd5865e8f..5384441838 100644
--- a/test/parallel/test-timers-unref-call.js
+++ b/test/parallel/test-timers-unref-call.js
@@ -1,12 +1,12 @@
 'use strict';
 require('../common');
 
-var Timer = process.binding('timer_wrap').Timer;
+const Timer = process.binding('timer_wrap').Timer;
 Timer.now = function() { return ++Timer.now.ticks; };
 Timer.now.ticks = 0;
 
-var t = setInterval(function() {}, 1);
-var o = { _idleStart: 0, _idleTimeout: 1 };
+const t = setInterval(function() {}, 1);
+const o = { _idleStart: 0, _idleTimeout: 1 };
 t.unref.call(o);
 
 setTimeout(clearInterval.bind(null, t), 2);
diff --git a/test/parallel/test-timers-unref-leak.js b/test/parallel/test-timers-unref-leak.js
index 6a1f275ae5..d970910b24 100644
--- a/test/parallel/test-timers-unref-leak.js
+++ b/test/parallel/test-timers-unref-leak.js
@@ -2,16 +2,16 @@
 require('../common');
 const assert = require('assert');
 
-var called = 0;
-var closed = 0;
+let called = 0;
+let closed = 0;
 
-var timeout = setTimeout(function() {
+const timeout = setTimeout(function() {
   called++;
 }, 10);
 timeout.unref();
 
 // Wrap `close` method to check if the handle was closed
-var close = timeout._handle.close;
+const close = timeout._handle.close;
 timeout._handle.close = function() {
   closed++;
   return close.apply(this, arguments);
diff --git a/test/parallel/test-timers-unref-remove-other-unref-timers-only-one-fires.js b/test/parallel/test-timers-unref-remove-other-unref-timers-only-one-fires.js
index 03c6cf7fd3..ce63ad8d29 100644
--- a/test/parallel/test-timers-unref-remove-other-unref-timers-only-one-fires.js
+++ b/test/parallel/test-timers-unref-remove-other-unref-timers-only-one-fires.js
@@ -14,7 +14,7 @@ require('../common');
 const timers = require('timers');
 const assert = require('assert');
 
-var nbTimersFired = 0;
+let nbTimersFired = 0;
 
 const foo = {
   _onTimeout: function() {
diff --git a/test/parallel/test-timers-unref.js b/test/parallel/test-timers-unref.js
index c47931b965..2bde08f914 100644
--- a/test/parallel/test-timers-unref.js
+++ b/test/parallel/test-timers-unref.js
@@ -9,8 +9,8 @@ let unref_timer = false;
 let unref_callbacks = 0;
 let checks = 0;
 
-var LONG_TIME = 10 * 1000;
-var SHORT_TIME = 100;
+const LONG_TIME = 10 * 1000;
+const SHORT_TIME = 100;
 
 assert.doesNotThrow(function() {
   setTimeout(function() {}, 10).unref().ref().unref();
diff --git a/test/parallel/test-timers-unrefd-interval-still-fires.js b/test/parallel/test-timers-unrefd-interval-still-fires.js
index 39bb2cdb13..bf16013f00 100644
--- a/test/parallel/test-timers-unrefd-interval-still-fires.js
+++ b/test/parallel/test-timers-unrefd-interval-still-fires.js
@@ -6,7 +6,7 @@ const common = require('../common');
 
 const TEST_DURATION = common.platformTimeout(1000);
 const N = 3;
-var nbIntervalFired = 0;
+let nbIntervalFired = 0;
 
 const keepOpen = setTimeout(() => {
   console.error('[FAIL] Interval fired %d/%d times.', nbIntervalFired, N);
diff --git a/test/parallel/test-timers-unrefed-in-beforeexit.js b/test/parallel/test-timers-unrefed-in-beforeexit.js
index 0ac3311816..530d97674d 100644
--- a/test/parallel/test-timers-unrefed-in-beforeexit.js
+++ b/test/parallel/test-timers-unrefed-in-beforeexit.js
@@ -3,7 +3,7 @@
 require('../common');
 const assert = require('assert');
 
-var once = 0;
+let once = 0;
 
 process.on('beforeExit', () => {
   if (once > 1)
diff --git a/test/parallel/test-timers.js b/test/parallel/test-timers.js
index 38d8fd86b1..33f1706e3d 100644
--- a/test/parallel/test-timers.js
+++ b/test/parallel/test-timers.js
@@ -2,7 +2,7 @@
 require('../common');
 const assert = require('assert');
 
-var inputs = [
+const inputs = [
   undefined,
   null,
   true,
@@ -29,15 +29,15 @@ var inputs = [
   12345678901234  // ditto
 ];
 
-var timeouts = [];
-var intervals = [];
+const timeouts = [];
+const intervals = [];
 
 inputs.forEach(function(value, index) {
   setTimeout(function() {
     timeouts[index] = true;
   }, value);
 
-  var handle = setInterval(function() {
+  const handle = setInterval(function() {
     clearInterval(handle); // disarm timer or we'll never finish
     intervals[index] = true;
   }, value);
diff --git a/test/parallel/test-tls-alert.js b/test/parallel/test-tls-alert.js
index 59cd355f35..a8c6016f62 100644
--- a/test/parallel/test-tls-alert.js
+++ b/test/parallel/test-tls-alert.js
@@ -16,7 +16,7 @@ const tls = require('tls');
 const fs = require('fs');
 const spawn = require('child_process').spawn;
 
-var success = false;
+let success = false;
 
 function filenamePEM(n) {
   return require('path').join(common.fixturesDir, 'keys', n + '.pem');
@@ -26,20 +26,20 @@ function loadPEM(n) {
   return fs.readFileSync(filenamePEM(n));
 }
 
-var server = tls.Server({
+const server = tls.Server({
   secureProtocol: 'TLSv1_2_server_method',
   key: loadPEM('agent2-key'),
   cert: loadPEM('agent2-cert')
 }, null).listen(0, function() {
-  var args = ['s_client', '-quiet', '-tls1_1',
-              '-connect', `127.0.0.1:${this.address().port}`];
+  const args = ['s_client', '-quiet', '-tls1_1',
+                '-connect', `127.0.0.1:${this.address().port}`];
 
   // for the performance and stability issue in s_client on Windows
   if (common.isWindows)
     args.push('-no_rand_screen');
 
-  var client = spawn(common.opensslCli, args);
-  var out = '';
+  const client = spawn(common.opensslCli, args);
+  let out = '';
   client.stderr.setEncoding('utf8');
   client.stderr.on('data', function(d) {
     out += d;
diff --git a/test/parallel/test-tls-alpn-server-client.js b/test/parallel/test-tls-alpn-server-client.js
index 0da2086f4d..ca5785b09a 100644
--- a/test/parallel/test-tls-alpn-server-client.js
+++ b/test/parallel/test-tls-alpn-server-client.js
@@ -24,7 +24,7 @@ function loadPEM(n) {
   return fs.readFileSync(filenamePEM(n));
 }
 
-var serverIP = common.localhostIPv4;
+const serverIP = common.localhostIPv4;
 
 function checkResults(result, expected) {
   assert.strictEqual(result.server.ALPN, expected.server.ALPN);
@@ -36,9 +36,9 @@ function checkResults(result, expected) {
 function runTest(clientsOptions, serverOptions, cb) {
   serverOptions.key = loadPEM('agent2-key');
   serverOptions.cert = loadPEM('agent2-cert');
-  var results = [];
-  var index = 0;
-  var server = tls.createServer(serverOptions, function(c) {
+  const results = [];
+  let index = 0;
+  const server = tls.createServer(serverOptions, function(c) {
     results[index].server = {ALPN: c.alpnProtocol, NPN: c.npnProtocol};
   });
 
@@ -47,13 +47,13 @@ function runTest(clientsOptions, serverOptions, cb) {
   });
 
   function connectClient(options) {
-    var opt = options.shift();
+    const opt = options.shift();
     opt.port = server.address().port;
     opt.host = serverIP;
     opt.rejectUnauthorized = false;
 
     results[index] = {};
-    var client = tls.connect(opt, function() {
+    const client = tls.connect(opt, function() {
       results[index].client = {ALPN: client.alpnProtocol,
                                NPN: client.npnProtocol};
       client.destroy();
@@ -71,12 +71,12 @@ function runTest(clientsOptions, serverOptions, cb) {
 
 // Server: ALPN/NPN, Client: ALPN/NPN
 function Test1() {
-  var serverOptions = {
+  const serverOptions = {
     ALPNProtocols: ['a', 'b', 'c'],
     NPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     ALPNProtocols: ['a', 'b', 'c'],
     NPNProtocols: ['a', 'b', 'c']
   }, {
@@ -107,12 +107,12 @@ function Test1() {
 
 // Server: ALPN/NPN, Client: ALPN
 function Test2() {
-  var serverOptions = {
+  const serverOptions = {
     ALPNProtocols: ['a', 'b', 'c'],
     NPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     ALPNProtocols: ['a', 'b', 'c']
   }, {
     ALPNProtocols: ['c', 'b', 'e']
@@ -140,12 +140,12 @@ function Test2() {
 
 // Server: ALPN/NPN, Client: NPN
 function Test3() {
-  var serverOptions = {
+  const serverOptions = {
     ALPNProtocols: ['a', 'b', 'c'],
     NPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     NPNProtocols: ['a', 'b', 'c']
   }, {
     NPPNProtocols: ['c', 'b', 'e']
@@ -173,12 +173,12 @@ function Test3() {
 
 // Server: ALPN/NPN, Client: Nothing
 function Test4() {
-  var serverOptions = {
+  const serverOptions = {
     ALPNProtocols: ['a', 'b', 'c'],
     NPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{}, {}, {}];
+  const clientsOptions = [{}, {}, {}];
 
   runTest(clientsOptions, serverOptions, function(results) {
     // nothing is selected by ALPN
@@ -200,11 +200,11 @@ function Test4() {
 
 // Server: ALPN, Client: ALPN/NPN
 function Test5() {
-  var serverOptions = {
+  const serverOptions = {
     ALPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     ALPNProtocols: ['a', 'b', 'c'],
     NPNProtocols: ['a', 'b', 'c']
   }, {
@@ -233,11 +233,11 @@ function Test5() {
 
 // Server: ALPN, Client: ALPN
 function Test6() {
-  var serverOptions = {
+  const serverOptions = {
     ALPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     ALPNProtocols: ['a', 'b', 'c']
   }, {
     ALPNProtocols: ['c', 'b', 'e']
@@ -262,11 +262,11 @@ function Test6() {
 
 // Server: ALPN, Client: NPN
 function Test7() {
-  var serverOptions = {
+  const serverOptions = {
     ALPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     NPNProtocols: ['a', 'b', 'c']
   }, {
     NPNProtocols: ['c', 'b', 'e']
@@ -292,11 +292,11 @@ function Test7() {
 
 // Server: ALPN, Client: Nothing
 function Test8() {
-  var serverOptions = {
+  const serverOptions = {
     ALPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{}, {}, {}];
+  const clientsOptions = [{}, {}, {}];
 
   runTest(clientsOptions, serverOptions, function(results) {
     // nothing is selected by ALPN
@@ -316,11 +316,11 @@ function Test8() {
 
 // Server: NPN, Client: ALPN/NPN
 function Test9() {
-  var serverOptions = {
+  const serverOptions = {
     NPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     ALPNrotocols: ['a', 'b', 'c'],
     NPNProtocols: ['a', 'b', 'c']
   }, {
@@ -349,11 +349,11 @@ function Test9() {
 
 // Server: NPN, Client: ALPN
 function Test10() {
-  var serverOptions = {
+  const serverOptions = {
     NPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     ALPNProtocols: ['a', 'b', 'c']
   }, {
     ALPNProtocols: ['c', 'b', 'e']
@@ -378,11 +378,11 @@ function Test10() {
 
 // Server: NPN, Client: NPN
 function Test11() {
-  var serverOptions = {
+  const serverOptions = {
     NPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     NPNProtocols: ['a', 'b', 'c']
   }, {
     NPNProtocols: ['c', 'b', 'e']
@@ -408,11 +408,11 @@ function Test11() {
 
 // Server: NPN, Client: Nothing
 function Test12() {
-  var serverOptions = {
+  const serverOptions = {
     NPNProtocols: ['a', 'b', 'c']
   };
 
-  var clientsOptions = [{}, {}, {}];
+  const clientsOptions = [{}, {}, {}];
 
   runTest(clientsOptions, serverOptions, function(results) {
     // nothing is selected
@@ -432,9 +432,9 @@ function Test12() {
 
 // Server: Nothing, Client: ALPN/NPN
 function Test13() {
-  var serverOptions = {};
+  const serverOptions = {};
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     ALPNrotocols: ['a', 'b', 'c'],
     NPNProtocols: ['a', 'b', 'c']
   }, {
@@ -463,9 +463,9 @@ function Test13() {
 
 // Server: Nothing, Client: ALPN
 function Test14() {
-  var serverOptions = {};
+  const serverOptions = {};
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     ALPNrotocols: ['a', 'b', 'c']
   }, {
     ALPNProtocols: ['c', 'b', 'e']
@@ -491,9 +491,9 @@ function Test14() {
 
 // Server: Nothing, Client: NPN
 function Test15() {
-  var serverOptions = {};
+  const serverOptions = {};
 
-  var clientsOptions = [{
+  const clientsOptions = [{
     NPNProtocols: ['a', 'b', 'c']
   }, {
     NPNProtocols: ['c', 'b', 'e']
@@ -519,9 +519,9 @@ function Test15() {
 
 // Server: Nothing, Client: Nothing
 function Test16() {
-  var serverOptions = {};
+  const serverOptions = {};
 
-  var clientsOptions = [{}, {}, {}];
+  const clientsOptions = [{}, {}, {}];
 
   runTest(clientsOptions, serverOptions, function(results) {
     // nothing is selected
diff --git a/test/parallel/test-tls-async-cb-after-socket-end.js b/test/parallel/test-tls-async-cb-after-socket-end.js
index dc03e74c65..be499b8450 100644
--- a/test/parallel/test-tls-async-cb-after-socket-end.js
+++ b/test/parallel/test-tls-async-cb-after-socket-end.js
@@ -12,17 +12,17 @@ const SSL_OP_NO_TICKET = require('crypto').constants.SSL_OP_NO_TICKET;
 
 const tls = require('tls');
 
-var options = {
+const options = {
   secureOptions: SSL_OP_NO_TICKET,
   key: fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem')),
   cert: fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'))
 };
 
-var server = tls.createServer(options, function(c) {
+const server = tls.createServer(options, function(c) {
 });
 
-var sessionCb = null;
-var client = null;
+let sessionCb = null;
+let client = null;
 
 server.on('newSession', function(key, session, done) {
   done();
@@ -35,18 +35,18 @@ server.on('resumeSession', function(id, cb) {
 });
 
 server.listen(0, function() {
-  var clientOpts = {
+  const clientOpts = {
     port: this.address().port,
     rejectUnauthorized: false,
     session: false
   };
 
-  var s1 = tls.connect(clientOpts, function() {
+  const s1 = tls.connect(clientOpts, function() {
     clientOpts.session = s1.getSession();
     console.log('1st secure');
 
     s1.destroy();
-    var s2 = tls.connect(clientOpts, function(s) {
+    const s2 = tls.connect(clientOpts, function(s) {
       console.log('2nd secure');
 
       s2.destroy();
diff --git a/test/parallel/test-tls-cert-regression.js b/test/parallel/test-tls-cert-regression.js
index e74201bc2e..ff5827bcf8 100644
--- a/test/parallel/test-tls-cert-regression.js
+++ b/test/parallel/test-tls-cert-regression.js
@@ -8,7 +8,7 @@ if (!common.hasCrypto) {
 const tls = require('tls');
 
 
-var cert =
+const cert =
 `-----BEGIN CERTIFICATE-----
 MIIBfjCCASgCCQDmmNjAojbDQjANBgkqhkiG9w0BAQUFADBFMQswCQYDVQQGEwJB
 VTETMBEGA1UECBMKU29tZS1TdGF0ZTEhMB8GA1UEChMYSW50ZXJuZXQgV2lkZ2l0
@@ -21,7 +21,7 @@ uvCyndyeB4d24FlfqEUlkfaWCZlNKRaV9YhLDiEg3BcIreFo4brtKQfZzTRs0GVm
 KHg=
 -----END CERTIFICATE-----`;
 
-var key =
+const key =
 `-----BEGIN RSA PRIVATE KEY-----
 MIIBPQIBAAJBAPKwlfMX6HGZIt1xm7fna72eWcOYfUfSxSugghvqYgJt2Oi3lH+w
 sU1O9FzRIVmpeIjDXhbpMjsa1HtzSiccPXsCAwEAAQJBAM4uU9aJE0OfdE1p/X+K
@@ -33,7 +33,7 @@ AOaJnkQrmurlRdePX6LvN/LgGAQoxwovfjcOYNnZsIVY
 -----END RSA PRIVATE KEY-----`;
 
 function test(cert, key, cb) {
-  var server = tls.createServer({
+  const server = tls.createServer({
     cert: cert,
     key: key
   }).listen(0, function() {
diff --git a/test/parallel/test-tls-cipher-list.js b/test/parallel/test-tls-cipher-list.js
index 44ebd3d2f8..4acb30bc1c 100644
--- a/test/parallel/test-tls-cipher-list.js
+++ b/test/parallel/test-tls-cipher-list.js
@@ -11,7 +11,7 @@ const spawn = require('child_process').spawn;
 const defaultCoreList = require('crypto').constants.defaultCoreCipherList;
 
 function doCheck(arg, check) {
-  var out = '';
+  let out = '';
   arg = arg.concat([
     '-pe',
     'require("crypto").constants.defaultCipherList'
diff --git a/test/parallel/test-tls-client-abort.js b/test/parallel/test-tls-client-abort.js
index f48d22c4fc..21ee41e927 100644
--- a/test/parallel/test-tls-client-abort.js
+++ b/test/parallel/test-tls-client-abort.js
@@ -11,10 +11,10 @@ const tls = require('tls');
 const fs = require('fs');
 const path = require('path');
 
-var cert = fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'));
-var key = fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem'));
+const cert = fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'));
+const key = fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem'));
 
-var conn = tls.connect({cert: cert, key: key, port: common.PORT}, function() {
+const conn = tls.connect({cert: cert, key: key, port: common.PORT}, function() {
   assert.ok(false); // callback should never be executed
 });
 conn.on('error', function() {
diff --git a/test/parallel/test-tls-client-abort2.js b/test/parallel/test-tls-client-abort2.js
index dd224be874..3247a2899a 100644
--- a/test/parallel/test-tls-client-abort2.js
+++ b/test/parallel/test-tls-client-abort2.js
@@ -8,7 +8,7 @@ if (!common.hasCrypto) {
 }
 const tls = require('tls');
 
-var conn = tls.connect(common.PORT, common.fail);
+const conn = tls.connect(common.PORT, common.fail);
 conn.on('error', common.mustCall(function() {
   assert.doesNotThrow(function() {
     conn.destroy();
diff --git a/test/parallel/test-tls-client-default-ciphers.js b/test/parallel/test-tls-client-default-ciphers.js
index f8fdab1102..618178d4a4 100644
--- a/test/parallel/test-tls-client-default-ciphers.js
+++ b/test/parallel/test-tls-client-default-ciphers.js
@@ -11,7 +11,7 @@ const tls = require('tls');
 function Done() {}
 
 function test1() {
-  var ciphers = '';
+  let ciphers = '';
 
   tls.createSecureContext = function(options) {
     ciphers = options.ciphers;
diff --git a/test/parallel/test-tls-client-destroy-soon.js b/test/parallel/test-tls-client-destroy-soon.js
index 3e79230514..016fcfa4da 100644
--- a/test/parallel/test-tls-client-destroy-soon.js
+++ b/test/parallel/test-tls-client-destroy-soon.js
@@ -14,29 +14,29 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem')
 };
 
-var big = Buffer.alloc(2 * 1024 * 1024, 'Y');
+const big = Buffer.alloc(2 * 1024 * 1024, 'Y');
 
 // create server
-var server = tls.createServer(options, common.mustCall(function(socket) {
+const server = tls.createServer(options, common.mustCall(function(socket) {
   socket.end(big);
   socket.destroySoon();
 }));
 
 // start listening
 server.listen(0, common.mustCall(function() {
-  var client = tls.connect({
+  const client = tls.connect({
     port: this.address().port,
     rejectUnauthorized: false
   }, common.mustCall(function() {
-    var bytesRead = 0;
+    let bytesRead = 0;
 
     client.on('readable', function() {
-      var d = client.read();
+      const d = client.read();
       if (d)
         bytesRead += d.length;
     });
diff --git a/test/parallel/test-tls-client-getephemeralkeyinfo.js b/test/parallel/test-tls-client-getephemeralkeyinfo.js
index fb42991e14..b49d7131c4 100644
--- a/test/parallel/test-tls-client-getephemeralkeyinfo.js
+++ b/test/parallel/test-tls-client-getephemeralkeyinfo.js
@@ -12,27 +12,27 @@ const fs = require('fs');
 const key = fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem');
 const cert = fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem');
 
-var ntests = 0;
-var nsuccess = 0;
+let ntests = 0;
+let nsuccess = 0;
 
 function loadDHParam(n) {
-  var path = common.fixturesDir;
+  let path = common.fixturesDir;
   if (n !== 'error') path += '/keys';
   return fs.readFileSync(path + '/dh' + n + '.pem');
 }
 
-var cipherlist = {
+const cipherlist = {
   'NOT_PFS': 'AES128-SHA256',
   'DH': 'DHE-RSA-AES128-GCM-SHA256',
   'ECDH': 'ECDHE-RSA-AES128-GCM-SHA256'
 };
 
 function test(size, type, name, next) {
-  var cipher = type ? cipherlist[type] : cipherlist['NOT_PFS'];
+  const cipher = type ? cipherlist[type] : cipherlist['NOT_PFS'];
 
   if (name) tls.DEFAULT_ECDH_CURVE = name;
 
-  var options = {
+  const options = {
     key: key,
     cert: cert,
     ciphers: cipher
@@ -40,7 +40,7 @@ function test(size, type, name, next) {
 
   if (type === 'DH') options.dhparam = loadDHParam(size);
 
-  var server = tls.createServer(options, function(conn) {
+  const server = tls.createServer(options, function(conn) {
     assert.strictEqual(conn.getEphemeralKeyInfo(), null);
     conn.end();
   });
@@ -51,11 +51,11 @@ function test(size, type, name, next) {
   }));
 
   server.listen(0, '127.0.0.1', common.mustCall(function() {
-    var client = tls.connect({
+    const client = tls.connect({
       port: this.address().port,
       rejectUnauthorized: false
     }, function() {
-      var ekeyinfo = client.getEphemeralKeyInfo();
+      const ekeyinfo = client.getEphemeralKeyInfo();
       assert.strictEqual(ekeyinfo.type, type);
       assert.strictEqual(ekeyinfo.size, size);
       assert.strictEqual(ekeyinfo.name, name);
diff --git a/test/parallel/test-tls-client-mindhsize.js b/test/parallel/test-tls-client-mindhsize.js
index 55a32ad6c2..76e1034621 100644
--- a/test/parallel/test-tls-client-mindhsize.js
+++ b/test/parallel/test-tls-client-mindhsize.js
@@ -9,27 +9,27 @@ if (!common.hasCrypto) {
 const tls = require('tls');
 
 const fs = require('fs');
-var key = fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem');
-var cert = fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem');
+const key = fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem');
+const cert = fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem');
 
-var nsuccess = 0;
-var nerror = 0;
+let nsuccess = 0;
+let nerror = 0;
 
 function loadDHParam(n) {
-  var path = common.fixturesDir;
+  let path = common.fixturesDir;
   if (n !== 'error') path += '/keys';
   return fs.readFileSync(path + '/dh' + n + '.pem');
 }
 
 function test(size, err, next) {
-  var options = {
+  const options = {
     key: key,
     cert: cert,
     dhparam: loadDHParam(size),
     ciphers: 'DHE-RSA-AES128-GCM-SHA256'
   };
 
-  var server = tls.createServer(options, function(conn) {
+  const server = tls.createServer(options, function(conn) {
     conn.end();
   });
 
@@ -42,7 +42,7 @@ function test(size, err, next) {
     // client set minimum DH parameter size to 2048 bits so that
     // it fails when it make a connection to the tls server where
     // dhparams is 1024 bits
-    var client = tls.connect({
+    const client = tls.connect({
       minDHSize: 2048,
       port: this.address().port,
       rejectUnauthorized: false
diff --git a/test/parallel/test-tls-client-resume.js b/test/parallel/test-tls-client-resume.js
index 029f10d449..96109d7656 100644
--- a/test/parallel/test-tls-client-resume.js
+++ b/test/parallel/test-tls-client-resume.js
@@ -14,21 +14,21 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem')
 };
 
 // create server
-var server = tls.Server(options, common.mustCall(function(socket) {
+const server = tls.Server(options, common.mustCall(function(socket) {
   socket.end('Goodbye');
 }, 2));
 
 // start listening
 server.listen(0, function() {
 
-  var session1 = null;
-  var client1 = tls.connect({
+  let session1 = null;
+  const client1 = tls.connect({
     port: this.address().port,
     rejectUnauthorized: false
   }, function() {
@@ -40,13 +40,13 @@ server.listen(0, function() {
   client1.on('close', function() {
     console.log('close1');
 
-    var opts = {
+    const opts = {
       port: server.address().port,
       rejectUnauthorized: false,
       session: session1
     };
 
-    var client2 = tls.connect(opts, function() {
+    const client2 = tls.connect(opts, function() {
       console.log('connect2');
       assert.ok(client2.isSessionReused(), 'Session *should* be reused.');
     });
diff --git a/test/parallel/test-tls-client-verify.js b/test/parallel/test-tls-client-verify.js
index a1fa35acf3..70d96ef154 100644
--- a/test/parallel/test-tls-client-verify.js
+++ b/test/parallel/test-tls-client-verify.js
@@ -106,7 +106,7 @@ function testServers(index, servers, clientOptions, cb) {
 
 
 function runTest(testIndex) {
-  var tcase = testCases[testIndex];
+  const tcase = testCases[testIndex];
   if (!tcase) return;
 
   const clientOptions = {
diff --git a/test/parallel/test-tls-close-error.js b/test/parallel/test-tls-close-error.js
index 6b37d5c1ce..6172be813e 100644
--- a/test/parallel/test-tls-close-error.js
+++ b/test/parallel/test-tls-close-error.js
@@ -11,12 +11,12 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var server = tls.createServer({
+const server = tls.createServer({
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 }, function(c) {
 }).listen(0, common.mustCall(function() {
-  var c = tls.connect(this.address().port, function() {
+  const c = tls.connect(this.address().port, function() {
     assert(false, 'should not be called');
   });
 
diff --git a/test/parallel/test-tls-close-notify.js b/test/parallel/test-tls-close-notify.js
index 3dd2afbd6e..5d5bc10077 100644
--- a/test/parallel/test-tls-close-notify.js
+++ b/test/parallel/test-tls-close-notify.js
@@ -9,7 +9,7 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var server = tls.createServer({
+const server = tls.createServer({
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 }, function(c) {
@@ -17,7 +17,7 @@ var server = tls.createServer({
   if (c._handle.shutdownSSL() !== 1)
     c._handle.shutdownSSL();
 }).listen(0, common.mustCall(function() {
-  var c = tls.connect(this.address().port, {
+  const c = tls.connect(this.address().port, {
     rejectUnauthorized: false
   }, common.mustCall(function() {
     // Ensure that we receive 'end' event anyway
diff --git a/test/parallel/test-tls-connect-no-host.js b/test/parallel/test-tls-connect-no-host.js
index 2b81698664..2e4bcfbc6d 100644
--- a/test/parallel/test-tls-connect-no-host.js
+++ b/test/parallel/test-tls-connect-no-host.js
@@ -11,8 +11,8 @@ const assert = require('assert');
 const fs = require('fs');
 const path = require('path');
 
-var cert = fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'));
-var key = fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem'));
+const cert = fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'));
+const key = fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem'));
 
 // https://github.com/nodejs/node/issues/1489
 // tls.connect(options) with no options.host should accept a cert with
@@ -21,7 +21,7 @@ tls.createServer({
   key: key,
   cert: cert
 }).listen(0, function() {
-  var socket = tls.connect({
+  const socket = tls.connect({
     port: this.address().port,
     ca: cert,
     // No host set here. 'localhost' is the default,
diff --git a/test/parallel/test-tls-connect-pipe.js b/test/parallel/test-tls-connect-pipe.js
index 67a53c4ebf..de60ae0846 100644
--- a/test/parallel/test-tls-connect-pipe.js
+++ b/test/parallel/test-tls-connect-pipe.js
@@ -9,19 +9,19 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
 common.refreshTmpDir();
 
-var server = tls.Server(options, common.mustCall(function(socket) {
+const server = tls.Server(options, common.mustCall(function(socket) {
   server.close();
 }));
 server.listen(common.PIPE, common.mustCall(function() {
-  var options = { rejectUnauthorized: false };
-  var client = tls.connect(common.PIPE, options, common.mustCall(function() {
+  const options = { rejectUnauthorized: false };
+  const client = tls.connect(common.PIPE, options, common.mustCall(function() {
     client.end();
   }));
 }));
diff --git a/test/parallel/test-tls-connect-stream-writes.js b/test/parallel/test-tls-connect-stream-writes.js
index 7070f63f49..2f4eebf229 100644
--- a/test/parallel/test-tls-connect-stream-writes.js
+++ b/test/parallel/test-tls-connect-stream-writes.js
@@ -11,34 +11,33 @@ const tls = require('tls');
 const stream = require('stream');
 const net = require('net');
 
-var server;
-var cert_dir = common.fixturesDir;
-var options = { key: fs.readFileSync(cert_dir + '/test_key.pem'),
-                cert: fs.readFileSync(cert_dir + '/test_cert.pem'),
-                ca: [ fs.readFileSync(cert_dir + '/test_ca.pem') ],
-                ciphers: 'AES256-GCM-SHA384' };
-var content = 'hello world';
-var recv_bufs = [];
-var send_data = '';
-server = tls.createServer(options, function(s) {
+const cert_dir = common.fixturesDir;
+const options = { key: fs.readFileSync(cert_dir + '/test_key.pem'),
+                  cert: fs.readFileSync(cert_dir + '/test_cert.pem'),
+                  ca: [ fs.readFileSync(cert_dir + '/test_ca.pem') ],
+                  ciphers: 'AES256-GCM-SHA384' };
+const content = 'hello world';
+const recv_bufs = [];
+let send_data = '';
+const server = tls.createServer(options, function(s) {
   s.on('data', function(c) {
     recv_bufs.push(c);
   });
 });
 server.listen(0, function() {
-  var raw = net.connect(this.address().port);
+  const raw = net.connect(this.address().port);
 
-  var pending = false;
+  let pending = false;
   raw.on('readable', function() {
     if (pending)
       p._read();
   });
 
-  var p = new stream.Duplex({
+  const p = new stream.Duplex({
     read: function read() {
       pending = false;
 
-      var chunk = raw.read();
+      const chunk = raw.read();
       if (chunk) {
         this.push(chunk);
       } else {
@@ -50,11 +49,11 @@ server.listen(0, function() {
     }
   });
 
-  var socket = tls.connect({
+  const socket = tls.connect({
     socket: p,
     rejectUnauthorized: false
   }, function() {
-    for (var i = 0; i < 50; ++i) {
+    for (let i = 0; i < 50; ++i) {
       socket.write(content);
       send_data += content;
     }
@@ -64,6 +63,6 @@ server.listen(0, function() {
 });
 
 process.on('exit', function() {
-  var recv_data = (Buffer.concat(recv_bufs)).toString();
+  const recv_data = (Buffer.concat(recv_bufs)).toString();
   assert.strictEqual(send_data, recv_data);
 });
diff --git a/test/parallel/test-tls-delayed-attach-error.js b/test/parallel/test-tls-delayed-attach-error.js
index e7cfd4fa6a..47be3f201b 100644
--- a/test/parallel/test-tls-delayed-attach-error.js
+++ b/test/parallel/test-tls-delayed-attach-error.js
@@ -9,16 +9,16 @@ const tls = require('tls');
 const fs = require('fs');
 const net = require('net');
 
-var bonkers = Buffer.alloc(1024, 42);
+const bonkers = Buffer.alloc(1024, 42);
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var server = net.createServer(common.mustCall(function(c) {
+const server = net.createServer(common.mustCall(function(c) {
   setTimeout(common.mustCall(function() {
-    var s = new tls.TLSSocket(c, {
+    const s = new tls.TLSSocket(c, {
       isServer: true,
       secureContext: tls.createSecureContext(options)
     });
@@ -31,7 +31,7 @@ var server = net.createServer(common.mustCall(function(c) {
     });
   }), 200);
 })).listen(0, function() {
-  var c = net.connect({port: this.address().port}, function() {
+  const c = net.connect({port: this.address().port}, function() {
     c.write(bonkers);
   });
 });
diff --git a/test/parallel/test-tls-delayed-attach.js b/test/parallel/test-tls-delayed-attach.js
index 867fc5edb8..d757ca756c 100644
--- a/test/parallel/test-tls-delayed-attach.js
+++ b/test/parallel/test-tls-delayed-attach.js
@@ -11,17 +11,17 @@ const tls = require('tls');
 const fs = require('fs');
 const net = require('net');
 
-var sent = 'hello world';
-var received = '';
+const sent = 'hello world';
+let received = '';
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var server = net.createServer(function(c) {
+const server = net.createServer(function(c) {
   setTimeout(function() {
-    var s = new tls.TLSSocket(c, {
+    const s = new tls.TLSSocket(c, {
       isServer: true,
       secureContext: tls.createSecureContext(options)
     });
@@ -36,7 +36,7 @@ var server = net.createServer(function(c) {
     });
   }, 200);
 }).listen(0, function() {
-  var c = tls.connect(this.address().port, {
+  const c = tls.connect(this.address().port, {
     rejectUnauthorized: false
   }, function() {
     c.end(sent);
diff --git a/test/parallel/test-tls-dhe.js b/test/parallel/test-tls-dhe.js
index 6340833765..4bc81af6e1 100644
--- a/test/parallel/test-tls-dhe.js
+++ b/test/parallel/test-tls-dhe.js
@@ -10,28 +10,28 @@ const tls = require('tls');
 
 const spawn = require('child_process').spawn;
 const fs = require('fs');
-var key = fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem');
-var cert = fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem');
-var nsuccess = 0;
-var ntests = 0;
-var ciphers = 'DHE-RSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256';
+const key = fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem');
+const cert = fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem');
+let nsuccess = 0;
+let ntests = 0;
+const ciphers = 'DHE-RSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256';
 
 
 function loadDHParam(n) {
-  var path = common.fixturesDir;
+  let path = common.fixturesDir;
   if (n !== 'error') path += '/keys';
   return fs.readFileSync(path + '/dh' + n + '.pem');
 }
 
 function test(keylen, expectedCipher, cb) {
-  var options = {
+  const options = {
     key: key,
     cert: cert,
     ciphers: ciphers,
     dhparam: loadDHParam(keylen)
   };
 
-  var server = tls.createServer(options, function(conn) {
+  const server = tls.createServer(options, function(conn) {
     conn.end();
   });
 
@@ -41,15 +41,15 @@ function test(keylen, expectedCipher, cb) {
   });
 
   server.listen(0, '127.0.0.1', function() {
-    var args = ['s_client', '-connect', `127.0.0.1:${this.address().port}`,
-                '-cipher', ciphers];
+    const args = ['s_client', '-connect', `127.0.0.1:${this.address().port}`,
+                  '-cipher', ciphers];
 
     // for the performance and stability issue in s_client on Windows
     if (common.isWindows)
       args.push('-no_rand_screen');
 
-    var client = spawn(common.opensslCli, args);
-    var out = '';
+    const client = spawn(common.opensslCli, args);
+    let out = '';
     client.stdout.setEncoding('utf8');
     client.stdout.on('data', function(d) {
       out += d;
@@ -57,7 +57,7 @@ function test(keylen, expectedCipher, cb) {
     client.stdout.on('end', function() {
       // DHE key length can be checked -brief option in s_client but it
       // is only supported in openssl 1.0.2 so we cannot check it.
-      var reg = new RegExp('Cipher    : ' + expectedCipher);
+      const reg = new RegExp('Cipher    : ' + expectedCipher);
       if (reg.test(out)) {
         nsuccess++;
         server.close();
diff --git a/test/parallel/test-tls-ecdh-disable.js b/test/parallel/test-tls-ecdh-disable.js
index df51c5d085..a0555e8778 100644
--- a/test/parallel/test-tls-ecdh-disable.js
+++ b/test/parallel/test-tls-ecdh-disable.js
@@ -11,17 +11,17 @@ const tls = require('tls');
 const exec = require('child_process').exec;
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem'),
   ciphers: 'ECDHE-RSA-RC4-SHA',
   ecdhCurve: false
 };
 
-var server = tls.createServer(options, common.fail);
+const server = tls.createServer(options, common.fail);
 
 server.listen(0, '127.0.0.1', common.mustCall(function() {
-  var cmd = '"' + common.opensslCli + '" s_client -cipher ' + options.ciphers +
+  let cmd = '"' + common.opensslCli + '" s_client -cipher ' + options.ciphers +
             ` -connect 127.0.0.1:${this.address().port}`;
 
   // for the performance and stability issue in s_client on Windows
diff --git a/test/parallel/test-tls-econnreset.js b/test/parallel/test-tls-econnreset.js
index 51f178198f..c0df9948f4 100644
--- a/test/parallel/test-tls-econnreset.js
+++ b/test/parallel/test-tls-econnreset.js
@@ -8,7 +8,7 @@ if (!common.hasCrypto) {
 }
 const tls = require('tls');
 
-var cacert =
+const cacert =
 `-----BEGIN CERTIFICATE-----
 MIIBxTCCAX8CAnXnMA0GCSqGSIb3DQEBBQUAMH0xCzAJBgNVBAYTAlVTMQswCQYD
 VQQIEwJDQTEWMBQGA1UEBxMNU2FuIEZyYW5jaXNjbzEZMBcGA1UEChMQU3Ryb25n
@@ -22,7 +22,7 @@ AVkCAwEAATANBgkqhkiG9w0BAQUFAAMxALA1uS4CqQXRSAyYTfio5oyLGz71a+NM
 +0AFLBwh5AQjhGd0FcenU4OfHxyDEOJT/Q==
 -----END CERTIFICATE-----`;
 
-var cert =
+const cert =
 `-----BEGIN CERTIFICATE-----
 MIIBfDCCATYCAgQaMA0GCSqGSIb3DQEBBQUAMH0xCzAJBgNVBAYTAlVTMQswCQYD
 VQQIEwJDQTEWMBQGA1UEBxMNU2FuIEZyYW5jaXNjbzEZMBcGA1UEChMQU3Ryb25n
@@ -34,7 +34,7 @@ AQABoxkwFzAVBgNVHREEDjAMhwQAAAAAhwR/AAABMA0GCSqGSIb3DQEBBQUAAzEA
 cGpYrhkrb7mIh9DNhV0qp7pGjqBzlHqB7KQXw2luLDp//6dyHBMexDCQznkhZKRU
 -----END CERTIFICATE-----`;
 
-var key =
+const key =
 `-----BEGIN RSA PRIVATE KEY-----
 MIH0AgEAAjEAx+QjQtZTXmk9TAhA0ydFWGE6JuLRNCYBq7wUG2DAR8YrTVal8ZRo
 mvVrjUrxSrvHAgMBAAECMBCGccvSwC2r8Z9Zh1JtirQVxaL1WWpAQfmVwLe0bAgg
@@ -44,18 +44,18 @@ pQIZAMMwxuS3XiO7two2sQF6W+JTYyX1DPCwAQIZAOYg1TvEGT38k8e8jygv8E8w
 YqrWTeQFNQ==
 -----END RSA PRIVATE KEY-----`;
 
-var ca = [ cert, cacert ];
+const ca = [ cert, cacert ];
 
-var clientError = null;
-var connectError = null;
+let clientError = null;
+let connectError = null;
 
-var server = tls.createServer({ ca: ca, cert: cert, key: key }, function(conn) {
+const server = tls.createServer({ ca: ca, cert: cert, key: key }, (conn) => {
   throw 'unreachable';
 }).on('tlsClientError', function(err, conn) {
   assert(!clientError && conn);
   clientError = err;
 }).listen(0, function() {
-  var options = {
+  const options = {
     ciphers: 'AES128-GCM-SHA256',
     port: this.address().port,
     ca: ca
diff --git a/test/parallel/test-tls-env-bad-extra-ca.js b/test/parallel/test-tls-env-bad-extra-ca.js
index 1862366e01..cb6c4ba489 100644
--- a/test/parallel/test-tls-env-bad-extra-ca.js
+++ b/test/parallel/test-tls-env-bad-extra-ca.js
@@ -22,11 +22,11 @@ const env = {
   NODE_EXTRA_CA_CERTS: common.fixturesDir + '/no-such-file-exists',
 };
 
-var opts = {
+const opts = {
   env: env,
   silent: true,
 };
-var stderr = '';
+let stderr = '';
 
 fork(__filename, opts)
   .on('exit', common.mustCall(function(status) {
diff --git a/test/parallel/test-tls-fast-writing.js b/test/parallel/test-tls-fast-writing.js
index 33665536ba..b2d6fe6eee 100644
--- a/test/parallel/test-tls-fast-writing.js
+++ b/test/parallel/test-tls-fast-writing.js
@@ -10,14 +10,14 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var dir = common.fixturesDir;
-var options = { key: fs.readFileSync(dir + '/test_key.pem'),
-                cert: fs.readFileSync(dir + '/test_cert.pem'),
-                ca: [ fs.readFileSync(dir + '/test_ca.pem') ] };
+const dir = common.fixturesDir;
+const options = { key: fs.readFileSync(dir + '/test_key.pem'),
+                  cert: fs.readFileSync(dir + '/test_cert.pem'),
+                  ca: [ fs.readFileSync(dir + '/test_ca.pem') ] };
 
-var server = tls.createServer(options, onconnection);
-var gotChunk = false;
-var gotDrain = false;
+const server = tls.createServer(options, onconnection);
+let gotChunk = false;
+let gotDrain = false;
 
 setTimeout(function() {
   console.log('not ok - timed out');
@@ -41,9 +41,9 @@ function onconnection(conn) {
 }
 
 server.listen(0, function() {
-  var chunk = Buffer.alloc(1024, 'x');
-  var opt = { port: this.address().port, rejectUnauthorized: false };
-  var conn = tls.connect(opt, function() {
+  const chunk = Buffer.alloc(1024, 'x');
+  const opt = { port: this.address().port, rejectUnauthorized: false };
+  const conn = tls.connect(opt, function() {
     conn.on('drain', ondrain);
     write();
   });
diff --git a/test/parallel/test-tls-friendly-error-message.js b/test/parallel/test-tls-friendly-error-message.js
index 9ae69f4016..3f3bfe9133 100644
--- a/test/parallel/test-tls-friendly-error-message.js
+++ b/test/parallel/test-tls-friendly-error-message.js
@@ -17,7 +17,7 @@ tls.createServer({ key: key, cert: cert }, common.mustCall(function(conn) {
   conn.end();
   this.close();
 })).listen(0, common.mustCall(function() {
-  var options = { port: this.address().port, rejectUnauthorized: true };
+  const options = { port: this.address().port, rejectUnauthorized: true };
   tls.connect(options).on('error', common.mustCall(function(err) {
     assert.strictEqual(err.code, 'UNABLE_TO_VERIFY_LEAF_SIGNATURE');
     assert.strictEqual(err.message, 'unable to verify the first certificate');
diff --git a/test/parallel/test-tls-handshake-error.js b/test/parallel/test-tls-handshake-error.js
index 7623f816f3..4909fa45e1 100644
--- a/test/parallel/test-tls-handshake-error.js
+++ b/test/parallel/test-tls-handshake-error.js
@@ -11,13 +11,13 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var server = tls.createServer({
+const server = tls.createServer({
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem'),
   rejectUnauthorized: true
 }, function(c) {
 }).listen(0, common.mustCall(function() {
-  var c = tls.connect({
+  const c = tls.connect({
     port: this.address().port,
     ciphers: 'RC4'
   }, function() {
diff --git a/test/parallel/test-tls-honorcipherorder.js b/test/parallel/test-tls-honorcipherorder.js
index 883abc199e..f7894e4670 100644
--- a/test/parallel/test-tls-honorcipherorder.js
+++ b/test/parallel/test-tls-honorcipherorder.js
@@ -9,19 +9,19 @@ if (!common.hasCrypto) {
 const tls = require('tls');
 
 const fs = require('fs');
-var nconns = 0;
+let nconns = 0;
 
 // We explicitly set TLS version to 1.2 so as to be safe when the
 // default method is updated in the future
-var SSL_Method = 'TLSv1_2_method';
-var localhost = '127.0.0.1';
+const SSL_Method = 'TLSv1_2_method';
+const localhost = '127.0.0.1';
 
 process.on('exit', function() {
   assert.equal(nconns, 6);
 });
 
 function test(honorCipherOrder, clientCipher, expectedCipher, cb) {
-  var soptions = {
+  const soptions = {
     secureProtocol: SSL_Method,
     key: fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem'),
     cert: fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem'),
@@ -30,7 +30,7 @@ function test(honorCipherOrder, clientCipher, expectedCipher, cb) {
     honorCipherOrder: !!honorCipherOrder
   };
 
-  var server = tls.createServer(soptions, function(cleartextStream) {
+  const server = tls.createServer(soptions, function(cleartextStream) {
     nconns++;
 
     // End socket to send CLOSE_NOTIFY and TCP FIN packet, otherwise
@@ -38,7 +38,7 @@ function test(honorCipherOrder, clientCipher, expectedCipher, cb) {
     cleartextStream.end();
   });
   server.listen(0, localhost, function() {
-    var coptions = {
+    const coptions = {
       rejectUnauthorized: false,
       secureProtocol: SSL_Method
     };
@@ -46,8 +46,8 @@ function test(honorCipherOrder, clientCipher, expectedCipher, cb) {
       coptions.ciphers = clientCipher;
     }
     const port = this.address().port;
-    var client = tls.connect(port, localhost, coptions, function() {
-      var cipher = client.getCipher();
+    const client = tls.connect(port, localhost, coptions, function() {
+      const cipher = client.getCipher();
       client.end();
       server.close();
       assert.equal(cipher.name, expectedCipher);
diff --git a/test/parallel/test-tls-inception.js b/test/parallel/test-tls-inception.js
index 760887d9f2..183fbe587b 100644
--- a/test/parallel/test-tls-inception.js
+++ b/test/parallel/test-tls-inception.js
@@ -54,7 +54,7 @@ a.listen(0, function() {
       rejectUnauthorized: false
     });
     ssl.setEncoding('utf8');
-    var buf = '';
+    let buf = '';
     ssl.on('data', function(data) {
       buf += data;
     });
diff --git a/test/parallel/test-tls-invoke-queued.js b/test/parallel/test-tls-invoke-queued.js
index b37b900d77..bd3ad43fe6 100644
--- a/test/parallel/test-tls-invoke-queued.js
+++ b/test/parallel/test-tls-invoke-queued.js
@@ -11,9 +11,9 @@ const tls = require('tls');
 const fs = require('fs');
 
 
-var received = '';
+let received = '';
 
-var server = tls.createServer({
+const server = tls.createServer({
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 }, function(c) {
@@ -26,7 +26,7 @@ var server = tls.createServer({
 
   server.close();
 }).listen(0, common.mustCall(function() {
-  var c = tls.connect(this.address().port, {
+  const c = tls.connect(this.address().port, {
     rejectUnauthorized: false
   }, common.mustCall(function() {
     c.on('data', function(chunk) {
diff --git a/test/parallel/test-tls-js-stream.js b/test/parallel/test-tls-js-stream.js
index 2f69a306cf..e1b580da30 100644
--- a/test/parallel/test-tls-js-stream.js
+++ b/test/parallel/test-tls-js-stream.js
@@ -12,12 +12,12 @@ const stream = require('stream');
 const fs = require('fs');
 const net = require('net');
 
-var connected = {
+const connected = {
   client: 0,
   server: 0
 };
 
-var server = tls.createServer({
+const server = tls.createServer({
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 }, function(c) {
@@ -25,19 +25,19 @@ var server = tls.createServer({
   connected.server++;
   c.end('ohai');
 }).listen(0, function() {
-  var raw = net.connect(this.address().port);
+  const raw = net.connect(this.address().port);
 
-  var pending = false;
+  let pending = false;
   raw.on('readable', function() {
     if (pending)
       p._read();
   });
 
-  var p = new stream.Duplex({
+  const p = new stream.Duplex({
     read: function read() {
       pending = false;
 
-      var chunk = raw.read();
+      const chunk = raw.read();
       if (chunk) {
         console.log('read', chunk);
         this.push(chunk);
@@ -51,7 +51,7 @@ var server = tls.createServer({
     }
   });
 
-  var socket = tls.connect({
+  const socket = tls.connect({
     socket: p,
     rejectUnauthorized: false
   }, function() {
diff --git a/test/parallel/test-tls-legacy-onselect.js b/test/parallel/test-tls-legacy-onselect.js
index 392df2e715..9af65c43a0 100644
--- a/test/parallel/test-tls-legacy-onselect.js
+++ b/test/parallel/test-tls-legacy-onselect.js
@@ -8,8 +8,8 @@ if (!common.hasCrypto) {
 const tls = require('tls');
 const net = require('net');
 
-var server = net.Server(common.mustCall(function(raw) {
-  var pair = tls.createSecurePair(null, true, false, false);
+const server = net.Server(common.mustCall(function(raw) {
+  const pair = tls.createSecurePair(null, true, false, false);
   pair.on('error', function() {});
   pair.ssl.setSNICallback(common.mustCall(function() {
     raw.destroy();
diff --git a/test/parallel/test-tls-max-send-fragment.js b/test/parallel/test-tls-max-send-fragment.js
index d96f42e6a2..4b81be35e3 100644
--- a/test/parallel/test-tls-max-send-fragment.js
+++ b/test/parallel/test-tls-max-send-fragment.js
@@ -10,11 +10,11 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var buf = Buffer.allocUnsafe(10000);
-var received = 0;
-var maxChunk = 768;
+const buf = Buffer.allocUnsafe(10000);
+let received = 0;
+const maxChunk = 768;
 
-var server = tls.createServer({
+const server = tls.createServer({
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 }, function(c) {
@@ -27,7 +27,7 @@ var server = tls.createServer({
 
   c.end(buf);
 }).listen(0, common.mustCall(function() {
-  var c = tls.connect(this.address().port, {
+  const c = tls.connect(this.address().port, {
     rejectUnauthorized: false
   }, common.mustCall(function() {
     c.on('data', function(chunk) {
diff --git a/test/parallel/test-tls-multi-key.js b/test/parallel/test-tls-multi-key.js
index 840deb59d4..e3d5d661a7 100644
--- a/test/parallel/test-tls-multi-key.js
+++ b/test/parallel/test-tls-multi-key.js
@@ -9,7 +9,7 @@ if (!common.hasCrypto) {
 const tls = require('tls');
 const fs = require('fs');
 
-var options = {
+const options = {
   key: [
     fs.readFileSync(common.fixturesDir + '/keys/ec-key.pem'),
     fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
@@ -20,17 +20,17 @@ var options = {
   ]
 };
 
-var ciphers = [];
+const ciphers = [];
 
-var server = tls.createServer(options, function(conn) {
+const server = tls.createServer(options, function(conn) {
   conn.end('ok');
 }).listen(0, function() {
-  var ecdsa = tls.connect(this.address().port, {
+  const ecdsa = tls.connect(this.address().port, {
     ciphers: 'ECDHE-ECDSA-AES256-GCM-SHA384',
     rejectUnauthorized: false
   }, function() {
     ciphers.push(ecdsa.getCipher());
-    var rsa = tls.connect(server.address().port, {
+    const rsa = tls.connect(server.address().port, {
       ciphers: 'ECDHE-RSA-AES256-GCM-SHA384',
       rejectUnauthorized: false
     }, function() {
diff --git a/test/parallel/test-tls-no-rsa-key.js b/test/parallel/test-tls-no-rsa-key.js
index 1e75b21eab..233d8453db 100644
--- a/test/parallel/test-tls-no-rsa-key.js
+++ b/test/parallel/test-tls-no-rsa-key.js
@@ -10,15 +10,15 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/ec-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/ec-cert.pem')
 };
 
-var server = tls.createServer(options, function(conn) {
+const server = tls.createServer(options, function(conn) {
   conn.end('ok');
 }).listen(0, common.mustCall(function() {
-  var c = tls.connect(this.address().port, {
+  const c = tls.connect(this.address().port, {
     rejectUnauthorized: false
   }, common.mustCall(function() {
     c.on('end', common.mustCall(function() {
diff --git a/test/parallel/test-tls-npn-server-client.js b/test/parallel/test-tls-npn-server-client.js
index fdd9fcbb32..1b622cc8f7 100644
--- a/test/parallel/test-tls-npn-server-client.js
+++ b/test/parallel/test-tls-npn-server-client.js
@@ -24,7 +24,7 @@ function loadPEM(n) {
   return fs.readFileSync(filenamePEM(n));
 }
 
-var serverOptions = {
+const serverOptions = {
   key: loadPEM('agent2-key'),
   cert: loadPEM('agent2-cert'),
   crl: loadPEM('ca2-crl'),
@@ -38,7 +38,7 @@ var serverOptions = {
   NPNProtocols: ['a', 'b', 'c']
 };
 
-var clientsOptions = [{
+const clientsOptions = [{
   port: undefined,
   key: serverOptions.key,
   cert: serverOptions.cert,
@@ -70,7 +70,7 @@ var clientsOptions = [{
 const serverResults = [];
 const clientsResults = [];
 
-var server = tls.createServer(serverOptions, function(c) {
+const server = tls.createServer(serverOptions, function(c) {
   serverResults.push(c.npnProtocol);
 });
 server.listen(0, startTest);
@@ -78,7 +78,7 @@ server.listen(0, startTest);
 function startTest() {
   function connectClient(options, callback) {
     options.port = server.address().port;
-    var client = tls.connect(options, function() {
+    const client = tls.connect(options, function() {
       clientsResults.push(client.npnProtocol);
       client.destroy();
 
diff --git a/test/parallel/test-tls-over-http-tunnel.js b/test/parallel/test-tls-over-http-tunnel.js
index f55f1c0ac0..4e03bcaf53 100644
--- a/test/parallel/test-tls-over-http-tunnel.js
+++ b/test/parallel/test-tls-over-http-tunnel.js
@@ -12,17 +12,17 @@ const fs = require('fs');
 const net = require('net');
 const http = require('http');
 
-var gotRequest = false;
+let gotRequest = false;
 
-var key = fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem');
-var cert = fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem');
+const key = fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem');
+const cert = fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem');
 
-var options = {
+const options = {
   key: key,
   cert: cert
 };
 
-var server = https.createServer(options, function(req, res) {
+const server = https.createServer(options, function(req, res) {
   console.log('SERVER: got request');
   res.writeHead(200, {
     'content-type': 'text/plain'
@@ -31,10 +31,10 @@ var server = https.createServer(options, function(req, res) {
   res.end('hello world\n');
 });
 
-var proxy = net.createServer(function(clientSocket) {
+const proxy = net.createServer(function(clientSocket) {
   console.log('PROXY: got a client connection');
 
-  var serverSocket = null;
+  let serverSocket = null;
 
   clientSocket.on('data', function(chunk) {
     if (!serverSocket) {
@@ -80,7 +80,7 @@ server.listen(0);
 proxy.listen(0, function() {
   console.log('CLIENT: Making CONNECT request');
 
-  var req = http.request({
+  const req = http.request({
     port: this.address().port,
     method: 'CONNECT',
     path: `localhost:${server.address().port}`,
diff --git a/test/parallel/test-tls-pause.js b/test/parallel/test-tls-pause.js
index aa48c04866..911ea7097b 100644
--- a/test/parallel/test-tls-pause.js
+++ b/test/parallel/test-tls-pause.js
@@ -11,16 +11,16 @@ const tls = require('tls');
 const fs = require('fs');
 const path = require('path');
 
-var options = {
+const options = {
   key: fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem')),
   cert: fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'))
 };
 
-var bufSize = 1024 * 1024;
-var sent = 0;
-var received = 0;
+const bufSize = 1024 * 1024;
+let sent = 0;
+let received = 0;
 
-var server = tls.Server(options, function(socket) {
+const server = tls.Server(options, function(socket) {
   socket.pipe(socket);
   socket.on('data', function(c) {
     console.error('data', c.length);
@@ -28,8 +28,8 @@ var server = tls.Server(options, function(socket) {
 });
 
 server.listen(0, function() {
-  var resumed = false;
-  var client = tls.connect({
+  let resumed = false;
+  const client = tls.connect({
     port: this.address().port,
     rejectUnauthorized: false
   }, function() {
@@ -39,7 +39,7 @@ server.listen(0, function() {
     send();
     function send() {
       console.error('sending');
-      var ret = client.write(Buffer.allocUnsafe(bufSize));
+      const ret = client.write(Buffer.allocUnsafe(bufSize));
       console.error('write => %j', ret);
       if (false !== ret) {
         console.error('write again');
diff --git a/test/parallel/test-tls-peer-certificate-encoding.js b/test/parallel/test-tls-peer-certificate-encoding.js
index 1062dab2af..545caf403e 100644
--- a/test/parallel/test-tls-peer-certificate-encoding.js
+++ b/test/parallel/test-tls-peer-certificate-encoding.js
@@ -12,21 +12,21 @@ const fs = require('fs');
 const util = require('util');
 const join = require('path').join;
 
-var options = {
+const options = {
   key: fs.readFileSync(join(common.fixturesDir, 'keys', 'agent5-key.pem')),
   cert: fs.readFileSync(join(common.fixturesDir, 'keys', 'agent5-cert.pem')),
   ca: [ fs.readFileSync(join(common.fixturesDir, 'keys', 'ca2-cert.pem')) ]
 };
 
-var server = tls.createServer(options, function(cleartext) {
+const server = tls.createServer(options, function(cleartext) {
   cleartext.end('World');
 });
 server.listen(0, common.mustCall(function() {
-  var socket = tls.connect({
+  const socket = tls.connect({
     port: this.address().port,
     rejectUnauthorized: false
   }, common.mustCall(function() {
-    var peerCert = socket.getPeerCertificate();
+    const peerCert = socket.getPeerCertificate();
 
     console.error(util.inspect(peerCert));
     assert.equal(peerCert.subject.CN, 'Ádám Lippai');
diff --git a/test/parallel/test-tls-peer-certificate-multi-keys.js b/test/parallel/test-tls-peer-certificate-multi-keys.js
index 6176a34a71..693bb4cb3f 100644
--- a/test/parallel/test-tls-peer-certificate-multi-keys.js
+++ b/test/parallel/test-tls-peer-certificate-multi-keys.js
@@ -12,20 +12,20 @@ const fs = require('fs');
 const util = require('util');
 const join = require('path').join;
 
-var options = {
+const options = {
   key: fs.readFileSync(join(common.fixturesDir, 'agent.key')),
   cert: fs.readFileSync(join(common.fixturesDir, 'multi-alice.crt'))
 };
 
-var server = tls.createServer(options, function(cleartext) {
+const server = tls.createServer(options, function(cleartext) {
   cleartext.end('World');
 });
 server.listen(0, common.mustCall(function() {
-  var socket = tls.connect({
+  const socket = tls.connect({
     port: this.address().port,
     rejectUnauthorized: false
   }, common.mustCall(function() {
-    var peerCert = socket.getPeerCertificate();
+    const peerCert = socket.getPeerCertificate();
     console.error(util.inspect(peerCert));
     assert.deepStrictEqual(
       peerCert.subject.OU,
diff --git a/test/parallel/test-tls-pfx-gh-5100-regr.js b/test/parallel/test-tls-pfx-gh-5100-regr.js
index 4670d9ea3d..142a7de10b 100644
--- a/test/parallel/test-tls-pfx-gh-5100-regr.js
+++ b/test/parallel/test-tls-pfx-gh-5100-regr.js
@@ -28,7 +28,7 @@ const server = tls.createServer({
   );
   c.end();
 })).listen(0, function() {
-  var client = tls.connect({
+  const client = tls.connect({
     port: this.address().port,
     pfx: pfx,
     passphrase: 'sample',
diff --git a/test/parallel/test-tls-request-timeout.js b/test/parallel/test-tls-request-timeout.js
index 2d1ddf34b3..4873a3c696 100644
--- a/test/parallel/test-tls-request-timeout.js
+++ b/test/parallel/test-tls-request-timeout.js
@@ -10,13 +10,13 @@ const tls = require('tls');
 
 const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var server = tls.Server(options, common.mustCall(function(socket) {
-  var s = socket.setTimeout(100);
+const server = tls.Server(options, common.mustCall(function(socket) {
+  const s = socket.setTimeout(100);
   assert.ok(s instanceof tls.TLSSocket);
 
   socket.on('timeout', common.mustCall(function(err) {
diff --git a/test/parallel/test-tls-securepair-fiftharg.js b/test/parallel/test-tls-securepair-fiftharg.js
index 99564055ef..c79b5cbede 100644
--- a/test/parallel/test-tls-securepair-fiftharg.js
+++ b/test/parallel/test-tls-securepair-fiftharg.js
@@ -15,7 +15,7 @@ const sslcontext = tls.createSecureContext({
   key: fs.readFileSync(common.fixturesDir + '/test_key.pem')
 });
 
-var catchedServername;
+let catchedServername;
 const pair = tls.createSecurePair(sslcontext, true, false, false, {
   SNICallback: common.mustCall(function(servername, cb) {
     catchedServername = servername;
diff --git a/test/parallel/test-tls-securepair-server.js b/test/parallel/test-tls-securepair-server.js
index afe76a0139..65a7d91a0c 100644
--- a/test/parallel/test-tls-securepair-server.js
+++ b/test/parallel/test-tls-securepair-server.js
@@ -13,19 +13,19 @@ const net = require('net');
 const fs = require('fs');
 const spawn = require('child_process').spawn;
 
-var key = fs.readFileSync(join(common.fixturesDir, 'agent.key')).toString();
-var cert = fs.readFileSync(join(common.fixturesDir, 'agent.crt')).toString();
+const key = fs.readFileSync(join(common.fixturesDir, 'agent.key')).toString();
+const cert = fs.readFileSync(join(common.fixturesDir, 'agent.crt')).toString();
 
 function log(a) {
   console.error('***server*** ' + a);
 }
 
-var server = net.createServer(common.mustCall(function(socket) {
+const server = net.createServer(common.mustCall(function(socket) {
   log('connection fd=' + socket.fd);
-  var sslcontext = tls.createSecureContext({key: key, cert: cert});
+  const sslcontext = tls.createSecureContext({key: key, cert: cert});
   sslcontext.context.setCiphers('RC4-SHA:AES128-SHA:AES256-SHA');
 
-  var pair = tls.createSecurePair(sslcontext, true);
+  const pair = tls.createSecurePair(sslcontext, true);
 
   assert.ok(pair.encrypted.writable);
   assert.ok(pair.cleartext.writable);
@@ -84,23 +84,23 @@ var server = net.createServer(common.mustCall(function(socket) {
   });
 }));
 
-var gotHello = false;
-var sentWorld = false;
-var gotWorld = false;
+let gotHello = false;
+let sentWorld = false;
+let gotWorld = false;
 
 server.listen(0, common.mustCall(function() {
   // To test use: openssl s_client -connect localhost:8000
 
-  var args = ['s_client', '-connect', `127.0.0.1:${this.address().port}`];
+  const args = ['s_client', '-connect', `127.0.0.1:${this.address().port}`];
 
   // for the performance and stability issue in s_client on Windows
   if (common.isWindows)
     args.push('-no_rand_screen');
 
-  var client = spawn(common.opensslCli, args);
+  const client = spawn(common.opensslCli, args);
 
 
-  var out = '';
+  let out = '';
 
   client.stdout.setEncoding('utf8');
   client.stdout.on('data', function(d) {
diff --git a/test/parallel/test-tls-session-cache.js b/test/parallel/test-tls-session-cache.js
index 60a8570179..5e3ae5128e 100644
--- a/test/parallel/test-tls-session-cache.js
+++ b/test/parallel/test-tls-session-cache.js
@@ -34,9 +34,9 @@ function doTest(testOptions, callback) {
     ca: [cert],
     requestCert: true
   };
-  var requestCount = 0;
-  var resumeCount = 0;
-  var session;
+  let requestCount = 0;
+  let resumeCount = 0;
+  let session;
 
   const server = tls.createServer(options, function(cleartext) {
     cleartext.on('error', function(er) {
@@ -90,7 +90,7 @@ function doTest(testOptions, callback) {
       const client = spawn(common.opensslCli, args, {
         stdio: [ 0, 1, 'pipe' ]
       });
-      var err = '';
+      let err = '';
       client.stderr.setEncoding('utf8');
       client.stderr.on('data', function(chunk) {
         err += chunk;
diff --git a/test/parallel/test-tls-set-ciphers.js b/test/parallel/test-tls-set-ciphers.js
index ef08c56460..a1af1a917e 100644
--- a/test/parallel/test-tls-set-ciphers.js
+++ b/test/parallel/test-tls-set-ciphers.js
@@ -34,7 +34,7 @@ const server = tls.createServer(options, common.mustCall(function(conn) {
 }));
 
 server.listen(0, '127.0.0.1', function() {
-  var cmd = '"' + common.opensslCli + '" s_client -cipher ' + options.ciphers +
+  let cmd = '"' + common.opensslCli + '" s_client -cipher ' + options.ciphers +
             ` -connect 127.0.0.1:${this.address().port}`;
 
   // for the performance and stability issue in s_client on Windows
diff --git a/test/parallel/test-tls-sni-option.js b/test/parallel/test-tls-sni-option.js
index 436618379a..2e7245336b 100644
--- a/test/parallel/test-tls-sni-option.js
+++ b/test/parallel/test-tls-sni-option.js
@@ -23,13 +23,13 @@ function loadPEM(n) {
   return fs.readFileSync(filenamePEM(n));
 }
 
-var serverOptions = {
+const serverOptions = {
   key: loadPEM('agent2-key'),
   cert: loadPEM('agent2-cert'),
   requestCert: true,
   rejectUnauthorized: false,
   SNICallback: function(servername, callback) {
-    var context = SNIContexts[servername];
+    const context = SNIContexts[servername];
 
     // Just to test asynchronous callback
     setTimeout(function() {
@@ -45,7 +45,7 @@ var serverOptions = {
   }
 };
 
-var SNIContexts = {
+const SNIContexts = {
   'a.example.com': {
     key: loadPEM('agent1-key'),
     cert: loadPEM('agent1-cert'),
@@ -60,7 +60,7 @@ var SNIContexts = {
   }
 };
 
-var clientsOptions = [{
+const clientsOptions = [{
   port: undefined,
   key: loadPEM('agent1-key'),
   cert: loadPEM('agent1-cert'),
@@ -104,7 +104,7 @@ const clientErrors = [];
 let serverError;
 let clientError;
 
-var server = tls.createServer(serverOptions, function(c) {
+const server = tls.createServer(serverOptions, function(c) {
   serverResults.push({ sni: c.servername, authorized: c.authorized });
 });
 
@@ -117,12 +117,12 @@ server.listen(0, startTest);
 
 function startTest() {
   function connectClient(i, callback) {
-    var options = clientsOptions[i];
+    const options = clientsOptions[i];
     clientError = null;
     serverError = null;
 
     options.port = server.address().port;
-    var client = tls.connect(options, function() {
+    const client = tls.connect(options, function() {
       clientResults.push(
           /Hostname\/IP doesn't/.test(client.authorizationError || ''));
       client.destroy();
diff --git a/test/parallel/test-tls-sni-server-client.js b/test/parallel/test-tls-sni-server-client.js
index a22b6a9726..93fffd3888 100644
--- a/test/parallel/test-tls-sni-server-client.js
+++ b/test/parallel/test-tls-sni-server-client.js
@@ -23,12 +23,12 @@ function loadPEM(n) {
   return fs.readFileSync(filenamePEM(n));
 }
 
-var serverOptions = {
+const serverOptions = {
   key: loadPEM('agent2-key'),
   cert: loadPEM('agent2-cert')
 };
 
-var SNIContexts = {
+const SNIContexts = {
   'a.example.com': {
     key: loadPEM('agent1-key'),
     cert: loadPEM('agent1-cert')
@@ -44,7 +44,7 @@ var SNIContexts = {
   }
 };
 
-var clientsOptions = [{
+const clientsOptions = [{
   port: undefined,
   ca: [loadPEM('ca1-cert')],
   servername: 'a.example.com',
@@ -74,7 +74,7 @@ var clientsOptions = [{
 const serverResults = [];
 const clientResults = [];
 
-var server = tls.createServer(serverOptions, function(c) {
+const server = tls.createServer(serverOptions, function(c) {
   serverResults.push(c.servername);
 });
 
@@ -85,15 +85,15 @@ server.addContext('chain.example.com', SNIContexts['chain.example.com']);
 server.listen(0, startTest);
 
 function startTest() {
-  var i = 0;
+  let i = 0;
   function start() {
     // No options left
     if (i === clientsOptions.length)
       return server.close();
 
-    var options = clientsOptions[i++];
+    const options = clientsOptions[i++];
     options.port = server.address().port;
-    var client = tls.connect(options, function() {
+    const client = tls.connect(options, function() {
       clientResults.push(
         client.authorizationError &&
         /Hostname\/IP doesn't/.test(client.authorizationError));
diff --git a/test/parallel/test-tls-wrap-timeout.js b/test/parallel/test-tls-wrap-timeout.js
index 0454242d92..d66e1f2d68 100644
--- a/test/parallel/test-tls-wrap-timeout.js
+++ b/test/parallel/test-tls-wrap-timeout.js
@@ -27,8 +27,8 @@ const server = tls.createServer(options, common.mustCall((c) => {
   });
 }));
 
-var socket;
-var lastIdleStart;
+let socket;
+let lastIdleStart;
 
 server.listen(0, () => {
   socket = net.connect(server.address().port, function() {
diff --git a/test/parallel/test-umask.js b/test/parallel/test-umask.js
index 397f26ce90..1c870893a3 100644
--- a/test/parallel/test-umask.js
+++ b/test/parallel/test-umask.js
@@ -3,14 +3,14 @@ const common = require('../common');
 const assert = require('assert');
 
 // Note in Windows one can only set the "user" bits.
-var mask;
+let mask;
 if (common.isWindows) {
   mask = '0600';
 } else {
   mask = '0664';
 }
 
-var old = process.umask(mask);
+const old = process.umask(mask);
 
 assert.equal(parseInt(mask, 8), process.umask(old));
 
diff --git a/test/parallel/test-url.js b/test/parallel/test-url.js
index 75e790cf8d..fc51b4b300 100644
--- a/test/parallel/test-url.js
+++ b/test/parallel/test-url.js
@@ -8,7 +8,7 @@ const url = require('url');
 
 // URLs to parse, and expected data
 // { url : parsed }
-var parseTests = {
+const parseTests = {
   '//some_path': {
     href: '//some_path',
     pathname: '//some_path',
@@ -942,7 +942,7 @@ function check(actual, expected) {
   });
 }
 
-var parseTestsWithQueryString = {
+const parseTestsWithQueryString = {
   '/foo/bar?baz=quux#frag': {
     href: '/foo/bar?baz=quux#frag',
     hash: '#frag',
@@ -1013,7 +1013,7 @@ for (const u in parseTestsWithQueryString) {
 
 // some extra formatting tests, just to verify
 // that it'll format slightly wonky content to a valid url.
-var formatTests = {
+const formatTests = {
   'http://example.com?': {
     href: 'http://example.com/?',
     protocol: 'http:',
@@ -1261,7 +1261,7 @@ for (const u in formatTests) {
 /*
  [from, path, expected]
 */
-var relativeTests = [
+const relativeTests = [
   ['/foo/bar/baz', 'quux', '/foo/bar/quux'],
   ['/foo/bar/baz', 'quux/asdf', '/foo/bar/quux/asdf'],
   ['/foo/bar/baz', 'quux/baz', '/foo/bar/quux/baz'],
@@ -1340,7 +1340,7 @@ relativeTests.forEach(function(relativeTest) {
 //
 // Changes marked with @isaacs
 
-var bases = [
+const bases = [
   'http://a/b/c/d;p?q',
   'http://a/b/c/d;p?q=1/2',
   'http://a/b/c/d;p=1/2?q',
@@ -1349,7 +1349,7 @@ var bases = [
 ];
 
 //[to, from, result]
-var relativeTests2 = [
+const relativeTests2 = [
   // http://lists.w3.org/Archives/Public/uri/2004Feb/0114.html
   ['../c', 'foo:a/b', 'foo:c'],
   ['foo:.', 'foo:a', 'foo:'],
@@ -1652,8 +1652,8 @@ relativeTests2.forEach(function(relativeTest) {
 
 //format: [from, path, expected]
 relativeTests.forEach(function(relativeTest) {
-  var actual = url.resolveObject(url.parse(relativeTest[0]), relativeTest[1]);
-  var expected = url.parse(relativeTest[2]);
+  let actual = url.resolveObject(url.parse(relativeTest[0]), relativeTest[1]);
+  let expected = url.parse(relativeTest[2]);
 
 
   assert.deepStrictEqual(actual, expected);
@@ -1680,8 +1680,8 @@ if (relativeTests2[181][0] === './/g' &&
   relativeTests2.splice(181, 1);
 }
 relativeTests2.forEach(function(relativeTest) {
-  var actual = url.resolveObject(url.parse(relativeTest[1]), relativeTest[0]);
-  var expected = url.parse(relativeTest[2]);
+  let actual = url.resolveObject(url.parse(relativeTest[1]), relativeTest[0]);
+  let expected = url.parse(relativeTest[2]);
 
   assert.deepStrictEqual(
     actual,
@@ -1699,7 +1699,7 @@ relativeTests2.forEach(function(relativeTest) {
 
 
 // https://github.com/nodejs/node/pull/1036
-var throws = [
+const throws = [
   undefined,
   null,
   true,
diff --git a/test/parallel/test-util-inspect.js b/test/parallel/test-util-inspect.js
index 0a2f929bb2..d1fa12902e 100644
--- a/test/parallel/test-util-inspect.js
+++ b/test/parallel/test-util-inspect.js
@@ -233,7 +233,7 @@ assert.strictEqual(util.inspect({get readwrite() {}, set readwrite(val) {}}),
 assert.strictEqual(util.inspect({set writeonly(val) {}}),
                    '{ writeonly: [Setter] }');
 
-var value = {};
+let value = {};
 value['a'] = value;
 assert.strictEqual(util.inspect(value), '{ a: [Circular] }');
 
@@ -272,10 +272,10 @@ assert.strictEqual(util.inspect(value), '{ 2010-02-14T11:48:40.000Z aprop: 42 }'
 
 // test the internal isDate implementation
 const Date2 = vm.runInNewContext('Date');
-var d = new Date2();
-var orig = util.inspect(d);
+const d = new Date2();
+const orig = util.inspect(d);
 Date2.prototype.foo = 'bar';
-var after = util.inspect(d);
+const after = util.inspect(d);
 assert.strictEqual(orig, after);
 
 // test positive/negative zero
@@ -283,7 +283,7 @@ assert.strictEqual(util.inspect(0), '0');
 assert.strictEqual(util.inspect(-0), '-0');
 
 // test for sparse array
-var a = ['foo', 'bar', 'baz'];
+const a = ['foo', 'bar', 'baz'];
 assert.strictEqual(util.inspect(a), '[ \'foo\', \'bar\', \'baz\' ]');
 delete a[1];
 assert.strictEqual(util.inspect(a), '[ \'foo\', , \'baz\' ]');
@@ -319,17 +319,17 @@ obj = vm.runInNewContext('fn=function(){};new Promise(fn,fn)', {});
 assert.strictEqual(util.inspect(obj), 'Promise { <pending> }');
 
 // test for property descriptors
-var getter = Object.create(null, {
+const getter = Object.create(null, {
   a: {
     get: function() { return 'aaa'; }
   }
 });
-var setter = Object.create(null, {
+const setter = Object.create(null, {
   b: {
     set: function() {}
   }
 });
-var getterAndSetter = Object.create(null, {
+const getterAndSetter = Object.create(null, {
   c: {
     get: function() { return 'ccc'; },
     set: function() {}
@@ -356,7 +356,7 @@ try {
 } catch (e) {
   assert.strictEqual(util.inspect(e), e.stack);
 }
-var ex = util.inspect(new Error('FAIL'), true);
+const ex = util.inspect(new Error('FAIL'), true);
 assert(ex.includes('Error: FAIL'));
 assert(ex.includes('[stack]'));
 assert(ex.includes('[message]'));
@@ -380,19 +380,19 @@ assert.strictEqual(util.inspect(Object.create(Date.prototype)), 'Date {}');
 
 // GH-1944
 assert.doesNotThrow(function() {
-  var d = new Date();
+  const d = new Date();
   d.toUTCString = null;
   util.inspect(d);
 });
 
 assert.doesNotThrow(function() {
-  var d = new Date();
+  const d = new Date();
   d.toISOString = null;
   util.inspect(d);
 });
 
 assert.doesNotThrow(function() {
-  var r = /regexp/;
+  const r = /regexp/;
   r.toString = null;
   util.inspect(r);
 });
@@ -411,14 +411,14 @@ assert.doesNotThrow(function() {
 }
 
 // util.inspect should not display the escaped value of a key.
-var w = {
+const w = {
   '\\': 1,
   '\\\\': 2,
   '\\\\\\': 3,
   '\\\\\\\\': 4,
 };
 
-var y = ['a', 'b', 'c'];
+const y = ['a', 'b', 'c'];
 y['\\\\\\'] = 'd';
 
 assert.strictEqual(
@@ -432,15 +432,15 @@ assert.strictEqual(
 
 // util.inspect.styles and util.inspect.colors
 function test_color_style(style, input, implicit) {
-  var color_name = util.inspect.styles[style];
-  var color = ['', ''];
+  const color_name = util.inspect.styles[style];
+  let color = ['', ''];
   if (util.inspect.colors[color_name])
     color = util.inspect.colors[color_name];
 
-  var without_color = util.inspect(input, false, 0, false);
-  var with_color = util.inspect(input, false, 0, true);
-  var expect = '\u001b[' + color[0] + 'm' + without_color +
-               '\u001b[' + color[1] + 'm';
+  const without_color = util.inspect(input, false, 0, false);
+  const with_color = util.inspect(input, false, 0, true);
+  const expect = '\u001b[' + color[0] + 'm' + without_color +
+                 '\u001b[' + color[1] + 'm';
   assert.strictEqual(
     with_color,
     expect,
@@ -593,19 +593,19 @@ assert.doesNotThrow(function() {
 
 // util.inspect with "colors" option should produce as many lines as without it
 function test_lines(input) {
-  var count_lines = function(str) {
+  const count_lines = function(str) {
     return (str.match(/\n/g) || []).length;
   };
 
-  var without_color = util.inspect(input);
-  var with_color = util.inspect(input, {colors: true});
+  const without_color = util.inspect(input);
+  const with_color = util.inspect(input, {colors: true});
   assert.strictEqual(count_lines(without_color), count_lines(with_color));
 }
 
 test_lines([1, 2, 3, 4, 5, 6, 7]);
 test_lines(function() {
-  var big_array = [];
-  for (var i = 0; i < 100; i++) {
+  const big_array = [];
+  for (let i = 0; i < 100; i++) {
     big_array.push(i);
   }
   return big_array;
@@ -633,15 +633,15 @@ assert.strictEqual(util.inspect(new Number(-1.1)), '[Number: -1.1]');
 assert.strictEqual(util.inspect(new Number(13.37)), '[Number: 13.37]');
 
 // test boxed primitives with own properties
-var str = new String('baz');
+const str = new String('baz');
 str.foo = 'bar';
 assert.strictEqual(util.inspect(str), '{ [String: \'baz\'] foo: \'bar\' }');
 
-var bool = new Boolean(true);
+const bool = new Boolean(true);
 bool.foo = 'bar';
 assert.strictEqual(util.inspect(bool), '{ [Boolean: true] foo: \'bar\' }');
 
-var num = new Number(13.37);
+const num = new Number(13.37);
 num.foo = 'bar';
 assert.strictEqual(util.inspect(num), '{ [Number: 13.37] foo: \'bar\' }');
 
@@ -675,7 +675,7 @@ if (typeof Symbol !== 'undefined') {
 // test Set
 assert.strictEqual(util.inspect(new Set()), 'Set {}');
 assert.strictEqual(util.inspect(new Set([1, 2, 3])), 'Set { 1, 2, 3 }');
-var set = new Set(['foo']);
+const set = new Set(['foo']);
 set.bar = 42;
 assert.strictEqual(
   util.inspect(set, true),
@@ -707,30 +707,30 @@ assert.strictEqual(
   util.inspect(new Promise(function() {})),
   'Promise { <pending> }'
 );
-var promise = Promise.resolve('foo');
+const promise = Promise.resolve('foo');
 promise.bar = 42;
 assert.strictEqual(util.inspect(promise), 'Promise { \'foo\', bar: 42 }');
 
 // Make sure it doesn't choke on polyfills. Unlike Set/Map, there is no standard
 // interface to synchronously inspect a Promise, so our techniques only work on
 // a bonafide native Promise.
-var oldPromise = Promise;
+const oldPromise = Promise;
 global.Promise = function() { this.bar = 42; };
 assert.strictEqual(util.inspect(new Promise()), '{ bar: 42 }');
 global.Promise = oldPromise;
 
 // Map/Set Iterators
-var m = new Map([['foo', 'bar']]);
+const m = new Map([['foo', 'bar']]);
 assert.strictEqual(util.inspect(m.keys()), 'MapIterator { \'foo\' }');
 assert.strictEqual(util.inspect(m.values()), 'MapIterator { \'bar\' }');
 assert.strictEqual(util.inspect(m.entries()),
                    'MapIterator { [ \'foo\', \'bar\' ] }');
 // make sure the iterator doesn't get consumed
-var keys = m.keys();
+let keys = m.keys();
 assert.strictEqual(util.inspect(keys), 'MapIterator { \'foo\' }');
 assert.strictEqual(util.inspect(keys), 'MapIterator { \'foo\' }');
 
-var s = new Set([1, 3]);
+const s = new Set([1, 3]);
 assert.strictEqual(util.inspect(s.keys()), 'SetIterator { 1, 3 }');
 assert.strictEqual(util.inspect(s.values()), 'SetIterator { 1, 3 }');
 assert.strictEqual(util.inspect(s.entries()),
@@ -744,10 +744,10 @@ assert.strictEqual(util.inspect(keys), 'SetIterator { 1, 3 }');
 // Assumes that the first numeric character is the start of an item.
 
 function checkAlignment(container) {
-  var lines = util.inspect(container).split('\n');
-  var pos;
+  const lines = util.inspect(container).split('\n');
+  let pos;
   lines.forEach(function(line) {
-    var npos = line.search(/\d/);
+    const npos = line.search(/\d/);
     if (npos !== -1) {
       if (pos !== undefined)
         assert.strictEqual(pos, npos, 'container items not aligned');
@@ -756,14 +756,14 @@ function checkAlignment(container) {
   });
 }
 
-var big_array = [];
-for (var i = 0; i < 100; i++) {
+const big_array = [];
+for (let i = 0; i < 100; i++) {
   big_array.push(i);
 }
 
 checkAlignment(big_array);
 checkAlignment(function() {
-  var obj = {};
+  const obj = {};
   big_array.forEach(function(v) {
     obj[v] = null;
   });
diff --git a/test/parallel/test-util-log.js b/test/parallel/test-util-log.js
index 8480bf83ea..d45ebf2fc0 100644
--- a/test/parallel/test-util-log.js
+++ b/test/parallel/test-util-log.js
@@ -6,14 +6,14 @@ const util = require('util');
 assert.ok(process.stdout.writable);
 assert.ok(process.stderr.writable);
 
-var stdout_write = global.process.stdout.write;
-var strings = [];
+const stdout_write = global.process.stdout.write;
+const strings = [];
 global.process.stdout.write = function(string) {
   strings.push(string);
 };
 console._stderr = process.stdout;
 
-var tests = [
+const tests = [
   {input: 'foo', output: 'foo'},
   {input: undefined, output: 'undefined'},
   {input: null, output: 'null'},
diff --git a/test/parallel/test-v8-stats.js b/test/parallel/test-v8-stats.js
index dc36beac13..5b756f3596 100644
--- a/test/parallel/test-v8-stats.js
+++ b/test/parallel/test-v8-stats.js
@@ -3,8 +3,8 @@ require('../common');
 const assert = require('assert');
 const v8 = require('v8');
 
-var s = v8.getHeapStatistics();
-var keys = [
+const s = v8.getHeapStatistics();
+const keys = [
   'does_zap_garbage',
   'heap_size_limit',
   'malloced_memory',
diff --git a/test/parallel/test-vm-basic.js b/test/parallel/test-vm-basic.js
index 75163d6cd3..2d57cc0d4b 100644
--- a/test/parallel/test-vm-basic.js
+++ b/test/parallel/test-vm-basic.js
@@ -4,8 +4,8 @@ const assert = require('assert');
 const vm = require('vm');
 
 // Test 1: vm.runInNewContext
-var sandbox = {};
-var result = vm.runInNewContext(
+const sandbox = {};
+let result = vm.runInNewContext(
   'foo = "bar"; this.typeofProcess = typeof process; typeof Object;',
   sandbox
 );
@@ -16,8 +16,8 @@ assert.deepStrictEqual(sandbox, {
 assert.strictEqual(result, 'function');
 
 // Test 2: vm.runInContext
-var sandbox2 = { foo: 'bar' };
-var context = vm.createContext(sandbox2);
+const sandbox2 = { foo: 'bar' };
+const context = vm.createContext(sandbox2);
 result = vm.runInContext(
   'baz = foo; this.typeofProcess = typeof process; typeof Object;',
   context
@@ -45,6 +45,6 @@ assert.strictEqual(global.vmResult, undefined);
 assert.strictEqual(result, 'undefined');
 
 // Test 5: vm.createContext
-var sandbox3 = {};
-var context2 = vm.createContext(sandbox3);
+const sandbox3 = {};
+const context2 = vm.createContext(sandbox3);
 assert.strictEqual(sandbox3, context2);
diff --git a/test/parallel/test-vm-context-async-script.js b/test/parallel/test-vm-context-async-script.js
index b4045fbfd3..1e9ed629fb 100644
--- a/test/parallel/test-vm-context-async-script.js
+++ b/test/parallel/test-vm-context-async-script.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const vm = require('vm');
 
-var sandbox = { setTimeout: setTimeout };
+const sandbox = { setTimeout: setTimeout };
 
-var ctx = vm.createContext(sandbox);
+const ctx = vm.createContext(sandbox);
 
 vm.runInContext('setTimeout(function() { x = 3; }, 0);', ctx);
 setTimeout(function() {
diff --git a/test/parallel/test-vm-context-property-forwarding.js b/test/parallel/test-vm-context-property-forwarding.js
index 03faef596f..b77fc45217 100644
--- a/test/parallel/test-vm-context-property-forwarding.js
+++ b/test/parallel/test-vm-context-property-forwarding.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const vm = require('vm');
 
-var sandbox = { x: 3 };
+const sandbox = { x: 3 };
 
-var ctx = vm.createContext(sandbox);
+const ctx = vm.createContext(sandbox);
 
 assert.strictEqual(vm.runInContext('x;', ctx), 3);
 vm.runInContext('y = 4;', ctx);
diff --git a/test/parallel/test-vm-context.js b/test/parallel/test-vm-context.js
index 48319be3a9..e4f6960259 100644
--- a/test/parallel/test-vm-context.js
+++ b/test/parallel/test-vm-context.js
@@ -3,12 +3,12 @@ require('../common');
 const assert = require('assert');
 
 const vm = require('vm');
-var Script = vm.Script;
-var script = new Script('"passed";');
+const Script = vm.Script;
+let script = new Script('"passed";');
 
 console.error('run in a new empty context');
-var context = vm.createContext();
-var result = script.runInContext(context);
+let context = vm.createContext();
+let result = script.runInContext(context);
 assert.equal('passed', result);
 
 console.error('create a new pre-populated context');
@@ -29,7 +29,7 @@ assert.throws(function() {
 
 // Issue GH-1140:
 console.error('test runInContext signature');
-var gh1140Exception;
+let gh1140Exception;
 try {
   vm.runInContext('throw new Error()', context, 'expected-filename.js');
 } catch (e) {
@@ -55,7 +55,7 @@ script.runInNewContext({ require: require });
 
 // Issue GH-7529
 script = vm.createScript('delete b');
-var ctx = {};
+let ctx = {};
 Object.defineProperty(ctx, 'b', { configurable: false });
 ctx = vm.createContext(ctx);
 assert.equal(script.runInContext(ctx), false);
diff --git a/test/parallel/test-vm-create-and-run-in-context.js b/test/parallel/test-vm-create-and-run-in-context.js
index b40491bb61..a0026102b5 100644
--- a/test/parallel/test-vm-create-and-run-in-context.js
+++ b/test/parallel/test-vm-create-and-run-in-context.js
@@ -6,8 +6,8 @@ const assert = require('assert');
 const vm = require('vm');
 
 console.error('run in a new empty context');
-var context = vm.createContext();
-var result = vm.runInContext('"passed";', context);
+let context = vm.createContext();
+let result = vm.runInContext('"passed";', context);
 assert.equal('passed', result);
 
 console.error('create a new pre-populated context');
@@ -22,7 +22,7 @@ assert.equal('lala', context.thing);
 
 // https://github.com/nodejs/node/issues/5768
 console.error('run in contextified sandbox without referencing the context');
-var sandbox = {x: 1};
+const sandbox = {x: 1};
 vm.createContext(sandbox);
 global.gc();
 vm.runInContext('x = 2', sandbox);
diff --git a/test/parallel/test-vm-create-context-accessors.js b/test/parallel/test-vm-create-context-accessors.js
index 3870b68247..27d7e6d7fc 100644
--- a/test/parallel/test-vm-create-context-accessors.js
+++ b/test/parallel/test-vm-create-context-accessors.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const vm = require('vm');
 
-var ctx = {};
+let ctx = {};
 
 Object.defineProperty(ctx, 'getter', {
   get: function() {
@@ -11,7 +11,7 @@ Object.defineProperty(ctx, 'getter', {
   }
 });
 
-var val;
+let val;
 Object.defineProperty(ctx, 'setter', {
   set: function(_val) {
     val = _val;
@@ -23,6 +23,6 @@ Object.defineProperty(ctx, 'setter', {
 
 ctx = vm.createContext(ctx);
 
-var result = vm.runInContext('setter = "test";[getter,setter]', ctx);
+const result = vm.runInContext('setter = "test";[getter,setter]', ctx);
 assert.strictEqual(result[0], 'ok');
 assert.strictEqual(result[1], 'ok=test');
diff --git a/test/parallel/test-vm-create-context-arg.js b/test/parallel/test-vm-create-context-arg.js
index 76634e5bef..91bbbc3b77 100644
--- a/test/parallel/test-vm-create-context-arg.js
+++ b/test/parallel/test-vm-create-context-arg.js
@@ -13,7 +13,7 @@ assert.doesNotThrow(function() {
 });
 
 assert.doesNotThrow(function() {
-  var sandbox = {};
+  const sandbox = {};
   vm.createContext(sandbox);
   vm.createContext(sandbox);
 });
diff --git a/test/parallel/test-vm-create-context-circular-reference.js b/test/parallel/test-vm-create-context-circular-reference.js
index e6c1ab8b85..13680c4c4e 100644
--- a/test/parallel/test-vm-create-context-circular-reference.js
+++ b/test/parallel/test-vm-create-context-circular-reference.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const vm = require('vm');
 
-var sbx = {};
+let sbx = {};
 sbx.window = sbx;
 
 sbx = vm.createContext(sbx);
diff --git a/test/parallel/test-vm-cross-context.js b/test/parallel/test-vm-cross-context.js
index c1d115b50b..bafb32accb 100644
--- a/test/parallel/test-vm-cross-context.js
+++ b/test/parallel/test-vm-cross-context.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 
 const vm = require('vm');
-var ctx = vm.createContext(global);
+const ctx = vm.createContext(global);
 
 assert.doesNotThrow(function() {
   vm.runInContext('!function() { var x = console.log; }()', ctx);
diff --git a/test/parallel/test-vm-function-declaration.js b/test/parallel/test-vm-function-declaration.js
index 976313b5f6..9f27723a04 100644
--- a/test/parallel/test-vm-function-declaration.js
+++ b/test/parallel/test-vm-function-declaration.js
@@ -3,10 +3,10 @@ require('../common');
 const assert = require('assert');
 
 const vm = require('vm');
-var o = vm.createContext({ console: console });
+const o = vm.createContext({ console: console });
 
 // This triggers the setter callback in node_contextify.cc
-var code = 'var a = function() {};\n';
+let code = 'var a = function() {};\n';
 
 // but this does not, since function decls are defineProperties,
 // not simple sets.
@@ -16,7 +16,7 @@ code += 'function b(){}\n';
 // we are getting the global function, and not some other thing
 code += '(function(){return this})().b;\n';
 
-var res = vm.runInContext(code, o, 'test');
+const res = vm.runInContext(code, o, 'test');
 
 assert.equal(typeof res, 'function', 'result should be function');
 assert.equal(res.name, 'b', 'res should be named b');
diff --git a/test/parallel/test-vm-global-define-property.js b/test/parallel/test-vm-global-define-property.js
index 31842ced2a..62a714615d 100644
--- a/test/parallel/test-vm-global-define-property.js
+++ b/test/parallel/test-vm-global-define-property.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 
 const vm = require('vm');
 
-var code =
+const code =
     'Object.defineProperty(this, "f", {\n' +
     '  get: function() { return x; },\n' +
     '  set: function(k) { x = k; },\n' +
@@ -14,10 +14,10 @@ var code =
     'g = f;\n' +
     'f;\n';
 
-var x = {};
-var o = vm.createContext({ console: console, x: x });
+const x = {};
+const o = vm.createContext({ console: console, x: x });
 
-var res = vm.runInContext(code, o, 'test');
+const res = vm.runInContext(code, o, 'test');
 
 assert(res);
 assert.equal(typeof res, 'object');
diff --git a/test/parallel/test-vm-global-identity.js b/test/parallel/test-vm-global-identity.js
index 876318aece..ae9fd71b31 100644
--- a/test/parallel/test-vm-global-identity.js
+++ b/test/parallel/test-vm-global-identity.js
@@ -3,9 +3,9 @@ require('../common');
 const assert = require('assert');
 const vm = require('vm');
 
-var ctx = vm.createContext();
+const ctx = vm.createContext();
 ctx.window = ctx;
 
-var thisVal = vm.runInContext('this;', ctx);
-var windowVal = vm.runInContext('window;', ctx);
+const thisVal = vm.runInContext('this;', ctx);
+const windowVal = vm.runInContext('window;', ctx);
 assert.strictEqual(thisVal, windowVal);
diff --git a/test/parallel/test-vm-harmony-symbols.js b/test/parallel/test-vm-harmony-symbols.js
index eeacdaed18..80c1a2127f 100644
--- a/test/parallel/test-vm-harmony-symbols.js
+++ b/test/parallel/test-vm-harmony-symbols.js
@@ -4,7 +4,7 @@ const assert = require('assert');
 const vm = require('vm');
 
 // The sandbox should have its own Symbol constructor.
-var sandbox = {};
+let sandbox = {};
 vm.runInNewContext('this.Symbol = Symbol', sandbox);
 assert.strictEqual(typeof sandbox.Symbol, 'function');
 assert.notStrictEqual(sandbox.Symbol, Symbol);
diff --git a/test/parallel/test-vm-is-context.js b/test/parallel/test-vm-is-context.js
index 474c0686a8..e58b896fcc 100644
--- a/test/parallel/test-vm-is-context.js
+++ b/test/parallel/test-vm-is-context.js
@@ -13,6 +13,6 @@ assert.strictEqual(vm.isContext([]), false);
 assert.strictEqual(vm.isContext(vm.createContext()), true);
 assert.strictEqual(vm.isContext(vm.createContext([])), true);
 
-var sandbox = { foo: 'bar' };
+const sandbox = { foo: 'bar' };
 vm.createContext(sandbox);
 assert.strictEqual(vm.isContext(sandbox), true);
diff --git a/test/parallel/test-vm-new-script-new-context.js b/test/parallel/test-vm-new-script-new-context.js
index 2af8f05f1c..a02627668f 100644
--- a/test/parallel/test-vm-new-script-new-context.js
+++ b/test/parallel/test-vm-new-script-new-context.js
@@ -6,7 +6,7 @@ const Script = require('vm').Script;
 common.globalCheck = false;
 
 console.error('run a string');
-var script = new Script('\'passed\';');
+let script = new Script('\'passed\';');
 console.error('script created');
 const result1 = script.runInNewContext();
 const result2 = script.runInNewContext();
@@ -41,7 +41,7 @@ global.foo = 2;
 global.obj = { foo: 0, baz: 3 };
 script = new Script(global.code);
 /* eslint-disable no-unused-vars */
-var baz = script.runInNewContext(global.obj);
+const baz = script.runInNewContext(global.obj);
 /* eslint-enable no-unused-vars */
 assert.equal(1, global.obj.foo);
 assert.equal(2, global.obj.bar);
@@ -55,7 +55,7 @@ assert.equal(global.foo, 100);
 
 console.error('modify an object by reference');
 script = new Script('f.a = 2');
-var f = { a: 1 };
+const f = { a: 1 };
 script.runInNewContext({ f: f });
 assert.equal(f.a, 2);
 
diff --git a/test/parallel/test-vm-new-script-this-context.js b/test/parallel/test-vm-new-script-this-context.js
index d225fb0d40..fb7e271985 100644
--- a/test/parallel/test-vm-new-script-this-context.js
+++ b/test/parallel/test-vm-new-script-this-context.js
@@ -6,7 +6,7 @@ const Script = require('vm').Script;
 common.globalCheck = false;
 
 console.error('run a string');
-var script = new Script('\'passed\';');
+let script = new Script('\'passed\';');
 const result = script.runInThisContext(script);
 assert.equal('passed', result);
 
diff --git a/test/parallel/test-vm-proxies.js b/test/parallel/test-vm-proxies.js
index 6532345336..25e16e8f5c 100644
--- a/test/parallel/test-vm-proxies.js
+++ b/test/parallel/test-vm-proxies.js
@@ -6,7 +6,7 @@ const vm = require('vm');
 
 // src/node_contextify.cc filters out the Proxy object from the parent
 // context.  Make sure that the new context has a Proxy object of its own.
-var sandbox = {};
+let sandbox = {};
 vm.runInNewContext('this.Proxy = Proxy', sandbox);
 assert.strictEqual(typeof sandbox.Proxy, 'function');
 assert.notStrictEqual(sandbox.Proxy, Proxy);
diff --git a/test/parallel/test-vm-run-in-new-context.js b/test/parallel/test-vm-run-in-new-context.js
index 956e02306d..b00e5a973e 100644
--- a/test/parallel/test-vm-run-in-new-context.js
+++ b/test/parallel/test-vm-run-in-new-context.js
@@ -30,7 +30,7 @@ global.code = 'foo = 1;' +
 global.foo = 2;
 global.obj = { foo: 0, baz: 3 };
 /* eslint-disable no-unused-vars */
-var baz = vm.runInNewContext(global.code, global.obj);
+const baz = vm.runInNewContext(global.code, global.obj);
 /* eslint-enable no-unused-vars */
 assert.equal(1, global.obj.foo);
 assert.equal(2, global.obj.bar);
@@ -42,12 +42,12 @@ vm.runInNewContext('f()', { f: changeFoo });
 assert.equal(global.foo, 100);
 
 console.error('modify an object by reference');
-var f = { a: 1 };
+const f = { a: 1 };
 vm.runInNewContext('f.a = 2', { f: f });
 assert.equal(f.a, 2);
 
 console.error('use function in context without referencing context');
-var fn = vm.runInNewContext('(function() { obj.p = {}; })', { obj: {} });
+const fn = vm.runInNewContext('(function() { obj.p = {}; })', { obj: {} });
 global.gc();
 fn();
 // Should not crash
diff --git a/test/parallel/test-vm-static-this.js b/test/parallel/test-vm-static-this.js
index 0f578ac5f8..076f81dbf6 100644
--- a/test/parallel/test-vm-static-this.js
+++ b/test/parallel/test-vm-static-this.js
@@ -6,7 +6,7 @@ const vm = require('vm');
 common.globalCheck = false;
 
 // Run a string
-var result = vm.runInThisContext('\'passed\';');
+const result = vm.runInThisContext('\'passed\';');
 assert.strictEqual('passed', result);
 
 // thrown error
@@ -20,13 +20,14 @@ assert.strictEqual(2, global.hello);
 
 
 // pass values
-var code = 'foo = 1;' +
-           'bar = 2;' +
-           'if (typeof baz !== \'undefined\') throw new Error(\'test fail\');';
+const code = 'foo = 1;' +
+             'bar = 2;' +
+             'if (typeof baz !== \'undefined\')' +
+             'throw new Error(\'test fail\');';
 global.foo = 2;
 global.obj = { foo: 0, baz: 3 };
 /* eslint-disable no-unused-vars */
-var baz = vm.runInThisContext(code);
+const baz = vm.runInThisContext(code);
 /* eslint-enable no-unused-vars */
 assert.strictEqual(0, global.obj.foo);
 assert.strictEqual(2, global.bar);
diff --git a/test/parallel/test-vm-syntax-error-message.js b/test/parallel/test-vm-syntax-error-message.js
index b4c101d580..1a97393d6d 100644
--- a/test/parallel/test-vm-syntax-error-message.js
+++ b/test/parallel/test-vm-syntax-error-message.js
@@ -3,7 +3,7 @@ require('../common');
 const assert = require('assert');
 const child_process = require('child_process');
 
-var p = child_process.spawn(process.execPath, [
+const p = child_process.spawn(process.execPath, [
   '-e',
   'vm = require("vm");' +
       'context = vm.createContext({});' +
@@ -15,7 +15,7 @@ p.stderr.on('data', function(data) {
   assert(false, 'Unexpected stderr data: ' + data);
 });
 
-var output = '';
+let output = '';
 
 p.stdout.on('data', function(data) {
   output += data;
diff --git a/test/parallel/test-vm-timeout.js b/test/parallel/test-vm-timeout.js
index 5b78c891e8..5260ca7dc2 100644
--- a/test/parallel/test-vm-timeout.js
+++ b/test/parallel/test-vm-timeout.js
@@ -23,7 +23,7 @@ vm.runInThisContext('', { timeout: 1000 });
 
 // Test 5: Nested vm timeouts, inner timeout propagates out
 assert.throws(function() {
-  var context = {
+  const context = {
     log: console.log,
     runInVM: function(timeout) {
       vm.runInNewContext('while(true) {}', context, { timeout: timeout });
diff --git a/test/parallel/test-whatwg-url-searchparams.js b/test/parallel/test-whatwg-url-searchparams.js
index 54743b97a3..ca8f4ff070 100644
--- a/test/parallel/test-whatwg-url-searchparams.js
+++ b/test/parallel/test-whatwg-url-searchparams.js
@@ -31,7 +31,7 @@ assert.strictEqual(m.search, `?${serialized}`);
 
 assert.strictEqual(sp[Symbol.iterator], sp.entries);
 
-var key, val, n = 0;
+let key, val, n = 0;
 for ([key, val] of sp) {
   assert.strictEqual(key, 'a');
   assert.strictEqual(val, String(values[n++]));
diff --git a/test/parallel/test-whatwg-url-setters.js b/test/parallel/test-whatwg-url-setters.js
index 3b2aca2afd..20544dbb7f 100644
--- a/test/parallel/test-whatwg-url-setters.js
+++ b/test/parallel/test-whatwg-url-setters.js
@@ -17,7 +17,7 @@ for (const attr in attrs) {
   if (attr === 'comment')
     continue;
   const tests = attrs[attr];
-  var n = 0;
+  let n = 0;
   for (const test of tests) {
     if (test.skip) continue;
     n++;
diff --git a/test/parallel/test-writedouble.js b/test/parallel/test-writedouble.js
index 107bf52134..935f9e7ae0 100644
--- a/test/parallel/test-writedouble.js
+++ b/test/parallel/test-writedouble.js
@@ -6,7 +6,7 @@ require('../common');
 const assert = require('assert');
 
 function test(clazz) {
-  var buffer = new clazz(16);
+  const buffer = new clazz(16);
 
   buffer.writeDoubleBE(2.225073858507201e-308, 0);
   buffer.writeDoubleLE(2.225073858507201e-308, 8);
diff --git a/test/parallel/test-writefloat.js b/test/parallel/test-writefloat.js
index 2f76dd57c7..113e97c6d3 100644
--- a/test/parallel/test-writefloat.js
+++ b/test/parallel/test-writefloat.js
@@ -6,7 +6,7 @@ require('../common');
 const assert = require('assert');
 
 function test(clazz) {
-  var buffer = new clazz(8);
+  const buffer = new clazz(8);
 
   buffer.writeFloatBE(1, 0);
   buffer.writeFloatLE(1, 4);
diff --git a/test/parallel/test-writeint.js b/test/parallel/test-writeint.js
index 43784fb390..ed95e7876e 100644
--- a/test/parallel/test-writeint.js
+++ b/test/parallel/test-writeint.js
@@ -6,7 +6,7 @@ require('../common');
 const assert = require('assert');
 
 function test8(clazz) {
-  var buffer = new clazz(2);
+  const buffer = new clazz(2);
 
   buffer.writeInt8(0x23, 0);
   buffer.writeInt8(-5, 1);
@@ -38,7 +38,7 @@ function test8(clazz) {
 
 
 function test16(clazz) {
-  var buffer = new clazz(6);
+  const buffer = new clazz(6);
 
   buffer.writeInt16BE(0x0023, 0);
   buffer.writeInt16LE(0x0023, 2);
@@ -91,7 +91,7 @@ function test16(clazz) {
 
 
 function test32(clazz) {
-  var buffer = new clazz(8);
+  const buffer = new clazz(8);
 
   buffer.writeInt32BE(0x23, 0);
   buffer.writeInt32LE(0x23, 4);
diff --git a/test/parallel/test-writeuint.js b/test/parallel/test-writeuint.js
index 0cccd4bc15..bddb954d24 100644
--- a/test/parallel/test-writeuint.js
+++ b/test/parallel/test-writeuint.js
@@ -13,7 +13,7 @@ const assert = require('assert');
  *  - Correctly interpreting values that are beyond the signed range as unsigned
  */
 function test8(clazz) {
-  var data = new clazz(4);
+  const data = new clazz(4);
 
   data.writeUInt8(23, 0);
   data.writeUInt8(23, 1);
@@ -42,8 +42,8 @@ function test8(clazz) {
 
 
 function test16(clazz) {
-  var value = 0x2343;
-  var data = new clazz(4);
+  let value = 0x2343;
+  const data = new clazz(4);
 
   data.writeUInt16BE(value, 0);
   assert.equal(0x23, data[0]);
@@ -81,8 +81,8 @@ function test16(clazz) {
 
 
 function test32(clazz) {
-  var data = new clazz(6);
-  var value = 0xe7f90a6d;
+  const data = new clazz(6);
+  const value = 0xe7f90a6d;
 
   data.writeUInt32BE(value, 0);
   assert.equal(0xe7, data[0]);
@@ -124,10 +124,10 @@ function test32(clazz) {
 
 function testUint(clazz) {
   const data = new clazz(8);
-  var val = 1;
+  let val = 1;
 
   // Test 0 to 5 bytes.
-  for (var i = 0; i <= 5; i++) {
+  for (let i = 0; i <= 5; i++) {
     const errmsg = `byteLength: ${i}`;
     assert.throws(function() {
       data.writeUIntBE(val, 0, i);
diff --git a/test/parallel/test-zerolengthbufferbug.js b/test/parallel/test-zerolengthbufferbug.js
index 07113d1e25..42e6994e1b 100644
--- a/test/parallel/test-zerolengthbufferbug.js
+++ b/test/parallel/test-zerolengthbufferbug.js
@@ -4,8 +4,8 @@
 const common = require('../common');
 const http = require('http');
 
-var server = http.createServer(function(req, res) {
-  var buffer = Buffer.alloc(0);
+const server = http.createServer(function(req, res) {
+  const buffer = Buffer.alloc(0);
   // FIXME: WTF gjslint want this?
   res.writeHead(200, {'Content-Type': 'text/html',
                       'Content-Length': buffer.length});
diff --git a/test/parallel/test-zlib-convenience-methods.js b/test/parallel/test-zlib-convenience-methods.js
index 3aac0142b5..b67b65d7d0 100644
--- a/test/parallel/test-zlib-convenience-methods.js
+++ b/test/parallel/test-zlib-convenience-methods.js
@@ -5,7 +5,7 @@ require('../common');
 const assert = require('assert');
 const zlib = require('zlib');
 
-var hadRun = 0;
+let hadRun = 0;
 
 const expect = 'blahblahblahblahblahblah';
 const opts = {
@@ -38,7 +38,7 @@ const opts = {
     });
   });
 
-  var result = zlib[method[0] + 'Sync'](expect, opts);
+  let result = zlib[method[0] + 'Sync'](expect, opts);
   result = zlib[method[1] + 'Sync'](result, opts);
   assert.equal(result, expect,
                'Should get original string after ' +
diff --git a/test/parallel/test-zlib-flush-drain.js b/test/parallel/test-zlib-flush-drain.js
index 45cca58485..afa4194ac6 100644
--- a/test/parallel/test-zlib-flush-drain.js
+++ b/test/parallel/test-zlib-flush-drain.js
@@ -13,8 +13,8 @@ const opts = {
 const deflater = zlib.createDeflate(opts);
 
 // shim deflater.flush so we can count times executed
-var flushCount = 0;
-var drainCount = 0;
+let flushCount = 0;
+let drainCount = 0;
 
 const flush = deflater.flush;
 deflater.flush = function(kind, callback) {
@@ -26,7 +26,7 @@ deflater.write(bigData);
 
 const ws = deflater._writableState;
 const beforeFlush = ws.needDrain;
-var afterFlush = ws.needDrain;
+let afterFlush = ws.needDrain;
 
 deflater.flush(function(err) {
   afterFlush = ws.needDrain;
diff --git a/test/parallel/test-zlib-flush.js b/test/parallel/test-zlib-flush.js
index 7f0021434f..0eec5da7dd 100644
--- a/test/parallel/test-zlib-flush.js
+++ b/test/parallel/test-zlib-flush.js
@@ -22,7 +22,8 @@ deflater.write(chunk, function() {
   deflater.flush(zlib.constants.Z_NO_FLUSH, function() {
     actualNone = deflater.read();
     deflater.flush(function() {
-      var bufs = [], buf;
+      const bufs = [];
+      let buf;
       while (buf = deflater.read())
         bufs.push(buf);
       actualFull = Buffer.concat(bufs);
diff --git a/test/parallel/test-zlib-from-gzip.js b/test/parallel/test-zlib-from-gzip.js
index 3739d8f9b4..3c7e96cef9 100644
--- a/test/parallel/test-zlib-from-gzip.js
+++ b/test/parallel/test-zlib-from-gzip.js
@@ -24,7 +24,7 @@ inp.pipe(gunzip).pipe(out);
 out.on('close', function() {
   const actual = fs.readFileSync(outputFile);
   assert.equal(actual.length, expect.length, 'length should match');
-  for (var i = 0, l = actual.length; i < l; i++) {
+  for (let i = 0, l = actual.length; i < l; i++) {
     assert.equal(actual[i], expect[i], 'byte[' + i + ']');
   }
 });
diff --git a/test/parallel/test-zlib-from-string.js b/test/parallel/test-zlib-from-string.js
index f3b8426eb4..0f885ac80e 100644
--- a/test/parallel/test-zlib-from-string.js
+++ b/test/parallel/test-zlib-from-string.js
@@ -51,7 +51,7 @@ zlib.gzip(inputString, function(err, buffer) {
   });
 });
 
-var buffer = Buffer.from(expectedBase64Deflate, 'base64');
+let buffer = Buffer.from(expectedBase64Deflate, 'base64');
 zlib.unzip(buffer, function(err, buffer) {
   assert.equal(buffer.toString(), inputString,
                'decoded inflated string should match');
diff --git a/test/parallel/test-zlib-invalid-input.js b/test/parallel/test-zlib-invalid-input.js
index eb9cd334c1..7b0e1fcaae 100644
--- a/test/parallel/test-zlib-invalid-input.js
+++ b/test/parallel/test-zlib-invalid-input.js
@@ -24,7 +24,7 @@ const unzips = [ zlib.Unzip(),
                  zlib.Gunzip(),
                  zlib.Inflate(),
                  zlib.InflateRaw() ];
-var hadError = [];
+const hadError = [];
 unzips.forEach(function(uz, i) {
   console.error('Error for ' + uz.constructor.name);
   uz.on('error', function(er) {
diff --git a/test/parallel/test-zlib-params.js b/test/parallel/test-zlib-params.js
index 0dce05be4d..40799637ec 100644
--- a/test/parallel/test-zlib-params.js
+++ b/test/parallel/test-zlib-params.js
@@ -21,7 +21,7 @@ deflater.write(chunk1, function() {
     while (deflater.read());
     deflater.end(chunk2, function() {
       const bufs = [];
-      var buf;
+      let buf;
       while (buf = deflater.read())
         bufs.push(buf);
       actual = Buffer.concat(bufs);
diff --git a/test/parallel/test-zlib-random-byte-pipes.js b/test/parallel/test-zlib-random-byte-pipes.js
index 6037703f7b..83855ebd1f 100644
--- a/test/parallel/test-zlib-random-byte-pipes.js
+++ b/test/parallel/test-zlib-random-byte-pipes.js
@@ -72,14 +72,14 @@ RandomReadStream.prototype._process = function() {
 
   // figure out how many bytes to output
   // if finished, then just emit end.
-  var block = this._opt.block;
+  let block = this._opt.block;
   const jitter = this._opt.jitter;
   if (jitter) {
     block += Math.ceil(Math.random() * jitter - (jitter / 2));
   }
   block = Math.min(block, this._remaining);
   const buf = Buffer.allocUnsafe(block);
-  for (var i = 0; i < block; i++) {
+  for (let i = 0; i < block; i++) {
     buf[i] = Math.random() * 256;
   }
 
@@ -129,10 +129,10 @@ HashStream.prototype.end = function(c) {
 };
 
 
-var inp = new RandomReadStream({ total: 1024, block: 256, jitter: 16 });
-var out = new HashStream();
-var gzip = zlib.createGzip();
-var gunz = zlib.createGunzip();
+const inp = new RandomReadStream({ total: 1024, block: 256, jitter: 16 });
+const out = new HashStream();
+const gzip = zlib.createGzip();
+const gunz = zlib.createGunzip();
 
 inp.pipe(gzip).pipe(gunz).pipe(out);
 
diff --git a/test/parallel/test-zlib-write-after-flush.js b/test/parallel/test-zlib-write-after-flush.js
index 89edaf9304..342c9143ba 100644
--- a/test/parallel/test-zlib-write-after-flush.js
+++ b/test/parallel/test-zlib-write-after-flush.js
@@ -8,7 +8,7 @@ const gunz = zlib.createUnzip();
 
 gzip.pipe(gunz);
 
-var output = '';
+let output = '';
 const input = 'A line of data\n';
 gunz.setEncoding('utf8');
 gunz.on('data', function(c) {
diff --git a/test/parallel/test-zlib-zero-byte.js b/test/parallel/test-zlib-zero-byte.js
index ca58d06ff2..cd4cf41f98 100644
--- a/test/parallel/test-zlib-zero-byte.js
+++ b/test/parallel/test-zlib-zero-byte.js
@@ -5,7 +5,7 @@ const assert = require('assert');
 const zlib = require('zlib');
 const gz = zlib.Gzip();
 const emptyBuffer = Buffer.alloc(0);
-var received = 0;
+let received = 0;
 gz.on('data', function(c) {
   received += c.length;
 });
diff --git a/test/parallel/test-zlib.js b/test/parallel/test-zlib.js
index 54a4bc4736..fdd5d0a940 100644
--- a/test/parallel/test-zlib.js
+++ b/test/parallel/test-zlib.js
@@ -7,7 +7,7 @@ const fs = require('fs');
 const util = require('util');
 const stream = require('stream');
 
-var zlibPairs = [
+let zlibPairs = [
   [zlib.Deflate, zlib.Inflate],
   [zlib.Gzip, zlib.Gunzip],
   [zlib.Deflate, zlib.Unzip],
@@ -16,18 +16,18 @@ var zlibPairs = [
 ];
 
 // how fast to trickle through the slowstream
-var trickle = [128, 1024, 1024 * 1024];
+let trickle = [128, 1024, 1024 * 1024];
 
 // tunable options for zlib classes.
 
 // several different chunk sizes
-var chunkSize = [128, 1024, 1024 * 16, 1024 * 1024];
+let chunkSize = [128, 1024, 1024 * 16, 1024 * 1024];
 
 // this is every possible value.
-var level = [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
-var windowBits = [8, 9, 10, 11, 12, 13, 14, 15];
-var memLevel = [1, 2, 3, 4, 5, 6, 7, 8, 9];
-var strategy = [0, 1, 2, 3, 4];
+let level = [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
+let windowBits = [8, 9, 10, 11, 12, 13, 14, 15];
+let memLevel = [1, 2, 3, 4, 5, 6, 7, 8, 9];
+let strategy = [0, 1, 2, 3, 4];
 
 // it's nice in theory to test every combination, but it
 // takes WAY too long.  Maybe a pummel test could do this?
@@ -40,7 +40,7 @@ if (!process.env.PUMMEL) {
   strategy = [0];
 }
 
-var testFiles = ['person.jpg', 'elipses.txt', 'empty.txt'];
+let testFiles = ['person.jpg', 'elipses.txt', 'empty.txt'];
 
 if (process.env.FAST) {
   zlibPairs = [[zlib.Gzip, zlib.Unzip]];
@@ -72,8 +72,8 @@ BufferStream.prototype.write = function(c) {
 BufferStream.prototype.end = function(c) {
   if (c) this.write(c);
   // flatten
-  var buf = Buffer.allocUnsafe(this.length);
-  var i = 0;
+  const buf = Buffer.allocUnsafe(this.length);
+  let i = 0;
   this.chunks.forEach(function(c) {
     c.copy(buf, i);
     i += c.length;
@@ -108,8 +108,8 @@ SlowStream.prototype.resume = function() {
       this.ended = true;
       return this.emit('end');
     }
-    var end = Math.min(this.offset + this.trickle, this.length);
-    var c = this.chunk.slice(this.offset, end);
+    const end = Math.min(this.offset + this.trickle, this.length);
+    const c = this.chunk.slice(this.offset, end);
     this.offset += c.length;
     this.emit('data', c);
     process.nextTick(emit);
@@ -134,12 +134,12 @@ SlowStream.prototype.end = function(chunk) {
 // for each of the files, make sure that compressing and
 // decompressing results in the same data, for every combination
 // of the options set above.
-var failures = 0;
-var total = 0;
-var done = 0;
+let failures = 0;
+let total = 0;
+let done = 0;
 
 Object.keys(tests).forEach(function(file) {
-  var test = tests[file];
+  const test = tests[file];
   chunkSize.forEach(function(chunkSize) {
     trickle.forEach(function(trickle) {
       windowBits.forEach(function(windowBits) {
@@ -147,29 +147,30 @@ Object.keys(tests).forEach(function(file) {
           memLevel.forEach(function(memLevel) {
             strategy.forEach(function(strategy) {
               zlibPairs.forEach(function(pair) {
-                var Def = pair[0];
-                var Inf = pair[1];
-                var opts = { level: level,
-                             windowBits: windowBits,
-                             memLevel: memLevel,
-                             strategy: strategy };
+                const Def = pair[0];
+                const Inf = pair[1];
+                const opts = { level: level,
+                               windowBits: windowBits,
+                               memLevel: memLevel,
+                               strategy: strategy };
 
                 total++;
 
-                var def = new Def(opts);
-                var inf = new Inf(opts);
-                var ss = new SlowStream(trickle);
-                var buf = new BufferStream();
+                const def = new Def(opts);
+                const inf = new Inf(opts);
+                const ss = new SlowStream(trickle);
+                const buf = new BufferStream();
 
                 // verify that the same exact buffer comes out the other end.
                 buf.on('data', function(c) {
-                  var msg = file + ' ' +
-                            chunkSize + ' ' +
-                            JSON.stringify(opts) + ' ' +
-                            Def.name + ' -> ' + Inf.name;
-                  var ok = true;
-                  var testNum = ++done;
-                  for (var i = 0; i < Math.max(c.length, test.length); i++) {
+                  const msg = file + ' ' +
+                              chunkSize + ' ' +
+                              JSON.stringify(opts) + ' ' +
+                              Def.name + ' -> ' + Inf.name;
+                  let ok = true;
+                  const testNum = ++done;
+                  let i;
+                  for (i = 0; i < Math.max(c.length, test.length); i++) {
                     if (c[i] !== test[i]) {
                       ok = false;
                       failures++;
diff --git a/test/pseudo-tty/no_dropped_stdio.js b/test/pseudo-tty/no_dropped_stdio.js
index 1a20889073..b24d234a38 100644
--- a/test/pseudo-tty/no_dropped_stdio.js
+++ b/test/pseudo-tty/no_dropped_stdio.js
@@ -6,7 +6,7 @@ require('../common');
 // 1000 bytes wrapped at 50 columns
 // \n turns into a double-byte character
 // (48 + {2}) * 20 = 1000
-var out = ('o'.repeat(48) + '\n').repeat(20);
+let out = ('o'.repeat(48) + '\n').repeat(20);
 // Add the remaining 24 bytes and terminate with an 'O'.
 // This results in 1025 bytes, just enough to overflow the 1kb OS X TTY buffer.
 out += 'o'.repeat(24) + 'O';
diff --git a/test/pseudo-tty/no_interleaved_stdio.js b/test/pseudo-tty/no_interleaved_stdio.js
index 778b2c0958..ff3ed8594a 100644
--- a/test/pseudo-tty/no_interleaved_stdio.js
+++ b/test/pseudo-tty/no_interleaved_stdio.js
@@ -6,7 +6,7 @@ require('../common');
 // 1000 bytes wrapped at 50 columns
 // \n turns into a double-byte character
 // (48 + {2}) * 20 = 1000
-var out = ('o'.repeat(48) + '\n').repeat(20);
+let out = ('o'.repeat(48) + '\n').repeat(20);
 // Add the remaining 24 bytes and terminate with an 'O'.
 // This results in 1025 bytes, just enough to overflow the 1kb OS X TTY buffer.
 out += 'o'.repeat(24) + 'O';
diff --git a/test/pummel/test-abort-fatal-error.js b/test/pummel/test-abort-fatal-error.js
index 1fed91d54a..d1a8d3daa6 100644
--- a/test/pummel/test-abort-fatal-error.js
+++ b/test/pummel/test-abort-fatal-error.js
@@ -9,7 +9,7 @@ if (common.isWindows) {
 
 const exec = require('child_process').exec;
 
-var cmdline = 'ulimit -c 0; ' + process.execPath;
+let cmdline = 'ulimit -c 0; ' + process.execPath;
 cmdline += ' --max-old-space-size=4 --max-semi-space-size=1';
 cmdline += ' -e "a = []; for (i = 0; i < 1e9; i++) { a.push({}) }"';
 
diff --git a/test/pummel/test-crypto-dh.js b/test/pummel/test-crypto-dh.js
index 7656d0ffa9..d441e80926 100644
--- a/test/pummel/test-crypto-dh.js
+++ b/test/pummel/test-crypto-dh.js
@@ -18,7 +18,7 @@ assert.throws(function() {
   crypto.getDiffieHellman('modp1').setPublicKey('');
 });
 
-var hashes = {
+const hashes = {
   modp1: '630e9acd2cc63f7e80d8507624ba60ac0757201a',
   modp2: '18f7aa964484137f57bca64b21917a385b6a0b60',
   modp5: 'c0a8eec0c2c8a5ec2f9c26f9661eb339a010ec61',
@@ -30,10 +30,10 @@ var hashes = {
 };
 
 for (const name in hashes) {
-  var group = crypto.getDiffieHellman(name);
-  var private_key = group.getPrime('hex');
-  var hash1 = hashes[name];
-  var hash2 = crypto.createHash('sha1')
+  const group = crypto.getDiffieHellman(name);
+  const private_key = group.getPrime('hex');
+  const hash1 = hashes[name];
+  const hash2 = crypto.createHash('sha1')
                     .update(private_key.toUpperCase()).digest('hex');
   assert.equal(hash1, hash2);
   assert.equal(group.getGenerator('hex'), '02');
@@ -43,11 +43,11 @@ for (const name in hashes) {
   // modp1 is 768 bits, FIPS requires >= 1024
   if (name === 'modp1' && common.hasFipsCrypto)
     continue;
-  var group1 = crypto.getDiffieHellman(name);
-  var group2 = crypto.getDiffieHellman(name);
+  const group1 = crypto.getDiffieHellman(name);
+  const group2 = crypto.getDiffieHellman(name);
   group1.generateKeys();
   group2.generateKeys();
-  var key1 = group1.computeSecret(group2.getPublicKey());
-  var key2 = group2.computeSecret(group1.getPublicKey());
+  const key1 = group1.computeSecret(group2.getPublicKey());
+  const key2 = group2.computeSecret(group1.getPublicKey());
   assert.deepStrictEqual(key1, key2);
 }
diff --git a/test/pummel/test-dh-regr.js b/test/pummel/test-dh-regr.js
index 8228332e5d..5be6abb9a8 100644
--- a/test/pummel/test-dh-regr.js
+++ b/test/pummel/test-dh-regr.js
@@ -8,9 +8,9 @@ if (!common.hasCrypto) {
 }
 const crypto = require('crypto');
 
-var p = crypto.createDiffieHellman(1024).getPrime();
+const p = crypto.createDiffieHellman(1024).getPrime();
 
-for (var i = 0; i < 2000; i++) {
+for (let i = 0; i < 2000; i++) {
   const a = crypto.createDiffieHellman(p);
   const b = crypto.createDiffieHellman(p);
 
diff --git a/test/pummel/test-dtrace-jsstack.js b/test/pummel/test-dtrace-jsstack.js
index af9c83bb3e..d1615a3855 100644
--- a/test/pummel/test-dtrace-jsstack.js
+++ b/test/pummel/test-dtrace-jsstack.js
@@ -11,20 +11,20 @@ if (!common.isSunOS) {
 /*
  * Some functions to create a recognizable stack.
  */
-var frames = [ 'stalloogle', 'bagnoogle', 'doogle' ];
+const frames = [ 'stalloogle', 'bagnoogle', 'doogle' ];
 
-var stalloogle = function(str) {
+const stalloogle = function(str) {
   global.expected = str;
   os.loadavg();
 };
 
-var bagnoogle = function(arg0, arg1) {
+const bagnoogle = function(arg0, arg1) {
   stalloogle(arg0 + ' is ' + arg1 + ' except that it is read-only');
 };
 
-var done = false;
+let done = false;
 
-var doogle = function() {
+const doogle = function() {
   if (!done)
     setTimeout(doogle, 10);
 
@@ -38,10 +38,10 @@ const spawn = require('child_process').spawn;
  * when we call getloadavg() -- with the implicit assumption that our
  * deepest function is the only caller of os.loadavg().
  */
-var dtrace = spawn('dtrace', [ '-qwn', 'syscall::getloadavg:entry/pid == ' +
+const dtrace = spawn('dtrace', [ '-qwn', 'syscall::getloadavg:entry/pid == ' +
   process.pid + '/{ustack(100, 8192); exit(0); }' ]);
 
-var output = '';
+let output = '';
 
 dtrace.stderr.on('data', function(data) {
   console.log('dtrace: ' + data);
@@ -59,17 +59,17 @@ dtrace.on('exit', function(code) {
 
   done = true;
 
-  var sentinel = '(anon) as ';
-  var lines = output.split('\n');
+  const sentinel = '(anon) as ';
+  const lines = output.split('\n');
 
-  for (var i = 0; i < lines.length; i++) {
-    var line = lines[i];
+  for (let i = 0; i < lines.length; i++) {
+    const line = lines[i];
 
     if (line.indexOf(sentinel) === -1 || frames.length === 0)
       continue;
 
-    var frame = line.substr(line.indexOf(sentinel) + sentinel.length);
-    var top = frames.shift();
+    const frame = line.substr(line.indexOf(sentinel) + sentinel.length);
+    const top = frames.shift();
 
     assert.equal(frame.indexOf(top), 0, 'unexpected frame where ' +
       top + ' was expected');
diff --git a/test/pummel/test-exec.js b/test/pummel/test-exec.js
index c87aa2d6e6..b7f5ddebe7 100644
--- a/test/pummel/test-exec.js
+++ b/test/pummel/test-exec.js
@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const exec = require('child_process').exec;
 
-var SLEEP3_COMMAND;
+let SLEEP3_COMMAND;
 if (!common.isWindows) {
   // Unix.
   SLEEP3_COMMAND = 'sleep 3';
@@ -14,8 +14,8 @@ if (!common.isWindows) {
 }
 
 
-var success_count = 0;
-var error_count = 0;
+let success_count = 0;
+let error_count = 0;
 
 
 exec(
@@ -55,9 +55,9 @@ exec('thisisnotavalidcommand', function(err, stdout, stderr) {
 });
 
 
-var sleeperStart = new Date();
+const sleeperStart = new Date();
 exec(SLEEP3_COMMAND, { timeout: 50 }, function(err, stdout, stderr) {
-  var diff = (new Date()) - sleeperStart;
+  const diff = (new Date()) - sleeperStart;
   console.log('\'sleep 3\' with timeout 50 took %d ms', diff);
   assert.ok(diff < 500);
   assert.ok(err);
@@ -66,8 +66,8 @@ exec(SLEEP3_COMMAND, { timeout: 50 }, function(err, stdout, stderr) {
 });
 
 
-var startSleep3 = new Date();
-var killMeTwice = exec(SLEEP3_COMMAND, {timeout: 1000}, killMeTwiceCallback);
+const startSleep3 = new Date();
+const killMeTwice = exec(SLEEP3_COMMAND, {timeout: 1000}, killMeTwiceCallback);
 
 process.nextTick(function() {
   console.log('kill pid %d', killMeTwice.pid);
@@ -79,7 +79,7 @@ process.nextTick(function() {
 });
 
 function killMeTwiceCallback(err, stdout, stderr) {
-  var diff = (new Date()) - startSleep3;
+  const diff = (new Date()) - startSleep3;
   // We should have already killed this process. Assert that the timeout still
   // works and that we are getting the proper callback parameters.
   assert.ok(err);
diff --git a/test/pummel/test-fs-watch-file-slow.js b/test/pummel/test-fs-watch-file-slow.js
index e64e00b32f..78c36152ec 100644
--- a/test/pummel/test-fs-watch-file-slow.js
+++ b/test/pummel/test-fs-watch-file-slow.js
@@ -4,10 +4,10 @@ const assert = require('assert');
 const path = require('path');
 const fs = require('fs');
 
-var FILENAME = path.join(common.tmpDir, 'watch-me');
-var TIMEOUT = 1300;
+const FILENAME = path.join(common.tmpDir, 'watch-me');
+const TIMEOUT = 1300;
 
-var nevents = 0;
+let nevents = 0;
 
 try {
   fs.unlinkSync(FILENAME);
diff --git a/test/pummel/test-fs-watch-file.js b/test/pummel/test-fs-watch-file.js
index 90acdccebe..1f5441f691 100644
--- a/test/pummel/test-fs-watch-file.js
+++ b/test/pummel/test-fs-watch-file.js
@@ -4,23 +4,23 @@ const assert = require('assert');
 const path = require('path');
 const fs = require('fs');
 
-var watchSeenOne = 0;
-var watchSeenTwo = 0;
-var watchSeenThree = 0;
-var watchSeenFour = 0;
+let watchSeenOne = 0;
+let watchSeenTwo = 0;
+let watchSeenThree = 0;
+let watchSeenFour = 0;
 
-var testDir = common.tmpDir;
+const testDir = common.tmpDir;
 
-var filenameOne = 'watch.txt';
-var filepathOne = path.join(testDir, filenameOne);
+const filenameOne = 'watch.txt';
+const filepathOne = path.join(testDir, filenameOne);
 
-var filenameTwo = 'hasOwnProperty';
-var filepathTwo = filenameTwo;
-var filepathTwoAbs = path.join(testDir, filenameTwo);
+const filenameTwo = 'hasOwnProperty';
+const filepathTwo = filenameTwo;
+const filepathTwoAbs = path.join(testDir, filenameTwo);
 
-var filenameThree = 'charm'; // because the third time is
+const filenameThree = 'charm'; // because the third time is
 
-var filenameFour = 'get';
+const filenameFour = 'get';
 
 process.on('exit', function() {
   fs.unlinkSync(filepathOne);
diff --git a/test/pummel/test-fs-watch-non-recursive.js b/test/pummel/test-fs-watch-non-recursive.js
index 7b40ef75ac..b7c19b9c36 100644
--- a/test/pummel/test-fs-watch-non-recursive.js
+++ b/test/pummel/test-fs-watch-non-recursive.js
@@ -3,9 +3,9 @@ const common = require('../common');
 const path = require('path');
 const fs = require('fs');
 
-var testDir = common.tmpDir;
-var testsubdir = path.join(testDir, 'testsubdir');
-var filepath = path.join(testsubdir, 'watch.txt');
+const testDir = common.tmpDir;
+const testsubdir = path.join(testDir, 'testsubdir');
+const filepath = path.join(testsubdir, 'watch.txt');
 
 function cleanup() {
   try { fs.unlinkSync(filepath); } catch (e) { }
@@ -18,7 +18,7 @@ try { fs.mkdirSync(testsubdir, 0o700); } catch (e) {}
 
 // Need a grace period, else the mkdirSync() above fires off an event.
 setTimeout(function() {
-  var watcher = fs.watch(testDir, { persistent: true }, common.fail);
+  const watcher = fs.watch(testDir, { persistent: true }, common.fail);
   setTimeout(function() {
     fs.writeFileSync(filepath, 'test');
   }, 100);
diff --git a/test/pummel/test-http-client-reconnect-bug.js b/test/pummel/test-http-client-reconnect-bug.js
index 395a538178..98d21c5cba 100644
--- a/test/pummel/test-http-client-reconnect-bug.js
+++ b/test/pummel/test-http-client-reconnect-bug.js
@@ -3,17 +3,17 @@ const common = require('../common');
 const net = require('net');
 const http = require('http');
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.end();
 });
 
 server.on('listening', common.mustCall(function() {
-  var client = http.createClient(common.PORT);
+  const client = http.createClient(common.PORT);
 
   client.on('error', common.mustCall(function(err) {}));
   client.on('end', common.mustCall(function() {}));
 
-  var request = client.request('GET', '/', {'host': 'localhost'});
+  const request = client.request('GET', '/', {'host': 'localhost'});
   request.end();
   request.on('response', function(response) {
     console.log('STATUS: ' + response.statusCode);
diff --git a/test/pummel/test-http-many-keep-alive-connections.js b/test/pummel/test-http-many-keep-alive-connections.js
index a7654f4524..e465c02a90 100644
--- a/test/pummel/test-http-many-keep-alive-connections.js
+++ b/test/pummel/test-http-many-keep-alive-connections.js
@@ -3,12 +3,12 @@ const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 
-var expected = 10000;
-var responses = 0;
-var requests = 0;
-var connection;
+const expected = 10000;
+let responses = 0;
+let requests = 0;
+let connection;
 
-var server = http.Server(function(req, res) {
+const server = http.Server(function(req, res) {
   requests++;
   assert.equal(req.connection, connection);
   res.writeHead(200);
@@ -20,7 +20,7 @@ server.once('connection', function(c) {
 });
 
 server.listen(common.PORT, function connect() {
-  var request = http.get({
+  const request = http.get({
     port: common.PORT,
     path: '/',
     headers: {
diff --git a/test/pummel/test-http-upload-timeout.js b/test/pummel/test-http-upload-timeout.js
index 15bc267ff6..da91ab3f09 100644
--- a/test/pummel/test-http-upload-timeout.js
+++ b/test/pummel/test-http-upload-timeout.js
@@ -24,18 +24,18 @@ server.on('request', function(req, res) {
 });
 
 server.listen(common.PORT, '127.0.0.1', function() {
-  for (var i = 0; i < 10; i++) {
+  for (let i = 0; i < 10; i++) {
     connections++;
 
     setTimeout(function() {
-      var request = http.request({
+      const request = http.request({
         port: common.PORT,
         method: 'POST',
         path: '/'
       });
 
       function ping() {
-        var nextPing = (Math.random() * 900).toFixed();
+        const nextPing = (Math.random() * 900).toFixed();
         if (nextPing > 600) {
           request.end();
           return;
diff --git a/test/pummel/test-https-ci-reneg-attack.js b/test/pummel/test-https-ci-reneg-attack.js
index 61e60c2673..850e0949c4 100644
--- a/test/pummel/test-https-ci-reneg-attack.js
+++ b/test/pummel/test-https-ci-reneg-attack.js
@@ -18,7 +18,7 @@ if (!common.opensslCli) {
 }
 
 // renegotiation limits to test
-var LIMITS = [0, 1, 2, 3, 5, 10, 16];
+const LIMITS = [0, 1, 2, 3, 5, 10, 16];
 
 {
   let n = 0;
@@ -31,15 +31,15 @@ var LIMITS = [0, 1, 2, 3, 5, 10, 16];
 }
 
 function test(next) {
-  var options = {
+  const options = {
     cert: fs.readFileSync(common.fixturesDir + '/test_cert.pem'),
     key: fs.readFileSync(common.fixturesDir + '/test_key.pem')
   };
 
-  var seenError = false;
+  let seenError = false;
 
-  var server = https.createServer(options, function(req, res) {
-    var conn = req.connection;
+  const server = https.createServer(options, function(req, res) {
+    const conn = req.connection;
     conn.on('error', function(err) {
       console.error('Caught exception: ' + err);
       assert(/TLS session renegotiation attack/.test(err));
@@ -50,8 +50,8 @@ function test(next) {
   });
 
   server.listen(common.PORT, function() {
-    var args = ('s_client -connect 127.0.0.1:' + common.PORT).split(' ');
-    var child = spawn(common.opensslCli, args);
+    const args = ('s_client -connect 127.0.0.1:' + common.PORT).split(' ');
+    const child = spawn(common.opensslCli, args);
 
     //child.stdout.pipe(process.stdout);
     //child.stderr.pipe(process.stderr);
@@ -60,8 +60,8 @@ function test(next) {
     child.stderr.resume();
 
     // count handshakes, start the attack after the initial handshake is done
-    var handshakes = 0;
-    var renegs = 0;
+    let handshakes = 0;
+    let renegs = 0;
 
     child.stderr.on('data', function(data) {
       if (seenError) return;
@@ -76,7 +76,7 @@ function test(next) {
       process.nextTick(next);
     });
 
-    var closed = false;
+    let closed = false;
     child.stdin.on('error', function(err) {
       switch (err.code) {
         case 'ECONNRESET':
diff --git a/test/pummel/test-https-large-response.js b/test/pummel/test-https-large-response.js
index 8a3aa022ea..39256aa630 100644
--- a/test/pummel/test-https-large-response.js
+++ b/test/pummel/test-https-large-response.js
@@ -10,16 +10,16 @@ if (!common.hasCrypto) {
 }
 const https = require('https');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
 process.stdout.write('build body...');
-var body = 'hello world\n'.repeat(1024 * 1024);
+const body = 'hello world\n'.repeat(1024 * 1024);
 process.stdout.write('done\n');
 
-var server = https.createServer(options, common.mustCall(function(req, res) {
+const server = https.createServer(options, common.mustCall(function(req, res) {
   console.log('got request');
   res.writeHead(200, { 'content-type': 'text/plain' });
   res.end(body);
@@ -32,7 +32,7 @@ server.listen(common.PORT, common.mustCall(function() {
   }, common.mustCall(function(res) {
     console.log('response!');
 
-    var count = 0;
+    let count = 0;
 
     res.on('data', function(d) {
       process.stdout.write('.');
diff --git a/test/pummel/test-https-no-reader.js b/test/pummel/test-https-no-reader.js
index b6cc32c999..a08676277a 100644
--- a/test/pummel/test-https-no-reader.js
+++ b/test/pummel/test-https-no-reader.js
@@ -12,23 +12,23 @@ const Buffer = require('buffer').Buffer;
 const fs = require('fs');
 const path = require('path');
 
-var options = {
+const options = {
   key: fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem')),
   cert: fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'))
 };
 
-var buf = Buffer.allocUnsafe(1024 * 1024);
+const buf = Buffer.allocUnsafe(1024 * 1024);
 
-var server = https.createServer(options, function(req, res) {
+const server = https.createServer(options, function(req, res) {
   res.writeHead(200);
-  for (var i = 0; i < 50; i++) {
+  for (let i = 0; i < 50; i++) {
     res.write(buf);
   }
   res.end();
 });
 
 server.listen(common.PORT, function() {
-  var req = https.request({
+  const req = https.request({
     method: 'POST',
     port: common.PORT,
     rejectUnauthorized: false
diff --git a/test/pummel/test-net-connect-econnrefused.js b/test/pummel/test-net-connect-econnrefused.js
index bd61dabc6b..30da36d983 100644
--- a/test/pummel/test-net-connect-econnrefused.js
+++ b/test/pummel/test-net-connect-econnrefused.js
@@ -5,17 +5,18 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var ROUNDS = 10;
-var ATTEMPTS_PER_ROUND = 100;
-var rounds = 1;
-var reqs = 0;
+const ROUNDS = 10;
+const ATTEMPTS_PER_ROUND = 100;
+let rounds = 1;
+let reqs = 0;
 
 pummel();
 
 function pummel() {
   console.log('Round', rounds, '/', ROUNDS);
 
-  for (var pending = 0; pending < ATTEMPTS_PER_ROUND; pending++) {
+  let pending;
+  for (pending = 0; pending < ATTEMPTS_PER_ROUND; pending++) {
     net.createConnection(common.PORT).on('error', function(err) {
       assert.equal(err.code, 'ECONNREFUSED');
       if (--pending > 0) return;
@@ -34,7 +35,7 @@ function check() {
     check_called = true;
   }, 0);
 }
-var check_called = false;
+let check_called = false;
 
 process.on('exit', function() {
   assert.equal(rounds, ROUNDS);
diff --git a/test/pummel/test-net-connect-memleak.js b/test/pummel/test-net-connect-memleak.js
index a5d8ff47eb..f16034e477 100644
--- a/test/pummel/test-net-connect-memleak.js
+++ b/test/pummel/test-net-connect-memleak.js
@@ -12,7 +12,7 @@ assert.strictEqual(
 );
 net.createServer(function() {}).listen(common.PORT);
 
-var before = 0;
+let before = 0;
 {
   // 2**26 == 64M entries
   global.gc();
@@ -29,8 +29,8 @@ var before = 0;
 
 function done() {
   global.gc();
-  var after = process.memoryUsage().rss;
-  var reclaimed = (before - after) / 1024;
+  const after = process.memoryUsage().rss;
+  const reclaimed = (before - after) / 1024;
   console.log('%d kB reclaimed', reclaimed);
   assert(reclaimed > 128 * 1024);  // It's around 256 MB on x64.
   process.exit();
diff --git a/test/pummel/test-net-many-clients.js b/test/pummel/test-net-many-clients.js
index db98e55433..a92ae033a0 100644
--- a/test/pummel/test-net-many-clients.js
+++ b/test/pummel/test-net-many-clients.js
@@ -4,16 +4,16 @@ const assert = require('assert');
 const net = require('net');
 
 // settings
-var bytes = 1024 * 40;
-var concurrency = 100;
-var connections_per_client = 5;
+const bytes = 1024 * 40;
+const concurrency = 100;
+const connections_per_client = 5;
 
 // measured
-var total_connections = 0;
+let total_connections = 0;
 
-var body = 'C'.repeat(bytes);
+const body = 'C'.repeat(bytes);
 
-var server = net.createServer(function(c) {
+const server = net.createServer(function(c) {
   console.log('connected');
   total_connections++;
   console.log('#');
@@ -22,7 +22,7 @@ var server = net.createServer(function(c) {
 });
 
 function runClient(callback) {
-  var client = net.createConnection(common.PORT);
+  const client = net.createConnection(common.PORT);
 
   client.connections = 0;
 
@@ -66,8 +66,8 @@ function runClient(callback) {
 }
 
 server.listen(common.PORT, function() {
-  var finished_clients = 0;
-  for (var i = 0; i < concurrency; i++) {
+  let finished_clients = 0;
+  for (let i = 0; i < concurrency; i++) {
     runClient(function() {
       if (++finished_clients === concurrency) server.close();
     });
diff --git a/test/pummel/test-net-pause.js b/test/pummel/test-net-pause.js
index 59749e75fa..db426edbe9 100644
--- a/test/pummel/test-net-pause.js
+++ b/test/pummel/test-net-pause.js
@@ -3,10 +3,10 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var N = 200;
-var recv = '', chars_recved = 0;
+const N = 200;
+let recv = '', chars_recved = 0;
 
-var server = net.createServer(function(connection) {
+const server = net.createServer(function(connection) {
   function write(j) {
     if (j >= N) {
       connection.end();
@@ -21,7 +21,7 @@ var server = net.createServer(function(connection) {
 });
 
 server.on('listening', function() {
-  var client = net.createConnection(common.PORT);
+  const client = net.createConnection(common.PORT);
   client.setEncoding('ascii');
   client.on('data', function(d) {
     console.log(d);
diff --git a/test/pummel/test-net-pingpong-delay.js b/test/pummel/test-net-pingpong-delay.js
index 4e478c87a6..9a598435d1 100644
--- a/test/pummel/test-net-pingpong-delay.js
+++ b/test/pummel/test-net-pingpong-delay.js
@@ -4,12 +4,12 @@ const assert = require('assert');
 const net = require('net');
 
 function pingPongTest(port, host, on_complete) {
-  var N = 100;
-  var DELAY = 1;
-  var count = 0;
-  var client_ended = false;
+  const N = 100;
+  const DELAY = 1;
+  let count = 0;
+  let client_ended = false;
 
-  var server = net.createServer({ allowHalfOpen: true }, function(socket) {
+  const server = net.createServer({ allowHalfOpen: true }, function(socket) {
     socket.setEncoding('utf8');
 
     socket.on('data', function(data) {
@@ -43,7 +43,7 @@ function pingPongTest(port, host, on_complete) {
   });
 
   server.listen(port, host, common.mustCall(function() {
-    var client = net.createConnection(port, host);
+    const client = net.createConnection(port, host);
 
     client.setEncoding('utf8');
 
diff --git a/test/pummel/test-net-pingpong.js b/test/pummel/test-net-pingpong.js
index b65d5d4ff9..6ede6f9fcb 100644
--- a/test/pummel/test-net-pingpong.js
+++ b/test/pummel/test-net-pingpong.js
@@ -3,17 +3,17 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var tests_run = 0;
+let tests_run = 0;
 
 function pingPongTest(port, host, on_complete) {
-  var N = 1000;
-  var count = 0;
-  var sent_final_ping = false;
+  const N = 1000;
+  let count = 0;
+  let sent_final_ping = false;
 
-  var server = net.createServer({ allowHalfOpen: true }, function(socket) {
+  const server = net.createServer({ allowHalfOpen: true }, function(socket) {
     assert.equal(true, socket.remoteAddress !== null);
     assert.equal(true, socket.remoteAddress !== undefined);
-    var address = socket.remoteAddress;
+    const address = socket.remoteAddress;
     if (host === '127.0.0.1') {
       assert.equal(address, '127.0.0.1');
     } else if (host == null || host === 'localhost') {
@@ -49,7 +49,7 @@ function pingPongTest(port, host, on_complete) {
   });
 
   server.listen(port, host, function() {
-    var client = net.createConnection(port, host);
+    const client = net.createConnection(port, host);
 
     client.setEncoding('utf8');
 
diff --git a/test/pummel/test-net-throttle.js b/test/pummel/test-net-throttle.js
index d2f32ec25b..23f05f4d5e 100644
--- a/test/pummel/test-net-throttle.js
+++ b/test/pummel/test-net-throttle.js
@@ -5,15 +5,15 @@ const net = require('net');
 
 const N = 1024 * 1024;
 const part_N = N / 3;
-var chars_recved = 0;
-var npauses = 0;
+let chars_recved = 0;
+let npauses = 0;
 
 console.log('build big string');
 const body = 'C'.repeat(N);
 
 console.log('start server on port ' + common.PORT);
 
-var server = net.createServer(function(connection) {
+const server = net.createServer(function(connection) {
   connection.write(body.slice(0, part_N));
   connection.write(body.slice(part_N, 2 * part_N));
   assert.equal(false, connection.write(body.slice(2 * part_N, N)));
@@ -24,8 +24,8 @@ var server = net.createServer(function(connection) {
 });
 
 server.listen(common.PORT, function() {
-  var paused = false;
-  var client = net.createConnection(common.PORT);
+  let paused = false;
+  const client = net.createConnection(common.PORT);
   client.setEncoding('ascii');
   client.on('data', function(d) {
     chars_recved += d.length;
@@ -35,7 +35,7 @@ server.listen(common.PORT, function() {
       npauses += 1;
       paused = true;
       console.log('pause');
-      var x = chars_recved;
+      const x = chars_recved;
       setTimeout(function() {
         assert.equal(chars_recved, x);
         client.resume();
diff --git a/test/pummel/test-net-timeout.js b/test/pummel/test-net-timeout.js
index e4f8664657..d6dd8e489c 100644
--- a/test/pummel/test-net-timeout.js
+++ b/test/pummel/test-net-timeout.js
@@ -3,12 +3,12 @@ const common = require('../common');
 const assert = require('assert');
 const net = require('net');
 
-var exchanges = 0;
-var starttime = null;
-var timeouttime = null;
-var timeout = 1000;
+let exchanges = 0;
+let starttime = null;
+let timeouttime = null;
+const timeout = 1000;
 
-var echo_server = net.createServer(function(socket) {
+const echo_server = net.createServer(function(socket) {
   socket.setTimeout(timeout);
 
   socket.on('timeout', function() {
@@ -36,7 +36,7 @@ var echo_server = net.createServer(function(socket) {
 echo_server.listen(common.PORT, function() {
   console.log('server listening at ' + common.PORT);
 
-  var client = net.createConnection(common.PORT);
+  const client = net.createConnection(common.PORT);
   client.setEncoding('UTF8');
   client.setTimeout(0); // disable the timeout for client
   client.on('connect', function() {
@@ -79,7 +79,7 @@ process.on('exit', function() {
   assert.ok(starttime != null);
   assert.ok(timeouttime != null);
 
-  var diff = timeouttime - starttime;
+  const diff = timeouttime - starttime;
   console.log('diff = ' + diff);
 
   assert.ok(timeout < diff);
diff --git a/test/pummel/test-net-timeout2.js b/test/pummel/test-net-timeout2.js
index 5567b128c7..a3d0abceb6 100644
--- a/test/pummel/test-net-timeout2.js
+++ b/test/pummel/test-net-timeout2.js
@@ -5,13 +5,13 @@
 const common = require('../common');
 const net = require('net');
 
-var seconds = 5;
-var counter = 0;
+const seconds = 5;
+let counter = 0;
 
-var server = net.createServer(function(socket) {
+const server = net.createServer(function(socket) {
   socket.setTimeout((seconds / 2) * 1000, common.fail);
 
-  var interval = setInterval(function() {
+  const interval = setInterval(function() {
     counter++;
 
     if (counter === seconds) {
@@ -28,6 +28,6 @@ var server = net.createServer(function(socket) {
 
 
 server.listen(common.PORT, function() {
-  var s = net.connect(common.PORT);
+  const s = net.connect(common.PORT);
   s.pipe(process.stdout);
 });
diff --git a/test/pummel/test-net-write-callbacks.js b/test/pummel/test-net-write-callbacks.js
index 54f8a67abe..6c3c1f202a 100644
--- a/test/pummel/test-net-write-callbacks.js
+++ b/test/pummel/test-net-write-callbacks.js
@@ -3,10 +3,10 @@ const common = require('../common');
 const net = require('net');
 const assert = require('assert');
 
-var cbcount = 0;
-var N = 500000;
+let cbcount = 0;
+const N = 500000;
 
-var server = net.Server(function(socket) {
+const server = net.Server(function(socket) {
   socket.on('data', function(d) {
     console.error('got %d bytes', d.length);
   });
@@ -18,9 +18,9 @@ var server = net.Server(function(socket) {
   });
 });
 
-var lastCalled = -1;
+let lastCalled = -1;
 function makeCallback(c) {
-  var called = false;
+  let called = false;
   return function() {
     if (called)
       throw new Error('called callback #' + c + ' more than once');
@@ -34,10 +34,10 @@ function makeCallback(c) {
 }
 
 server.listen(common.PORT, function() {
-  var client = net.createConnection(common.PORT);
+  const client = net.createConnection(common.PORT);
 
   client.on('connect', function() {
-    for (var i = 0; i < N; i++) {
+    for (let i = 0; i < N; i++) {
       client.write('hello world', makeCallback(i));
     }
     client.end();
diff --git a/test/pummel/test-next-tick-infinite-calls.js b/test/pummel/test-next-tick-infinite-calls.js
index 18223885d6..afce6ef846 100644
--- a/test/pummel/test-next-tick-infinite-calls.js
+++ b/test/pummel/test-next-tick-infinite-calls.js
@@ -1,7 +1,7 @@
 'use strict';
 require('../common');
 
-var complete = 0;
+let complete = 0;
 
 // This will fail with:
 //  FATAL ERROR: JS Allocation failed - process out of memory
diff --git a/test/pummel/test-process-hrtime.js b/test/pummel/test-process-hrtime.js
index 0149ef2bf5..d02b9c7982 100644
--- a/test/pummel/test-process-hrtime.js
+++ b/test/pummel/test-process-hrtime.js
@@ -2,17 +2,17 @@
 require('../common');
 const assert = require('assert');
 
-var start = process.hrtime();
+const start = process.hrtime();
 
 // process.hrtime() should return an Array
 assert(Array.isArray(start));
 
 // busy-loop for 2 seconds
-var now = Date.now();
+const now = Date.now();
 while (Date.now() - now < 2000);
 
 // get a diff reading
-var diff = process.hrtime(start);
+const diff = process.hrtime(start);
 
 // should be at least 1 second, at most 2 seconds later
 // (the while loop will usually exit a few nanoseconds before 2)
diff --git a/test/pummel/test-process-uptime.js b/test/pummel/test-process-uptime.js
index 90343e91a1..5c09d59b06 100644
--- a/test/pummel/test-process-uptime.js
+++ b/test/pummel/test-process-uptime.js
@@ -5,10 +5,10 @@ const assert = require('assert');
 console.error(process.uptime());
 assert.ok(process.uptime() <= 2);
 
-var original = process.uptime();
+const original = process.uptime();
 
 setTimeout(function() {
-  var uptime = process.uptime();
+  const uptime = process.uptime();
   // some wiggle room to account for timer
   // granularity, processor speed, and scheduling
   assert.ok(uptime >= original + 2);
diff --git a/test/pummel/test-regress-GH-814.js b/test/pummel/test-regress-GH-814.js
index 4c890e1705..20323360de 100644
--- a/test/pummel/test-regress-GH-814.js
+++ b/test/pummel/test-regress-GH-814.js
@@ -5,7 +5,7 @@ const common = require('../common');
 const assert = require('assert');
 
 function newBuffer(size, value) {
-  var buffer = Buffer.allocUnsafe(size);
+  const buffer = Buffer.allocUnsafe(size);
   while (size--) {
     buffer[size] = value;
   }
@@ -16,14 +16,14 @@ function newBuffer(size, value) {
 
 const fs = require('fs');
 const testFileName = require('path').join(common.tmpDir, 'GH-814_testFile.txt');
-var testFileFD = fs.openSync(testFileName, 'w');
+const testFileFD = fs.openSync(testFileName, 'w');
 console.log(testFileName);
 
 
-var kBufSize = 128 * 1024;
-var PASS = true;
-var neverWrittenBuffer = newBuffer(kBufSize, 0x2e); //0x2e === '.'
-var bufPool = [];
+const kBufSize = 128 * 1024;
+let PASS = true;
+const neverWrittenBuffer = newBuffer(kBufSize, 0x2e); //0x2e === '.'
+const bufPool = [];
 
 
 const tail = require('child_process').spawn('tail', ['-f', testFileName]);
@@ -34,13 +34,13 @@ function tailCB(data) {
 }
 
 
-var timeToQuit = Date.now() + 8e3; //Test during no more than this seconds.
+const timeToQuit = Date.now() + 8e3; //Test during no more than this seconds.
 (function main() {
 
   if (PASS) {
     fs.write(testFileFD, newBuffer(kBufSize, 0x61), 0, kBufSize, -1, cb);
     global.gc();
-    var nuBuf = Buffer.allocUnsafe(kBufSize);
+    const nuBuf = Buffer.allocUnsafe(kBufSize);
     neverWrittenBuffer.copy(nuBuf);
     if (bufPool.push(nuBuf) > 100) {
       bufPool.length = 0;
diff --git a/test/pummel/test-regress-GH-814_2.js b/test/pummel/test-regress-GH-814_2.js
index afba742e60..233e853045 100644
--- a/test/pummel/test-regress-GH-814_2.js
+++ b/test/pummel/test-regress-GH-814_2.js
@@ -6,7 +6,7 @@ const assert = require('assert');
 
 const fs = require('fs');
 const testFileName = require('path').join(common.tmpDir, 'GH-814_test.txt');
-var testFD = fs.openSync(testFileName, 'w');
+const testFD = fs.openSync(testFileName, 'w');
 console.error(testFileName + '\n');
 
 
@@ -27,12 +27,12 @@ function tailCB(data) {
 }
 
 
-var PASS = true;
-var bufPool = [];
-var kBufSize = 16 * 1024 * 1024;
-var neverWrittenBuffer = newBuffer(kBufSize, 0x2e); //0x2e === '.'
+let PASS = true;
+const bufPool = [];
+const kBufSize = 16 * 1024 * 1024;
+const neverWrittenBuffer = newBuffer(kBufSize, 0x2e); //0x2e === '.'
 
-var timeToQuit = Date.now() + 5e3; //Test should last no more than this.
+const timeToQuit = Date.now() + 5e3; //Test should last no more than this.
 writer();
 
 function writer() {
@@ -51,7 +51,7 @@ function writer() {
       global.gc();
       global.gc();
       global.gc();
-      var nuBuf = Buffer.allocUnsafe(kBufSize);
+      const nuBuf = Buffer.allocUnsafe(kBufSize);
       neverWrittenBuffer.copy(nuBuf);
       if (bufPool.push(nuBuf) > 100) {
         bufPool.length = 0;
@@ -73,7 +73,7 @@ function writerCB(err, written) {
 
 
 function newBuffer(size, value) {
-  var buffer = Buffer.allocUnsafe(size);
+  const buffer = Buffer.allocUnsafe(size);
   while (size--) {
     buffer[size] = value;
   }
diff --git a/test/pummel/test-regress-GH-892.js b/test/pummel/test-regress-GH-892.js
index 9a2fce08a7..0619fd84de 100644
--- a/test/pummel/test-regress-GH-892.js
+++ b/test/pummel/test-regress-GH-892.js
@@ -17,9 +17,9 @@ const https = require('https');
 
 const fs = require('fs');
 
-var bytesExpected = 1024 * 1024 * 32;
+const bytesExpected = 1024 * 1024 * 32;
 
-var started = false;
+let started = false;
 
 const childScript = require('path').join(common.fixturesDir,
                                          'GH-892-request.js');
@@ -28,17 +28,17 @@ function makeRequest() {
   if (started) return;
   started = true;
 
-  var stderrBuffer = '';
+  let stderrBuffer = '';
 
   // Pass along --trace-deprecation/--throw-deprecation in
   // process.execArgv to track down nextTick recursion errors
   // more easily.  Also, this is handy when using this test to
   // view V8 opt/deopt behavior.
-  var args = process.execArgv.concat([ childScript,
-                                       common.PORT,
-                                       bytesExpected ]);
+  const args = process.execArgv.concat([ childScript,
+                                         common.PORT,
+                                         bytesExpected ]);
 
-  var child = spawn(process.execPath, args);
+  const child = spawn(process.execPath, args);
 
   child.on('exit', function(code) {
     assert.ok(/DONE/.test(stderrBuffer));
@@ -59,14 +59,14 @@ function makeRequest() {
 }
 
 
-var serverOptions = {
+const serverOptions = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
 
-var uploadCount = 0;
+let uploadCount = 0;
 
-var server = https.Server(serverOptions, function(req, res) {
+const server = https.Server(serverOptions, function(req, res) {
   // Close the server immediately. This test is only doing a single upload.
   // We need to make sure the server isn't keeping the event loop alive
   // while the upload is in progress.
diff --git a/test/pummel/test-stream-pipe-multi.js b/test/pummel/test-stream-pipe-multi.js
index 3c62548e5b..e1e5e01102 100644
--- a/test/pummel/test-stream-pipe-multi.js
+++ b/test/pummel/test-stream-pipe-multi.js
@@ -5,14 +5,14 @@
 require('../common');
 const assert = require('assert');
 const Stream = require('stream').Stream;
-var rr = [];
-var ww = [];
-var cnt = 100;
-var chunks = 1000;
-var chunkSize = 250;
-var data = Buffer.allocUnsafe(chunkSize);
-var wclosed = 0;
-var rclosed = 0;
+const rr = [];
+const ww = [];
+const cnt = 100;
+const chunks = 1000;
+const chunkSize = 250;
+const data = Buffer.allocUnsafe(chunkSize);
+let wclosed = 0;
+let rclosed = 0;
 
 function FakeStream() {
   Stream.apply(this);
@@ -54,14 +54,14 @@ for (let i = 0; i < chunkSize; i++) {
 }
 
 for (let i = 0; i < cnt; i++) {
-  var r = new FakeStream();
+  const r = new FakeStream();
   r.on('close', function() {
     console.error(this.ID, 'read close');
     rclosed++;
   });
   rr.push(r);
 
-  var w = new FakeStream();
+  const w = new FakeStream();
   w.on('close', function() {
     console.error(this.ID, 'write close');
     wclosed++;
@@ -75,8 +75,8 @@ for (let i = 0; i < cnt; i++) {
 // now start passing through data
 // simulate a relatively fast async stream.
 rr.forEach(function(r) {
-  var cnt = chunks;
-  var paused = false;
+  let cnt = chunks;
+  let paused = false;
 
   r.on('pause', function() {
     paused = true;
diff --git a/test/pummel/test-stream2-basic.js b/test/pummel/test-stream2-basic.js
index 3659b267fa..073c1d8e35 100644
--- a/test/pummel/test-stream2-basic.js
+++ b/test/pummel/test-stream2-basic.js
@@ -16,9 +16,9 @@ function TestReader(n) {
 util.inherits(TestReader, R);
 
 TestReader.prototype._read = function(n) {
-  var max = this._buffer.length - this._pos;
+  const max = this._buffer.length - this._pos;
   n = Math.max(n, 0);
-  var toRead = Math.min(n, max);
+  const toRead = Math.min(n, max);
   if (toRead === 0) {
     // simulate the read buffer filling up with some more bytes some time
     // in the future.
@@ -39,7 +39,7 @@ TestReader.prototype._read = function(n) {
     return;
   }
 
-  var ret = this._buffer.slice(this._pos, this._pos + toRead);
+  const ret = this._buffer.slice(this._pos, this._pos + toRead);
   this._pos += toRead;
   this.push(ret);
 };
@@ -68,8 +68,8 @@ TestWriter.prototype.end = function(c) {
 ////////
 
 // tiny node-tap lookalike.
-var tests = [];
-var count = 0;
+const tests = [];
+let count = 0;
 
 function test(name, fn) {
   count++;
@@ -77,12 +77,12 @@ function test(name, fn) {
 }
 
 function run() {
-  var next = tests.shift();
+  const next = tests.shift();
   if (!next)
     return console.error('ok');
 
-  var name = next[0];
-  var fn = next[1];
+  const name = next[0];
+  const fn = next[1];
   console.log('# %s', name);
   fn({
     same: assert.deepStrictEqual,
@@ -104,34 +104,34 @@ process.nextTick(run);
 
 
 test('a most basic test', function(t) {
-  var r = new TestReader(20);
-
-  var reads = [];
-  var expect = [ 'x',
-                 'xx',
-                 'xxx',
-                 'xxxx',
-                 'xxxxx',
-                 'xxxxxxxxx',
-                 'xxxxxxxxxx',
-                 'xxxxxxxxxxxx',
-                 'xxxxxxxxxxxxx',
-                 'xxxxxxxxxxxxxxx',
-                 'xxxxxxxxxxxxxxxxx',
-                 'xxxxxxxxxxxxxxxxxxx',
-                 'xxxxxxxxxxxxxxxxxxxxx',
-                 'xxxxxxxxxxxxxxxxxxxxxxx',
-                 'xxxxxxxxxxxxxxxxxxxxxxxxx',
-                 'xxxxxxxxxxxxxxxxxxxxx' ];
+  const r = new TestReader(20);
+
+  const reads = [];
+  const expect = [ 'x',
+                   'xx',
+                   'xxx',
+                   'xxxx',
+                   'xxxxx',
+                   'xxxxxxxxx',
+                   'xxxxxxxxxx',
+                   'xxxxxxxxxxxx',
+                   'xxxxxxxxxxxxx',
+                   'xxxxxxxxxxxxxxx',
+                   'xxxxxxxxxxxxxxxxx',
+                   'xxxxxxxxxxxxxxxxxxx',
+                   'xxxxxxxxxxxxxxxxxxxxx',
+                   'xxxxxxxxxxxxxxxxxxxxxxx',
+                   'xxxxxxxxxxxxxxxxxxxxxxxxx',
+                   'xxxxxxxxxxxxxxxxxxxxx' ];
 
   r.on('end', function() {
     t.same(reads, expect);
     t.end();
   });
 
-  var readSize = 1;
+  let readSize = 1;
   function flow() {
-    var res;
+    let res;
     while (null !== (res = r.read(readSize++))) {
       reads.push(res.toString());
     }
@@ -142,20 +142,20 @@ test('a most basic test', function(t) {
 });
 
 test('pipe', function(t) {
-  var r = new TestReader(5);
+  const r = new TestReader(5);
 
-  var expect = [ 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx' ];
+  const expect = [ 'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx' ];
 
-  var w = new TestWriter();
+  const w = new TestWriter();
 
   w.on('end', function(received) {
     t.same(received, expect);
@@ -168,10 +168,10 @@ test('pipe', function(t) {
 
 [1, 2, 3, 4, 5, 6, 7, 8, 9].forEach(function(SPLIT) {
   test('unpipe', function(t) {
-    var r = new TestReader(5);
+    const r = new TestReader(5);
 
     // unpipe after 3 writes, then write to another stream instead.
-    var expect = [ 'xxxxx',
+    let expect = [ 'xxxxx',
                    'xxxxx',
                    'xxxxx',
                    'xxxxx',
@@ -183,9 +183,9 @@ test('pipe', function(t) {
                    'xxxxx' ];
     expect = [ expect.slice(0, SPLIT), expect.slice(SPLIT) ];
 
-    var w = [ new TestWriter(), new TestWriter() ];
+    const w = [ new TestWriter(), new TestWriter() ];
 
-    var writes = SPLIT;
+    let writes = SPLIT;
     w[0].on('write', function() {
       if (--writes === 0) {
         r.unpipe();
@@ -196,10 +196,10 @@ test('pipe', function(t) {
       }
     });
 
-    var ended = 0;
+    let ended = 0;
 
-    var ended0 = false;
-    var ended1 = false;
+    let ended0 = false;
+    let ended1 = false;
     w[0].on('end', function(results) {
       t.equal(ended0, false);
       ended0 = true;
@@ -223,21 +223,21 @@ test('pipe', function(t) {
 
 // both writers should get the same exact data.
 test('multipipe', function(t) {
-  var r = new TestReader(5);
-  var w = [ new TestWriter(), new TestWriter() ];
-
-  var expect = [ 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx',
-                 'xxxxx' ];
-
-  var c = 2;
+  const r = new TestReader(5);
+  const w = [ new TestWriter(), new TestWriter() ];
+
+  const expect = [ 'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx',
+                   'xxxxx' ];
+
+  let c = 2;
   w[0].on('end', function(received) {
     t.same(received, expect, 'first');
     if (--c === 0) t.end();
@@ -254,10 +254,10 @@ test('multipipe', function(t) {
 
 [1, 2, 3, 4, 5, 6, 7, 8, 9].forEach(function(SPLIT) {
   test('multi-unpipe', function(t) {
-    var r = new TestReader(5);
+    const r = new TestReader(5);
 
     // unpipe after 3 writes, then write to another stream instead.
-    var expect = [ 'xxxxx',
+    let expect = [ 'xxxxx',
                    'xxxxx',
                    'xxxxx',
                    'xxxxx',
@@ -269,9 +269,9 @@ test('multipipe', function(t) {
                    'xxxxx' ];
     expect = [ expect.slice(0, SPLIT), expect.slice(SPLIT) ];
 
-    var w = [ new TestWriter(), new TestWriter(), new TestWriter() ];
+    const w = [ new TestWriter(), new TestWriter(), new TestWriter() ];
 
-    var writes = SPLIT;
+    let writes = SPLIT;
     w[0].on('write', function() {
       if (--writes === 0) {
         r.unpipe();
@@ -280,7 +280,7 @@ test('multipipe', function(t) {
       }
     });
 
-    var ended = 0;
+    let ended = 0;
 
     w[0].on('end', function(results) {
       ended++;
@@ -302,16 +302,16 @@ test('multipipe', function(t) {
 test('back pressure respected', function(t) {
   function noop() {}
 
-  var r = new R({ objectMode: true });
+  const r = new R({ objectMode: true });
   r._read = noop;
-  var counter = 0;
+  let counter = 0;
   r.push(['one']);
   r.push(['two']);
   r.push(['three']);
   r.push(['four']);
   r.push(null);
 
-  var w1 = new R();
+  const w1 = new R();
   w1.write = function(chunk) {
     console.error('w1.emit("close")');
     assert.equal(chunk[0], 'one');
@@ -325,9 +325,9 @@ test('back pressure respected', function(t) {
 
   r.pipe(w1);
 
-  var expected = ['two', 'two', 'three', 'three', 'four', 'four'];
+  const expected = ['two', 'two', 'three', 'three', 'four', 'four'];
 
-  var w2 = new R();
+  const w2 = new R();
   w2.write = function(chunk) {
     console.error('w2 write', chunk, counter);
     assert.equal(chunk[0], expected.shift());
@@ -349,7 +349,7 @@ test('back pressure respected', function(t) {
   };
   w2.end = noop;
 
-  var w3 = new R();
+  const w3 = new R();
   w3.write = function(chunk) {
     console.error('w3 write', chunk, counter);
     assert.equal(chunk[0], expected.shift());
@@ -377,19 +377,19 @@ test('back pressure respected', function(t) {
 });
 
 test('read(0) for ended streams', function(t) {
-  var r = new R();
-  var written = false;
-  var ended = false;
+  const r = new R();
+  let written = false;
+  let ended = false;
   r._read = function(n) {};
 
   r.push(Buffer.from('foo'));
   r.push(null);
 
-  var v = r.read(0);
+  const v = r.read(0);
 
   assert.equal(v, null);
 
-  var w = new R();
+  const w = new R();
 
   w.write = function(buffer) {
     written = true;
@@ -407,8 +407,8 @@ test('read(0) for ended streams', function(t) {
 });
 
 test('sync _read ending', function(t) {
-  var r = new R();
-  var called = false;
+  const r = new R();
+  let called = false;
   r._read = function(n) {
     r.push(null);
   };
@@ -426,9 +426,9 @@ test('sync _read ending', function(t) {
 });
 
 test('adding readable triggers data flow', function(t) {
-  var r = new R({ highWaterMark: 5 });
-  var onReadable = false;
-  var readCalled = 0;
+  const r = new R({ highWaterMark: 5 });
+  let onReadable = false;
+  let readCalled = 0;
 
   r._read = function(n) {
     if (readCalled++ === 2)
@@ -450,9 +450,9 @@ test('adding readable triggers data flow', function(t) {
 });
 
 test('chainable', function(t) {
-  var r = new R();
+  const r = new R();
   r._read = function() {};
-  var r2 = r.setEncoding('utf8').pause().resume().pause();
+  const r2 = r.setEncoding('utf8').pause().resume().pause();
   t.equal(r, r2);
   t.end();
 });
diff --git a/test/pummel/test-timer-wrap.js b/test/pummel/test-timer-wrap.js
index 659912d1fc..d30661716b 100644
--- a/test/pummel/test-timer-wrap.js
+++ b/test/pummel/test-timer-wrap.js
@@ -1,10 +1,10 @@
 'use strict';
 const common = require('../common');
 
-var Timer = process.binding('timer_wrap').Timer;
-var kOnTimeout = Timer.kOnTimeout;
+const Timer = process.binding('timer_wrap').Timer;
+const kOnTimeout = Timer.kOnTimeout;
 
-var t = new Timer();
+const t = new Timer();
 
 t.start(1000);
 
diff --git a/test/pummel/test-timer-wrap2.js b/test/pummel/test-timer-wrap2.js
index 87497b2685..4bae86f8ce 100644
--- a/test/pummel/test-timer-wrap2.js
+++ b/test/pummel/test-timer-wrap2.js
@@ -4,5 +4,5 @@ require('../common');
 // Test that allocating a timer does not increase the loop's reference
 // count.
 
-var Timer = process.binding('timer_wrap').Timer;
+const Timer = process.binding('timer_wrap').Timer;
 new Timer();
diff --git a/test/pummel/test-timers.js b/test/pummel/test-timers.js
index 83a8eeffeb..4555582371 100644
--- a/test/pummel/test-timers.js
+++ b/test/pummel/test-timers.js
@@ -2,20 +2,20 @@
 const common = require('../common');
 const assert = require('assert');
 
-var WINDOW = 200; // why is does this need to be so big?
+const WINDOW = 200; // why is does this need to be so big?
 
-var interval_count = 0;
+let interval_count = 0;
 
 // check that these don't blow up.
 clearTimeout(null);
 clearInterval(null);
 
 assert.equal(true, setTimeout instanceof Function);
-var starttime = new Date();
+const starttime = new Date();
 setTimeout(common.mustCall(function() {
-  var endtime = new Date();
+  const endtime = new Date();
 
-  var diff = endtime - starttime;
+  const diff = endtime - starttime;
   assert.ok(diff > 0);
   console.error('diff: ' + diff);
 
@@ -23,18 +23,18 @@ setTimeout(common.mustCall(function() {
 }), 1000);
 
 // this timer shouldn't execute
-var id = setTimeout(function() { assert.equal(true, false); }, 500);
+const id = setTimeout(function() { assert.equal(true, false); }, 500);
 clearTimeout(id);
 
 setInterval(function() {
   interval_count += 1;
-  var endtime = new Date();
+  const endtime = new Date();
 
-  var diff = endtime - starttime;
+  const diff = endtime - starttime;
   assert.ok(diff > 0);
   console.error('diff: ' + diff);
 
-  var t = interval_count * 1000;
+  const t = interval_count * 1000;
 
   assert.equal(true, t - WINDOW < diff && diff < t + WINDOW);
 
@@ -49,7 +49,7 @@ setTimeout(function(param) {
   assert.equal('test param', param);
 }, 1000, 'test param');
 
-var interval_count2 = 0;
+let interval_count2 = 0;
 setInterval(function(param) {
   ++interval_count2;
   assert.equal('test param', param);
@@ -65,7 +65,7 @@ setTimeout(function(param1, param2) {
   assert.equal('param2', param2);
 }, 1000, 'param1', 'param2');
 
-var interval_count3 = 0;
+let interval_count3 = 0;
 setInterval(function(param1, param2) {
   ++interval_count3;
   assert.equal('param1', param1);
@@ -76,14 +76,14 @@ setInterval(function(param1, param2) {
 }, 1000, 'param1', 'param2');
 
 // setInterval(cb, 0) should be called multiple times.
-var count4 = 0;
-var interval4 = setInterval(function() {
+let count4 = 0;
+const interval4 = setInterval(function() {
   if (++count4 > 10) clearInterval(interval4);
 }, 0);
 
 
 // we should be able to clearTimeout multiple times without breakage.
-var expectedTimeouts = 3;
+let expectedTimeouts = 3;
 
 function t() {
   expectedTimeouts--;
@@ -91,7 +91,7 @@ function t() {
 
 setTimeout(t, 200);
 setTimeout(t, 200);
-var y = setTimeout(t, 200);
+const y = setTimeout(t, 200);
 
 clearTimeout(y);
 setTimeout(t, 200);
diff --git a/test/pummel/test-tls-ci-reneg-attack.js b/test/pummel/test-tls-ci-reneg-attack.js
index c8f78e0609..dd2b43ed60 100644
--- a/test/pummel/test-tls-ci-reneg-attack.js
+++ b/test/pummel/test-tls-ci-reneg-attack.js
@@ -17,7 +17,7 @@ if (!common.opensslCli) {
 }
 
 // renegotiation limits to test
-var LIMITS = [0, 1, 2, 3, 5, 10, 16];
+const LIMITS = [0, 1, 2, 3, 5, 10, 16];
 
 {
   let n = 0;
@@ -30,14 +30,14 @@ var LIMITS = [0, 1, 2, 3, 5, 10, 16];
 }
 
 function test(next) {
-  var options = {
+  const options = {
     cert: fs.readFileSync(common.fixturesDir + '/test_cert.pem'),
     key: fs.readFileSync(common.fixturesDir + '/test_key.pem')
   };
 
-  var seenError = false;
+  let seenError = false;
 
-  var server = tls.createServer(options, function(conn) {
+  const server = tls.createServer(options, function(conn) {
     conn.on('error', function(err) {
       console.error('Caught exception: ' + err);
       assert(/TLS session renegotiation attack/.test(err));
@@ -48,8 +48,8 @@ function test(next) {
   });
 
   server.listen(common.PORT, function() {
-    var args = ('s_client -connect 127.0.0.1:' + common.PORT).split(' ');
-    var child = spawn(common.opensslCli, args);
+    const args = ('s_client -connect 127.0.0.1:' + common.PORT).split(' ');
+    const child = spawn(common.opensslCli, args);
 
     //child.stdout.pipe(process.stdout);
     //child.stderr.pipe(process.stderr);
@@ -58,8 +58,8 @@ function test(next) {
     child.stderr.resume();
 
     // count handshakes, start the attack after the initial handshake is done
-    var handshakes = 0;
-    var renegs = 0;
+    let handshakes = 0;
+    let renegs = 0;
 
     child.stderr.on('data', function(data) {
       if (seenError) return;
@@ -74,7 +74,7 @@ function test(next) {
       process.nextTick(next);
     });
 
-    var closed = false;
+    let closed = false;
     child.stdin.on('error', function(err) {
       switch (err.code) {
         case 'ECONNRESET':
diff --git a/test/pummel/test-tls-connect-memleak.js b/test/pummel/test-tls-connect-memleak.js
index 46e7f425df..b1d1f05190 100644
--- a/test/pummel/test-tls-connect-memleak.js
+++ b/test/pummel/test-tls-connect-memleak.js
@@ -38,10 +38,10 @@ tls.createServer({
 }
 
 function done() {
-  var before = process.memoryUsage().rss;
+  const before = process.memoryUsage().rss;
   global.gc();
-  var after = process.memoryUsage().rss;
-  var reclaimed = (before - after) / 1024;
+  const after = process.memoryUsage().rss;
+  const reclaimed = (before - after) / 1024;
   console.log('%d kB reclaimed', reclaimed);
   assert(reclaimed > 256 * 1024);  // it's more like 512M on x64
   process.exit();
diff --git a/test/pummel/test-tls-securepair-client.js b/test/pummel/test-tls-securepair-client.js
index c47cc60301..75e7b40059 100644
--- a/test/pummel/test-tls-securepair-client.js
+++ b/test/pummel/test-tls-securepair-client.js
@@ -40,22 +40,22 @@ function test2() {
 
 function test(keyfn, certfn, check, next) {
   keyfn = join(common.fixturesDir, keyfn);
-  var key = fs.readFileSync(keyfn).toString();
+  const key = fs.readFileSync(keyfn).toString();
 
   certfn = join(common.fixturesDir, certfn);
-  var cert = fs.readFileSync(certfn).toString();
+  const cert = fs.readFileSync(certfn).toString();
 
-  var server = spawn(common.opensslCli, ['s_server',
-                                         '-accept', common.PORT,
-                                         '-cert', certfn,
-                                         '-key', keyfn]);
+  const server = spawn(common.opensslCli, ['s_server',
+                                           '-accept', common.PORT,
+                                           '-cert', certfn,
+                                           '-key', keyfn]);
   server.stdout.pipe(process.stdout);
   server.stderr.pipe(process.stdout);
 
 
-  var state = 'WAIT-ACCEPT';
+  let state = 'WAIT-ACCEPT';
 
-  var serverStdoutBuffer = '';
+  let serverStdoutBuffer = '';
   server.stdout.setEncoding('utf8');
   server.stdout.on('data', function(s) {
     serverStdoutBuffer += s;
@@ -86,13 +86,13 @@ function test(keyfn, certfn, check, next) {
   });
 
 
-  var timeout = setTimeout(function() {
+  const timeout = setTimeout(function() {
     server.kill();
     process.exit(1);
   }, 5000);
 
-  var gotWriteCallback = false;
-  var serverExitCode = -1;
+  let gotWriteCallback = false;
+  let serverExitCode = -1;
 
   server.on('exit', function(code) {
     serverExitCode = code;
@@ -102,12 +102,12 @@ function test(keyfn, certfn, check, next) {
 
 
   function startClient() {
-    var s = new net.Stream();
+    const s = new net.Stream();
 
-    var sslcontext = tls.createSecureContext({key: key, cert: cert});
+    const sslcontext = tls.createSecureContext({key: key, cert: cert});
     sslcontext.context.setCiphers('RC4-SHA:AES128-SHA:AES256-SHA');
 
-    var pair = tls.createSecurePair(sslcontext, false);
+    const pair = tls.createSecurePair(sslcontext, false);
 
     assert.ok(pair.encrypted.writable);
     assert.ok(pair.cleartext.writable);
diff --git a/test/pummel/test-tls-server-large-request.js b/test/pummel/test-tls-server-large-request.js
index 1b43a55835..f02eb51a2f 100644
--- a/test/pummel/test-tls-server-large-request.js
+++ b/test/pummel/test-tls-server-large-request.js
@@ -12,9 +12,9 @@ const fs = require('fs');
 const stream = require('stream');
 const util = require('util');
 
-var request = Buffer.from(new Array(1024 * 256).join('ABCD')); // 1mb
+const request = Buffer.from(new Array(1024 * 256).join('ABCD')); // 1mb
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
 };
@@ -35,14 +35,14 @@ Mediator.prototype._write = function _write(data, enc, cb) {
   }
 };
 
-var mediator = new Mediator();
+const mediator = new Mediator();
 
-var server = tls.Server(options, common.mustCall(function(socket) {
+const server = tls.Server(options, common.mustCall(function(socket) {
   socket.pipe(mediator);
 }));
 
 server.listen(common.PORT, common.mustCall(function() {
-  var client1 = tls.connect({
+  const client1 = tls.connect({
     port: common.PORT,
     rejectUnauthorized: false
   }, common.mustCall(function() {
diff --git a/test/pummel/test-tls-session-timeout.js b/test/pummel/test-tls-session-timeout.js
index ffa44a9702..c194188b36 100644
--- a/test/pummel/test-tls-session-timeout.js
+++ b/test/pummel/test-tls-session-timeout.js
@@ -28,13 +28,13 @@ function doTest() {
   const join = require('path').join;
   const spawn = require('child_process').spawn;
 
-  var SESSION_TIMEOUT = 1;
+  const SESSION_TIMEOUT = 1;
 
-  var keyFile = join(common.fixturesDir, 'agent.key');
-  var certFile = join(common.fixturesDir, 'agent.crt');
-  var key = fs.readFileSync(keyFile);
-  var cert = fs.readFileSync(certFile);
-  var options = {
+  const keyFile = join(common.fixturesDir, 'agent.key');
+  const certFile = join(common.fixturesDir, 'agent.crt');
+  const key = fs.readFileSync(keyFile);
+  const cert = fs.readFileSync(certFile);
+  const options = {
     key: key,
     cert: cert,
     ca: [cert],
@@ -47,41 +47,41 @@ function doTest() {
   // file containing a proper serialization of a session ticket.
   // To avoid a source control diff, we copy the ticket to a temporary file.
 
-  var sessionFileName = (function() {
-    var ticketFileName = 'tls-session-ticket.txt';
-    var fixturesPath = join(common.fixturesDir, ticketFileName);
-    var tmpPath = join(common.tmpDir, ticketFileName);
+  const sessionFileName = (function() {
+    const ticketFileName = 'tls-session-ticket.txt';
+    const fixturesPath = join(common.fixturesDir, ticketFileName);
+    const tmpPath = join(common.tmpDir, ticketFileName);
     fs.writeFileSync(tmpPath, fs.readFileSync(fixturesPath));
     return tmpPath;
   }());
 
   // Expects a callback -- cb(connectionType : enum ['New'|'Reused'])
 
-  var Client = function(cb) {
-    var flags = [
+  const Client = function(cb) {
+    const flags = [
       's_client',
       '-connect', 'localhost:' + common.PORT,
       '-sess_in', sessionFileName,
       '-sess_out', sessionFileName
     ];
-    var client = spawn(common.opensslCli, flags, {
+    const client = spawn(common.opensslCli, flags, {
       stdio: ['ignore', 'pipe', 'ignore']
     });
 
-    var clientOutput = '';
+    let clientOutput = '';
     client.stdout.on('data', function(data) {
       clientOutput += data.toString();
     });
     client.on('exit', function(code) {
-      var connectionType;
-      var grepConnectionType = function(line) {
-        var matches = line.match(/(New|Reused), /);
+      let connectionType;
+      const grepConnectionType = function(line) {
+        const matches = line.match(/(New|Reused), /);
         if (matches) {
           connectionType = matches[1];
           return true;
         }
       };
-      var lines = clientOutput.split('\n');
+      const lines = clientOutput.split('\n');
       if (!lines.some(grepConnectionType)) {
         throw new Error('unexpected output from openssl client');
       }
@@ -89,7 +89,7 @@ function doTest() {
     });
   };
 
-  var server = tls.createServer(options, function(cleartext) {
+  const server = tls.createServer(options, function(cleartext) {
     cleartext.on('error', function(er) {
       if (er.code !== 'ECONNRESET')
         throw er;
diff --git a/test/pummel/test-tls-throttle.js b/test/pummel/test-tls-throttle.js
index 581355d61b..574a83b18f 100644
--- a/test/pummel/test-tls-throttle.js
+++ b/test/pummel/test-tls-throttle.js
@@ -13,22 +13,22 @@ const tls = require('tls');
 const fs = require('fs');
 
 process.stdout.write('build body...');
-var body = 'hello world\n'.repeat(1024 * 1024);
+const body = 'hello world\n'.repeat(1024 * 1024);
 process.stdout.write('done\n');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem')
 };
 
-var server = tls.Server(options, common.mustCall(function(socket) {
+const server = tls.Server(options, common.mustCall(function(socket) {
   socket.end(body);
 }));
 
-var recvCount = 0;
+let recvCount = 0;
 
 server.listen(common.PORT, function() {
-  var client = tls.connect({
+  const client = tls.connect({
     port: common.PORT,
     rejectUnauthorized: false
   });
@@ -58,7 +58,7 @@ function displayCounts() {
 }
 
 
-var timeout = setTimeout(displayCounts, 10 * 1000);
+const timeout = setTimeout(displayCounts, 10 * 1000);
 
 
 process.on('exit', function() {
diff --git a/test/pummel/test-vm-memleak.js b/test/pummel/test-vm-memleak.js
index 6f26c9a590..8503e2389b 100644
--- a/test/pummel/test-vm-memleak.js
+++ b/test/pummel/test-vm-memleak.js
@@ -4,21 +4,21 @@
 require('../common');
 const assert = require('assert');
 
-var start = Date.now();
-var maxMem = 0;
+const start = Date.now();
+let maxMem = 0;
 
-var ok = process.execArgv.some(function(arg) {
+const ok = process.execArgv.some(function(arg) {
   return arg === '--max_old_space_size=32';
 });
 assert(ok, 'Run this test with --max_old_space_size=32.');
 
-var interval = setInterval(function() {
+const interval = setInterval(function() {
   try {
     require('vm').runInNewContext('throw 1;');
   } catch (e) {
   }
 
-  var rss = process.memoryUsage().rss;
+  const rss = process.memoryUsage().rss;
   maxMem = Math.max(rss, maxMem);
 
   if (Date.now() - start > 5 * 1000) {
@@ -30,7 +30,7 @@ var interval = setInterval(function() {
 }, 1);
 
 function testContextLeak() {
-  for (var i = 0; i < 1000; i++)
+  for (let i = 0; i < 1000; i++)
     require('vm').createContext({});
 }
 
diff --git a/test/sequential/test-child-process-execsync.js b/test/sequential/test-child-process-execsync.js
index 710137e419..fd9970db7d 100644
--- a/test/sequential/test-child-process-execsync.js
+++ b/test/sequential/test-child-process-execsync.js
@@ -5,12 +5,12 @@ const assert = require('assert');
 const execSync = require('child_process').execSync;
 const execFileSync = require('child_process').execFileSync;
 
-var TIMER = 200;
-var SLEEP = 2000;
+const TIMER = 200;
+const SLEEP = 2000;
 
-var start = Date.now();
-var err;
-var caught = false;
+const start = Date.now();
+let err;
+let caught = false;
 
 // Verify that stderr is not accessed when a bad shell is used
 assert.throws(
@@ -24,9 +24,10 @@ assert.throws(
   'execFileSync did not throw the expected exception!'
 );
 
+let cmd, ret;
 try {
-  var cmd = `"${process.execPath}" -e "setTimeout(function(){}, ${SLEEP});"`;
-  var ret = execSync(cmd, {timeout: TIMER});
+  cmd = `"${process.execPath}" -e "setTimeout(function(){}, ${SLEEP});"`;
+  ret = execSync(cmd, {timeout: TIMER});
 } catch (e) {
   caught = true;
   assert.strictEqual(e.errno, 'ETIMEDOUT');
@@ -34,7 +35,7 @@ try {
 } finally {
   assert.strictEqual(ret, undefined, 'we should not have a return value');
   assert.strictEqual(caught, true, 'execSync should throw');
-  var end = Date.now() - start;
+  const end = Date.now() - start;
   assert(end < SLEEP);
   assert(err.status > 128 || err.signal);
 }
@@ -43,8 +44,8 @@ assert.throws(function() {
   execSync('iamabadcommand');
 }, /Command failed: iamabadcommand/);
 
-var msg = 'foobar';
-var msgBuf = Buffer.from(msg + '\n');
+const msg = 'foobar';
+const msgBuf = Buffer.from(msg + '\n');
 
 // console.log ends every line with just '\n', even on Windows.
 
@@ -59,7 +60,7 @@ ret = execSync(cmd, { encoding: 'utf8' });
 
 assert.strictEqual(ret, msg + '\n', 'execSync encoding result should match');
 
-var args = [
+const args = [
   '-e',
   `console.log("${msg}");`
 ];
diff --git a/test/sequential/test-debugger-debug-brk.js b/test/sequential/test-debugger-debug-brk.js
index 5454aaa08a..f04af4544f 100644
--- a/test/sequential/test-debugger-debug-brk.js
+++ b/test/sequential/test-debugger-debug-brk.js
@@ -3,14 +3,14 @@ const common = require('../common');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-var script = common.fixturesDir + '/empty.js';
+const script = common.fixturesDir + '/empty.js';
 
 function fail() {
   assert(0); // `node --debug-brk script.js` should not quit
 }
 
 function test(arg) {
-  var child = spawn(process.execPath, [arg, script]);
+  const child = spawn(process.execPath, [arg, script]);
   child.on('exit', fail);
 
   // give node time to start up the debugger
diff --git a/test/sequential/test-deprecation-flags.js b/test/sequential/test-deprecation-flags.js
index e9f5ee673d..eb5c1a3ae6 100644
--- a/test/sequential/test-deprecation-flags.js
+++ b/test/sequential/test-deprecation-flags.js
@@ -38,7 +38,7 @@ execFile(node, traceDep, function(er, stdout, stderr) {
   console.error('--trace-deprecation: show stack');
   assert.equal(er, null);
   assert.equal(stdout, '');
-  var stack = stderr.trim().split('\n');
+  const stack = stderr.trim().split('\n');
   // just check the top and bottom.
   assert(/util.debug is deprecated. Use console.error instead./.test(stack[1]));
   assert(/DEBUG: This is deprecated/.test(stack[0]));
diff --git a/test/sequential/test-http-regr-gh-2928.js b/test/sequential/test-http-regr-gh-2928.js
index b4ae7054d7..190966083e 100644
--- a/test/sequential/test-http-regr-gh-2928.js
+++ b/test/sequential/test-http-regr-gh-2928.js
@@ -11,11 +11,11 @@ const net = require('net');
 const COUNT = httpCommon.parsers.max + 1;
 
 const parsers = new Array(COUNT);
-for (var i = 0; i < parsers.length; i++)
+for (let i = 0; i < parsers.length; i++)
   parsers[i] = httpCommon.parsers.alloc();
 
-var gotRequests = 0;
-var gotResponses = 0;
+let gotRequests = 0;
+let gotResponses = 0;
 
 function execAndClose() {
   if (parsers.length === 0)
@@ -50,7 +50,7 @@ function execAndClose() {
   };
 }
 
-var server = net.createServer(function(c) {
+const server = net.createServer(function(c) {
   if (++gotRequests === COUNT)
     server.close();
   c.end('HTTP/1.1 200 OK\r\n\r\n', function() {
diff --git a/test/sequential/test-module-loading.js b/test/sequential/test-module-loading.js
index 54fcc5c06a..dd4e7ba76b 100644
--- a/test/sequential/test-module-loading.js
+++ b/test/sequential/test-module-loading.js
@@ -101,7 +101,7 @@ assert.ok(my_path.path_func instanceof Function);
 // this one does not exist and should throw
 assert.throws(function() { require('./utils'); });
 
-var errorThrown = false;
+let errorThrown = false;
 try {
   require('../fixtures/throws_error');
 } catch (e) {
@@ -113,7 +113,7 @@ assert.strictEqual(require('path').dirname(__filename), __dirname);
 
 console.error('load custom file types with extensions');
 require.extensions['.test'] = function(module, filename) {
-  var content = fs.readFileSync(filename).toString();
+  let content = fs.readFileSync(filename).toString();
   assert.strictEqual('this is custom source\n', content);
   content = content.replace('this is custom source',
                             'exports.test = \'passed\'');
@@ -193,8 +193,8 @@ assert.deepStrictEqual(json, {
 // modules that we've required, and that all of them contain
 // the appropriate children, and so on.
 
-var children = module.children.reduce(function red(set, child) {
-  var id = path.relative(path.dirname(__dirname), child.id);
+const children = module.children.reduce(function red(set, child) {
+  let id = path.relative(path.dirname(__dirname), child.id);
   id = id.replace(/\\/g, '/');
   set[id] = child.children.reduce(red, {});
   return set;
diff --git a/test/sequential/test-net-GH-5504.js b/test/sequential/test-net-GH-5504.js
index b40cfb400b..a7f64c2e73 100644
--- a/test/sequential/test-net-GH-5504.js
+++ b/test/sequential/test-net-GH-5504.js
@@ -19,7 +19,7 @@ switch (process.argv[2]) {
 
 function server() {
   const net = require('net');
-  var content = Buffer.alloc(64 * 1024 * 1024, '#');
+  const content = Buffer.alloc(64 * 1024 * 1024, '#');
   net.createServer(function(socket) {
     this.close();
     socket.on('end', function() {
@@ -36,7 +36,7 @@ function server() {
 
 function client() {
   const net = require('net');
-  var client = net.connect({
+  const client = net.connect({
     host: common.localhostIPv4,
     port: common.PORT
   }, function() {
@@ -46,14 +46,14 @@ function client() {
 
 function parent() {
   const spawn = require('child_process').spawn;
-  var node = process.execPath;
+  const node = process.execPath;
 
-  var s = spawn(node, [__filename, 'server'], {
+  const s = spawn(node, [__filename, 'server'], {
     env: Object.assign(process.env, {
       NODE_DEBUG: 'net'
     })
   });
-  var c;
+  let c;
 
   wrap(s.stderr, process.stderr, 'SERVER 2>');
   wrap(s.stdout, process.stdout, 'SERVER 1>');
diff --git a/test/sequential/test-regress-GH-1697.js b/test/sequential/test-regress-GH-1697.js
index 1511cfd1cf..d99acf9e46 100644
--- a/test/sequential/test-regress-GH-1697.js
+++ b/test/sequential/test-regress-GH-1697.js
@@ -6,7 +6,7 @@ const cp = require('child_process');
 if (process.argv[2] === 'server') {
   // Server
 
-  var server = net.createServer(function(conn) {
+  const server = net.createServer(function(conn) {
     conn.on('data', function(data) {
       console.log('server received ' + data.length + ' bytes');
     });
@@ -23,22 +23,22 @@ if (process.argv[2] === 'server') {
 } else {
   // Client
 
-  var serverProcess = cp.spawn(process.execPath, [process.argv[1], 'server']);
+  const serverProcess = cp.spawn(process.execPath, [process.argv[1], 'server']);
   serverProcess.stdout.pipe(process.stdout);
   serverProcess.stderr.pipe(process.stdout);
 
   serverProcess.stdout.once('data', function() {
-    var client = net.createConnection(common.PORT, '127.0.0.1');
+    const client = net.createConnection(common.PORT, '127.0.0.1');
     client.on('connect', function() {
       const alot = Buffer.allocUnsafe(1024);
       const alittle = Buffer.allocUnsafe(1);
 
-      for (var i = 0; i < 100; i++) {
+      for (let i = 0; i < 100; i++) {
         client.write(alot);
       }
 
       // Block the event loop for 1 second
-      var start = (new Date()).getTime();
+      const start = (new Date()).getTime();
       while ((new Date()).getTime() < start + 1000) {}
 
       client.write(alittle);
diff --git a/test/sequential/test-regress-GH-1726.js b/test/sequential/test-regress-GH-1726.js
index 0097ed913d..c464193b23 100644
--- a/test/sequential/test-regress-GH-1726.js
+++ b/test/sequential/test-regress-GH-1726.js
@@ -8,8 +8,8 @@ require('../common');
 const assert = require('assert');
 const ch = require('child_process');
 
-var gen = +(process.argv[2] || 0);
-var maxGen = 5;
+const gen = +(process.argv[2] || 0);
+const maxGen = 5;
 
 
 if (gen === maxGen) {
@@ -17,7 +17,7 @@ if (gen === maxGen) {
   return;
 }
 
-var child = ch.spawn(process.execPath, [__filename, gen + 1], {
+const child = ch.spawn(process.execPath, [__filename, gen + 1], {
   stdio: [ 'ignore', 'pipe', 'ignore' ]
 });
 assert.ok(!child.stdin);
diff --git a/test/sequential/test-regress-GH-4015.js b/test/sequential/test-regress-GH-4015.js
index 87b2b8561d..249817de1d 100644
--- a/test/sequential/test-regress-GH-4015.js
+++ b/test/sequential/test-regress-GH-4015.js
@@ -3,8 +3,8 @@ const common = require('../common');
 const assert = require('assert');
 const exec = require('child_process').exec;
 
-var cmd = '"' + process.execPath + '" ' +
-          '"' + common.fixturesDir + '/test-regress-GH-4015.js"';
+const cmd = '"' + process.execPath + '" ' +
+            '"' + common.fixturesDir + '/test-regress-GH-4015.js"';
 
 exec(cmd, function(err, stdout, stderr) {
   assert(/RangeError: Maximum call stack size exceeded/.test(stderr));
diff --git a/test/sequential/test-regress-GH-4027.js b/test/sequential/test-regress-GH-4027.js
index 7653dda182..87a8861ac3 100644
--- a/test/sequential/test-regress-GH-4027.js
+++ b/test/sequential/test-regress-GH-4027.js
@@ -6,7 +6,7 @@ const fs = require('fs');
 
 common.refreshTmpDir();
 
-var filename = path.join(common.tmpDir, 'watched');
+const filename = path.join(common.tmpDir, 'watched');
 fs.writeFileSync(filename, 'quis custodiet ipsos custodes');
 setTimeout(fs.unlinkSync, 100, filename);
 
diff --git a/test/sequential/test-regress-GH-784.js b/test/sequential/test-regress-GH-784.js
index 645ffeb650..c9b630b0c8 100644
--- a/test/sequential/test-regress-GH-784.js
+++ b/test/sequential/test-regress-GH-784.js
@@ -12,8 +12,8 @@ const http = require('http');
 const assert = require('assert');
 
 
-var server = http.createServer(function(req, res) {
-  var body = '';
+const server = http.createServer(function(req, res) {
+  let body = '';
 
   req.setEncoding('utf8');
   req.on('data', function(chunk) {
@@ -43,7 +43,7 @@ function serverOff() {
   pingping();
 }
 
-var responses = [];
+const responses = [];
 
 
 function afterPing(result) {
@@ -81,14 +81,14 @@ function afterPing(result) {
 function ping() {
   console.error('making req');
 
-  var opt = {
+  const opt = {
     port: common.PORT,
     path: '/ping',
     method: 'POST'
   };
 
-  var req = http.request(opt, function(res) {
-    var body = '';
+  const req = http.request(opt, function(res) {
+    let body = '';
 
     res.setEncoding('utf8');
     res.on('data', function(chunk) {
@@ -105,8 +105,8 @@ function ping() {
 
   req.end('PING');
 
-  var gotEnd = false;
-  var hadError = false;
+  let gotEnd = false;
+  let hadError = false;
 
   req.on('error', function(error) {
     console.log('Error making ping req: ' + error);
diff --git a/test/sequential/test-regress-GH-877.js b/test/sequential/test-regress-GH-877.js
index 82229524ce..06c839aa48 100644
--- a/test/sequential/test-regress-GH-877.js
+++ b/test/sequential/test-regress-GH-877.js
@@ -3,28 +3,28 @@ const common = require('../common');
 const http = require('http');
 const assert = require('assert');
 
-var N = 20;
-var responses = 0;
-var maxQueued = 0;
+const N = 20;
+let responses = 0;
+let maxQueued = 0;
 
-var agent = http.globalAgent;
+const agent = http.globalAgent;
 agent.maxSockets = 10;
 
-var server = http.createServer(function(req, res) {
+const server = http.createServer(function(req, res) {
   res.writeHead(200);
   res.end('Hello World\n');
 });
 
-var addrString = agent.getName({ host: '127.0.0.1', port: common.PORT });
+const addrString = agent.getName({ host: '127.0.0.1', port: common.PORT });
 
 server.listen(common.PORT, '127.0.0.1', function() {
-  for (var i = 0; i < N; i++) {
-    var options = {
+  for (let i = 0; i < N; i++) {
+    const options = {
       host: '127.0.0.1',
       port: common.PORT
     };
 
-    var req = http.get(options, function(res) {
+    const req = http.get(options, function(res) {
       if (++responses === N) {
         server.close();
       }
@@ -37,7 +37,7 @@ server.listen(common.PORT, '127.0.0.1', function() {
                 agent.maxSockets + ' queued: ' + (agent.requests[addrString] ?
                 agent.requests[addrString].length : 0));
 
-    var agentRequests = agent.requests[addrString] ?
+    const agentRequests = agent.requests[addrString] ?
         agent.requests[addrString].length : 0;
 
     if (maxQueued < agentRequests) {
diff --git a/test/sequential/test-repl-timeout-throw.js b/test/sequential/test-repl-timeout-throw.js
index 344845e020..6966419556 100644
--- a/test/sequential/test-repl-timeout-throw.js
+++ b/test/sequential/test-repl-timeout-throw.js
@@ -8,7 +8,7 @@ const child = spawn(process.execPath, [ '-i' ], {
   stdio: [null, null, 2]
 });
 
-var stdout = '';
+let stdout = '';
 child.stdout.setEncoding('utf8');
 child.stdout.on('data', function(c) {
   process.stdout.write(c);
@@ -30,7 +30,7 @@ child.stdout.once('data', function() {
 
   setTimeout(fsTest, 50);
   function fsTest() {
-    var f = JSON.stringify(__filename);
+    const f = JSON.stringify(__filename);
     child.stdin.write('fs.readFile(' + f + ', thrower);\n');
     setTimeout(eeTest, 50);
   }
@@ -54,6 +54,6 @@ child.stdout.once('data', function() {
 child.on('close', function(c) {
   assert.strictEqual(c, 0);
   // make sure we got 3 throws, in the end.
-  var lastLine = stdout.trim().split(/\r?\n/).pop();
+  const lastLine = stdout.trim().split(/\r?\n/).pop();
   assert.strictEqual(lastLine, '> 3');
 });
diff --git a/test/sequential/test-require-cache-without-stat.js b/test/sequential/test-require-cache-without-stat.js
index 0549e5f72e..b591f1746b 100644
--- a/test/sequential/test-require-cache-without-stat.js
+++ b/test/sequential/test-require-cache-without-stat.js
@@ -6,13 +6,13 @@ const common = require('../common');
 const fs = require('fs');
 const assert = require('assert');
 
-var counter = 0;
+let counter = 0;
 
 // Switch out the two stat implementations so that they increase a counter
 // each time they are called.
 
-var _statSync = fs.statSync;
-var _stat = fs.stat;
+const _statSync = fs.statSync;
+const _stat = fs.stat;
 
 fs.statSync = function() {
   counter++;
@@ -31,7 +31,7 @@ require('./../fixtures/a.js');
 require('http');
 
 console.log('counterBefore = %d', counter);
-var counterBefore = counter;
+const counterBefore = counter;
 
 // Now load the module a bunch of times with equivalent paths.
 // stat should not be called.
@@ -47,6 +47,6 @@ for (let i = 0; i < 100; i++) {
 }
 
 console.log('counterAfter = %d', counter);
-var counterAfter = counter;
+const counterAfter = counter;
 
 assert.equal(counterBefore, counterAfter);
diff --git a/test/sequential/test-stream2-fs.js b/test/sequential/test-stream2-fs.js
index 3640749179..2891335c81 100644
--- a/test/sequential/test-stream2-fs.js
+++ b/test/sequential/test-stream2-fs.js
@@ -3,14 +3,14 @@ const common = require('../common');
 const assert = require('assert');
 
 const fs = require('fs');
-var FSReadable = fs.ReadStream;
+const FSReadable = fs.ReadStream;
 
 const path = require('path');
-var file = path.resolve(common.fixturesDir, 'x1024.txt');
+const file = path.resolve(common.fixturesDir, 'x1024.txt');
 
-var size = fs.statSync(file).size;
+const size = fs.statSync(file).size;
 
-var expectLengths = [1024];
+const expectLengths = [1024];
 
 const util = require('util');
 const Stream = require('stream');
@@ -34,8 +34,8 @@ TestWriter.prototype.end = function(c) {
   this.emit('results', this.buffer);
 };
 
-var r = new FSReadable(file);
-var w = new TestWriter();
+const r = new FSReadable(file);
+const w = new TestWriter();
 
 w.on('results', function(res) {
   console.error(res, w.length);
diff --git a/test/sequential/test-stream2-stderr-sync.js b/test/sequential/test-stream2-stderr-sync.js
index 2855a5f097..e37a5f2cd1 100644
--- a/test/sequential/test-stream2-stderr-sync.js
+++ b/test/sequential/test-stream2-stderr-sync.js
@@ -6,10 +6,10 @@ require('../common');
 function parent() {
   const spawn = require('child_process').spawn;
   const assert = require('assert');
-  var i = 0;
+  let i = 0;
   children.forEach(function(_, c) {
-    var child = spawn(process.execPath, [__filename, '' + c]);
-    var err = '';
+    const child = spawn(process.execPath, [__filename, '' + c]);
+    let err = '';
 
     child.stderr.on('data', function(c) {
       err += c;
@@ -43,7 +43,7 @@ function child1() {
 // using a net socket
 function child2() {
   const net = require('net');
-  var socket = new net.Socket({
+  const socket = new net.Socket({
     fd: 2,
     readable: false,
     writable: true});
@@ -62,7 +62,7 @@ function child4() {
   process.stderr.write('child 4\nfoo\nbar\nbaz\n');
 }
 
-var children = [ child0, child1, child2, child3, child4 ];
+const children = [ child0, child1, child2, child3, child4 ];
 
 if (!process.argv[2]) {
   parent();
diff --git a/test/sequential/test-timers-blocking-callback.js b/test/sequential/test-timers-blocking-callback.js
index e4e85cddc6..73b0f13997 100644
--- a/test/sequential/test-timers-blocking-callback.js
+++ b/test/sequential/test-timers-blocking-callback.js
@@ -28,9 +28,9 @@ const Timer = process.binding('timer_wrap').Timer;
 
 const TIMEOUT = 100;
 
-var nbBlockingCallbackCalls = 0;
-var latestDelay = 0;
-var timeCallbackScheduled = 0;
+let nbBlockingCallbackCalls = 0;
+let latestDelay = 0;
+let timeCallbackScheduled = 0;
 
 function initTest() {
   nbBlockingCallbackCalls = 0;
diff --git a/test/sequential/test-vm-timeout-rethrow.js b/test/sequential/test-vm-timeout-rethrow.js
index d55a9b310c..a434e2ae3e 100644
--- a/test/sequential/test-vm-timeout-rethrow.js
+++ b/test/sequential/test-vm-timeout-rethrow.js
@@ -5,11 +5,11 @@ const vm = require('vm');
 const spawn = require('child_process').spawn;
 
 if (process.argv[2] === 'child') {
-  var code = 'var j = 0;\n' +
-             'for (var i = 0; i < 1000000; i++) j += add(i, i + 1);\n' +
-             'j;';
+  const code = 'let j = 0;\n' +
+               'for (let i = 0; i < 1000000; i++) j += add(i, i + 1);\n' +
+               'j;';
 
-  var ctx = vm.createContext({
+  const ctx = vm.createContext({
     add: function(x, y) {
       return x + y;
     }
@@ -17,8 +17,8 @@ if (process.argv[2] === 'child') {
 
   vm.runInContext(code, ctx, { timeout: 1 });
 } else {
-  var proc = spawn(process.execPath, process.argv.slice(1).concat('child'));
-  var err = '';
+  const proc = spawn(process.execPath, process.argv.slice(1).concat('child'));
+  let err = '';
   proc.stderr.on('data', function(data) {
     err += data;
   });
diff --git a/test/timers/test-timers-reliability.js b/test/timers/test-timers-reliability.js
index 11dd0a225d..9a9834aefe 100644
--- a/test/timers/test-timers-reliability.js
+++ b/test/timers/test-timers-reliability.js
@@ -3,11 +3,11 @@
 
 require('../common');
 
-var Timer = process.binding('timer_wrap').Timer;
+const Timer = process.binding('timer_wrap').Timer;
 const assert = require('assert');
 
-var timerFired = false;
-var intervalFired = false;
+let timerFired = false;
+let intervalFired = false;
 
 /*
  * This test case aims at making sure that timing utilities such
@@ -31,7 +31,7 @@ var intervalFired = false;
  * is vulnerable to time drift or inconsistent time changes.
  */
 
-var monoTimer = new Timer();
+const monoTimer = new Timer();
 monoTimer[Timer.kOnTimeout] = function() {
     /*
      * Make sure that setTimeout's and setInterval's callbacks have
@@ -48,7 +48,7 @@ setTimeout(function() {
   timerFired = true;
 }, 200);
 
-var interval = setInterval(function() {
+const interval = setInterval(function() {
   intervalFired = true;
   clearInterval(interval);
 }, 200);