lib: reduce util.is*() usage

Many of the util.is*() methods used to check data types
simply compare against a single value or the result of
typeof. This commit replaces calls to these methods with
equivalent checks. This commit does not touch calls to the
more complex methods (isRegExp(), isDate(), etc.).

Fixes: https://github.com/iojs/io.js/issues/607
PR-URL: https://github.com/iojs/io.js/pull/647
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

37 files changed, 451 insertions, 445 deletions

diff --git a/lib/_debugger.js b/lib/_debugger.js
index 0dc9c95c2c..955080ff2f 100644
--- a/lib/_debugger.js
+++ b/lib/_debugger.js
@@ -164,7 +164,8 @@ exports.Client = Client;
 
 
 Client.prototype._addHandle = function(desc) {
-  if (!util.isObject(desc) || !util.isNumber(desc.handle)) {
+  if (desc === null || typeof desc !== 'object' ||
+      typeof desc.handle !== 'number') {
     return;
   }
 
@@ -282,7 +283,7 @@ Client.prototype.reqLookup = function(refs, cb) {
   this.req(req, function(err, res) {
     if (err) return cb(err);
     for (var ref in res) {
-      if (util.isObject(res[ref])) {
+      if (res[ref] !== null && typeof res[ref] === 'object') {
         self._addHandle(res[ref]);
       }
     }
@@ -544,8 +545,7 @@ Client.prototype.mirrorObject = function(handle, depth, cb) {
           mirrorValue = '[?]';
         }
 
-
-        if (util.isArray(mirror) && !util.isNumber(prop.name)) {
+        if (Array.isArray(mirror) && typeof prop.name !== 'number') {
           // Skip the 'length' property.
           return;
         }
@@ -578,7 +578,7 @@ Client.prototype.mirrorObject = function(handle, depth, cb) {
     val = function() {};
   } else if (handle.type === 'null') {
     val = null;
-  } else if (!util.isUndefined(handle.value)) {
+  } else if (handle.value !== undefined) {
     val = handle.value;
   } else if (handle.type === 'undefined') {
     val = undefined;
@@ -879,7 +879,7 @@ Interface.prototype.print = function(text, oneline) {
   if (this.killed) return;
   this.clearline();
 
-  this.stdout.write(util.isString(text) ? text : util.inspect(text));
+  this.stdout.write(typeof text === 'string' ? text : util.inspect(text));
 
   if (oneline !== true) {
     this.stdout.write('\n');
@@ -1194,7 +1194,7 @@ Interface.prototype.scripts = function() {
   this.pause();
   for (var id in client.scripts) {
     var script = client.scripts[id];
-    if (util.isObject(script) && script.name) {
+    if (script !== null && typeof script === 'object' && script.name) {
       if (displayNatives ||
           script.name == client.currentScript ||
           !script.isNative) {
@@ -1332,13 +1332,13 @@ Interface.prototype.setBreakpoint = function(script, line,
       ambiguous;
 
   // setBreakpoint() should insert breakpoint on current line
-  if (util.isUndefined(script)) {
+  if (script === undefined) {
     script = this.client.currentScript;
     line = this.client.currentSourceLine + 1;
   }
 
   // setBreakpoint(line-number) should insert breakpoint in current script
-  if (util.isUndefined(line) && util.isNumber(script)) {
+  if (line === undefined && typeof script === 'number') {
     line = script;
     script = this.client.currentScript;
   }
@@ -1442,7 +1442,7 @@ Interface.prototype.clearBreakpoint = function(script, line) {
     if (bp.scriptId === script ||
         bp.scriptReq === script ||
         (bp.script && bp.script.indexOf(script) !== -1)) {
-      if (!util.isUndefined(index)) {
+      if (index !== undefined) {
         ambiguous = true;
       }
       scriptId = script;
@@ -1470,11 +1470,11 @@ Interface.prototype.clearBreakpoint = function(script, line) {
 
   if (ambiguous) return this.error('Script name is ambiguous');
 
-  if (util.isUndefined(scriptId)) {
+  if (scriptId === undefined) {
     return this.error('Script ' + script + ' not found');
   }
 
-  if (util.isUndefined(breakpoint)) {
+  if (breakpoint === undefined) {
     return this.error('Breakpoint not found on line ' + line);
   }
 
diff --git a/lib/_http_client.js b/lib/_http_client.js
index 9212c0a1ba..6804157ee0 100644
--- a/lib/_http_client.js
+++ b/lib/_http_client.js
@@ -19,7 +19,7 @@ function ClientRequest(options, cb) {
   var self = this;
   OutgoingMessage.call(self);
 
-  if (util.isString(options)) {
+  if (typeof options === 'string') {
     options = url.parse(options);
   }
 
@@ -27,7 +27,8 @@ function ClientRequest(options, cb) {
   var defaultAgent = options._defaultAgent || Agent.globalAgent;
   if (agent === false) {
     agent = new defaultAgent.constructor();
-  } else if (util.isNullOrUndefined(agent) && !options.createConnection) {
+  } else if ((agent === null || agent === undefined) &&
+             !options.createConnection) {
     agent = defaultAgent;
   }
   self.agent = agent;
@@ -56,7 +57,7 @@ function ClientRequest(options, cb) {
   var port = options.port = options.port || defaultPort || 80;
   var host = options.host = options.hostname || options.host || 'localhost';
 
-  if (util.isUndefined(options.setHost)) {
+  if (options.setHost === undefined) {
     var setHost = true;
   }
 
@@ -68,7 +69,7 @@ function ClientRequest(options, cb) {
     self.once('response', cb);
   }
 
-  if (!util.isArray(options.headers)) {
+  if (!Array.isArray(options.headers)) {
     if (options.headers) {
       var keys = Object.keys(options.headers);
       for (var i = 0, l = keys.length; i < l; i++) {
@@ -101,7 +102,7 @@ function ClientRequest(options, cb) {
     self.useChunkedEncodingByDefault = true;
   }
 
-  if (util.isArray(options.headers)) {
+  if (Array.isArray(options.headers)) {
     self._storeHeader(self.method + ' ' + self.path + ' HTTP/1.1\r\n',
                       options.headers);
   } else if (self.getHeader('expect')) {
@@ -446,7 +447,7 @@ function tickOnSocket(req, socket) {
   httpSocketSetup(socket);
 
   // Propagate headers limit from request object to parser
-  if (util.isNumber(req.maxHeadersCount)) {
+  if (typeof req.maxHeadersCount === 'number') {
     parser.maxHeaderPairs = req.maxHeadersCount << 1;
   } else {
     // Set default value because parser may be reused from FreeList
diff --git a/lib/_http_common.js b/lib/_http_common.js
index 5f5c2d7fb2..209bd77bf4 100644
--- a/lib/_http_common.js
+++ b/lib/_http_common.js
@@ -8,7 +8,6 @@ const IncomingMessage = incoming.IncomingMessage;
 const readStart = incoming.readStart;
 const readStop = incoming.readStop;
 
-const isNumber = require('util').isNumber;
 const debug = require('util').debuglog('http');
 exports.debug = debug;
 
@@ -69,7 +68,7 @@ function parserOnHeadersComplete(versionMajor, versionMinor, headers, method,
 
   parser.incoming._addHeaderLines(headers, n);
 
-  if (isNumber(method)) {
+  if (typeof method === 'number') {
     // server only
     parser.incoming.method = HTTPParser.methods[method];
   } else {
diff --git a/lib/_http_incoming.js b/lib/_http_incoming.js
index e35457ce5e..41ece371cd 100644
--- a/lib/_http_incoming.js
+++ b/lib/_http_incoming.js
@@ -130,7 +130,7 @@ IncomingMessage.prototype._addHeaderLine = function(field, value, dest) {
   switch (field) {
     // Array headers:
     case 'set-cookie':
-      if (!util.isUndefined(dest[field])) {
+      if (dest[field] !== undefined) {
         dest[field].push(value);
       } else {
         dest[field] = [value];
@@ -152,13 +152,13 @@ IncomingMessage.prototype._addHeaderLine = function(field, value, dest) {
     case 'location':
     case 'max-forwards':
       // drop duplicates
-      if (util.isUndefined(dest[field]))
+      if (dest[field] === undefined)
         dest[field] = value;
       break;
 
     default:
       // make comma-separated list
-      if (!util.isUndefined(dest[field]))
+      if (dest[field] !== undefined)
         dest[field] += ', ' + value;
       else {
         dest[field] = value;
diff --git a/lib/_http_outgoing.js b/lib/_http_outgoing.js
index a93b7e0683..25157d34a4 100644
--- a/lib/_http_outgoing.js
+++ b/lib/_http_outgoing.js
@@ -106,7 +106,7 @@ OutgoingMessage.prototype._send = function(data, encoding, callback) {
   // the same packet. Future versions of Node are going to take care of
   // this at a lower level and in a more general way.
   if (!this._headerSent) {
-    if (util.isString(data) &&
+    if (typeof data === 'string' &&
         encoding !== 'hex' &&
         encoding !== 'base64') {
       data = this._header + data;
@@ -122,13 +122,13 @@ OutgoingMessage.prototype._send = function(data, encoding, callback) {
 
 
 OutgoingMessage.prototype._writeRaw = function(data, encoding, callback) {
-  if (util.isFunction(encoding)) {
+  if (typeof encoding === 'function') {
     callback = encoding;
     encoding = null;
   }
 
   if (data.length === 0) {
-    if (util.isFunction(callback))
+    if (typeof callback === 'function')
       process.nextTick(callback);
     return true;
   }
@@ -187,7 +187,7 @@ OutgoingMessage.prototype._storeHeader = function(firstLine, headers) {
 
   if (headers) {
     var keys = Object.keys(headers);
-    var isArray = util.isArray(headers);
+    var isArray = Array.isArray(headers);
     var field, value;
 
     for (var i = 0, l = keys.length; i < l; i++) {
@@ -200,7 +200,7 @@ OutgoingMessage.prototype._storeHeader = function(firstLine, headers) {
         value = headers[key];
       }
 
-      if (util.isArray(value)) {
+      if (Array.isArray(value)) {
         for (var j = 0; j < value.length; j++) {
           storeHeader(this, state, field, value[j]);
         }
@@ -408,7 +408,7 @@ OutgoingMessage.prototype.write = function(chunk, encoding, callback) {
     return true;
   }
 
-  if (!util.isString(chunk) && !util.isBuffer(chunk)) {
+  if (typeof chunk !== 'string' && !(chunk instanceof Buffer)) {
     throw new TypeError('first argument must be a string or Buffer');
   }
 
@@ -419,7 +419,7 @@ OutgoingMessage.prototype.write = function(chunk, encoding, callback) {
 
   var len, ret;
   if (this.chunkedEncoding) {
-    if (util.isString(chunk) &&
+    if (typeof chunk === 'string' &&
         encoding !== 'hex' &&
         encoding !== 'base64' &&
         encoding !== 'binary') {
@@ -428,7 +428,7 @@ OutgoingMessage.prototype.write = function(chunk, encoding, callback) {
       ret = this._send(chunk, encoding, callback);
     } else {
       // buffer, or a non-toString-friendly encoding
-      if (util.isString(chunk))
+      if (typeof chunk === 'string')
         len = Buffer.byteLength(chunk, encoding);
       else
         len = chunk.length;
@@ -458,7 +458,7 @@ OutgoingMessage.prototype.write = function(chunk, encoding, callback) {
 OutgoingMessage.prototype.addTrailers = function(headers) {
   this._trailer = '';
   var keys = Object.keys(headers);
-  var isArray = util.isArray(headers);
+  var isArray = Array.isArray(headers);
   var field, value;
   for (var i = 0, l = keys.length; i < l; i++) {
     var key = keys[i];
@@ -479,15 +479,15 @@ const crlf_buf = new Buffer('\r\n');
 
 
 OutgoingMessage.prototype.end = function(data, encoding, callback) {
-  if (util.isFunction(data)) {
+  if (typeof data === 'function') {
     callback = data;
     data = null;
-  } else if (util.isFunction(encoding)) {
+  } else if (typeof encoding === 'function') {
     callback = encoding;
     encoding = null;
   }
 
-  if (data && !util.isString(data) && !util.isBuffer(data)) {
+  if (data && typeof data !== 'string' && !(data instanceof Buffer)) {
     throw new TypeError('first argument must be a string or Buffer');
   }
 
@@ -500,10 +500,9 @@ OutgoingMessage.prototype.end = function(data, encoding, callback) {
     self.emit('finish');
   }
 
-  if (util.isFunction(callback))
+  if (typeof callback === 'function')
     this.once('finish', callback);
 
-
   if (!this._header) {
     this._implicitHeader();
   }
diff --git a/lib/_http_server.js b/lib/_http_server.js
index 20012a5ab1..d8b9799438 100644
--- a/lib/_http_server.js
+++ b/lib/_http_server.js
@@ -153,7 +153,7 @@ ServerResponse.prototype._implicitHeader = function() {
 ServerResponse.prototype.writeHead = function(statusCode, reason, obj) {
   var headers;
 
-  if (util.isString(reason)) {
+  if (typeof reason === 'string') {
     // writeHead(statusCode, reasonPhrase[, headers])
     this.statusMessage = reason;
   } else {
@@ -297,7 +297,7 @@ function connectionListener(socket) {
   parser.incoming = null;
 
   // Propagate headers limit from server instance to parser
-  if (util.isNumber(this.maxHeadersCount)) {
+  if (typeof this.maxHeadersCount === 'number') {
     parser.maxHeaderPairs = this.maxHeadersCount << 1;
   } else {
     // Set default value because parser may be reused from FreeList
@@ -455,7 +455,7 @@ function connectionListener(socket) {
       }
     }
 
-    if (!util.isUndefined(req.headers.expect) &&
+    if (req.headers.expect !== undefined &&
         (req.httpVersionMajor == 1 && req.httpVersionMinor == 1) &&
         continueExpression.test(req.headers['expect'])) {
       res._expect_continue = true;
diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
index da16420828..2418648b69 100644
--- a/lib/_stream_readable.js
+++ b/lib/_stream_readable.js
@@ -95,7 +95,7 @@ function Readable(options) {
 Readable.prototype.push = function(chunk, encoding) {
   var state = this._readableState;
 
-  if (util.isString(chunk) && !state.objectMode) {
+  if (!state.objectMode && typeof chunk === 'string') {
     encoding = encoding || state.defaultEncoding;
     if (encoding !== state.encoding) {
       chunk = new Buffer(chunk, encoding);
@@ -209,7 +209,7 @@ function howMuchToRead(n, state) {
   if (state.objectMode)
     return n === 0 ? 0 : 1;
 
-  if (util.isNull(n) || isNaN(n)) {
+  if (n === null || isNaN(n)) {
     // only flow one buffer at a time
     if (state.flowing && state.buffer.length)
       return state.buffer[0].length;
@@ -245,7 +245,7 @@ Readable.prototype.read = function(n) {
   var state = this._readableState;
   var nOrig = n;
 
-  if (!util.isNumber(n) || n > 0)
+  if (typeof n !== 'number' || n > 0)
     state.emittedReadable = false;
 
   // if we're doing read(0) to trigger a readable event, but we
@@ -333,7 +333,7 @@ Readable.prototype.read = function(n) {
   else
     ret = null;
 
-  if (util.isNull(ret)) {
+  if (ret === null) {
     state.needReadable = true;
     n = 0;
   }
@@ -349,7 +349,7 @@ Readable.prototype.read = function(n) {
   if (nOrig !== n && state.ended && state.length === 0)
     endReadable(this);
 
-  if (!util.isNull(ret))
+  if (ret !== null)
     this.emit('data', ret);
 
   return ret;
@@ -357,9 +357,10 @@ Readable.prototype.read = function(n) {
 
 function chunkInvalid(state, chunk) {
   var er = null;
-  if (!util.isBuffer(chunk) &&
-      !util.isString(chunk) &&
-      !util.isNullOrUndefined(chunk) &&
+  if (!(chunk instanceof Buffer) &&
+      typeof chunk !== 'string' &&
+      chunk !== null &&
+      chunk !== undefined &&
       !state.objectMode) {
     er = new TypeError('Invalid non-string/buffer chunk');
   }
@@ -757,7 +758,6 @@ Readable.prototype.wrap = function(stream) {
       chunk = state.decoder.write(chunk);
 
     // don't skip over falsy values in objectMode
-    //if (state.objectMode && util.isNullOrUndefined(chunk))
     if (state.objectMode && (chunk === null || chunk === undefined))
       return;
     else if (!state.objectMode && (!chunk || !chunk.length))
@@ -773,7 +773,7 @@ Readable.prototype.wrap = function(stream) {
   // proxy all the other methods.
   // important when wrapping filters and duplexes.
   for (var i in stream) {
-    if (util.isFunction(stream[i]) && util.isUndefined(this[i])) {
+    if (this[i] === undefined && typeof stream[i] === 'function') {
       this[i] = function(method) { return function() {
         return stream[method].apply(stream, arguments);
       }}(i);
diff --git a/lib/_stream_transform.js b/lib/_stream_transform.js
index f836208633..d3e7a13486 100644
--- a/lib/_stream_transform.js
+++ b/lib/_stream_transform.js
@@ -72,7 +72,7 @@ function afterTransform(stream, er, data) {
   ts.writechunk = null;
   ts.writecb = null;
 
-  if (!util.isNullOrUndefined(data))
+  if (data !== null && data !== undefined)
     stream.push(data);
 
   if (cb)
@@ -106,7 +106,7 @@ function Transform(options) {
   this._readableState.sync = false;
 
   this.once('prefinish', function() {
-    if (util.isFunction(this._flush))
+    if (typeof this._flush === 'function')
       this._flush(function(er) {
         done(stream, er);
       });
@@ -154,7 +154,7 @@ Transform.prototype._write = function(chunk, encoding, cb) {
 Transform.prototype._read = function(n) {
   var ts = this._transformState;
 
-  if (!util.isNull(ts.writechunk) && ts.writecb && !ts.transforming) {
+  if (ts.writechunk !== null && ts.writecb && !ts.transforming) {
     ts.transforming = true;
     this._transform(ts.writechunk, ts.writeencoding, ts.afterTransform);
   } else {
diff --git a/lib/_stream_writable.js b/lib/_stream_writable.js
index a3c65f4595..0176f4095f 100644
--- a/lib/_stream_writable.js
+++ b/lib/_stream_writable.js
@@ -162,9 +162,11 @@ function writeAfterEnd(stream, state, cb) {
 // how many bytes or characters.
 function validChunk(stream, state, chunk, cb) {
   var valid = true;
-  if (!util.isBuffer(chunk) &&
-      !util.isString(chunk) &&
-      !util.isNullOrUndefined(chunk) &&
+
+  if (!(chunk instanceof Buffer) &&
+      typeof chunk !== 'string' &&
+      chunk !== null &&
+      chunk !== undefined &&
       !state.objectMode) {
     var er = new TypeError('Invalid non-string/buffer chunk');
     stream.emit('error', er);
@@ -180,17 +182,17 @@ Writable.prototype.write = function(chunk, encoding, cb) {
   var state = this._writableState;
   var ret = false;
 
-  if (util.isFunction(encoding)) {
+  if (typeof encoding === 'function') {
     cb = encoding;
     encoding = null;
   }
 
-  if (util.isBuffer(chunk))
+  if (chunk instanceof Buffer)
     encoding = 'buffer';
   else if (!encoding)
     encoding = state.defaultEncoding;
 
-  if (!util.isFunction(cb))
+  if (typeof cb !== 'function')
     cb = nop;
 
   if (state.ended)
@@ -236,7 +238,7 @@ Writable.prototype.setDefaultEncoding = function setDefaultEncoding(encoding) {
 function decodeChunk(state, chunk, encoding) {
   if (!state.objectMode &&
       state.decodeStrings !== false &&
-      util.isString(chunk)) {
+      typeof chunk === 'string') {
     chunk = new Buffer(chunk, encoding);
   }
   return chunk;
@@ -247,7 +249,8 @@ function decodeChunk(state, chunk, encoding) {
 // If we return false, then we need a drain event, so set that flag.
 function writeOrBuffer(stream, state, chunk, encoding, cb) {
   chunk = decodeChunk(state, chunk, encoding);
-  if (util.isBuffer(chunk))
+
+  if (chunk instanceof Buffer)
     encoding = 'buffer';
   var len = state.objectMode ? 1 : chunk.length;
 
@@ -418,16 +421,16 @@ Writable.prototype._writev = null;
 Writable.prototype.end = function(chunk, encoding, cb) {
   var state = this._writableState;
 
-  if (util.isFunction(chunk)) {
+  if (typeof chunk === 'function') {
     cb = chunk;
     chunk = null;
     encoding = null;
-  } else if (util.isFunction(encoding)) {
+  } else if (typeof encoding === 'function') {
     cb = encoding;
     encoding = null;
   }
 
-  if (!util.isNullOrUndefined(chunk))
+  if (chunk !== null && chunk !== undefined)
     this.write(chunk, encoding);
 
   // .end() fully uncorks
diff --git a/lib/_tls_common.js b/lib/_tls_common.js
index 3db0d6969f..2c15d91df8 100644
--- a/lib/_tls_common.js
+++ b/lib/_tls_common.js
@@ -1,6 +1,5 @@
 'use strict';
 
-const util = require('util');
 const constants = require('constants');
 const tls = require('tls');
 
@@ -47,7 +46,7 @@ exports.createSecureContext = function createSecureContext(options, context) {
   // NOTE: It's important to add CA before the cert to be able to load
   // cert's issuer in C++ code.
   if (options.ca) {
-    if (util.isArray(options.ca)) {
+    if (Array.isArray(options.ca)) {
       for (var i = 0, len = options.ca.length; i < len; i++) {
         c.context.addCACert(options.ca[i]);
       }
@@ -95,7 +94,7 @@ exports.createSecureContext = function createSecureContext(options, context) {
   else
     c.context.setCiphers(tls.DEFAULT_CIPHERS);
 
-  if (util.isUndefined(options.ecdhCurve))
+  if (options.ecdhCurve === undefined)
     c.context.setECDHCurve(tls.DEFAULT_ECDH_CURVE);
   else if (options.ecdhCurve)
     c.context.setECDHCurve(options.ecdhCurve);
@@ -103,7 +102,7 @@ exports.createSecureContext = function createSecureContext(options, context) {
   if (options.dhparam) c.context.setDHParam(options.dhparam);
 
   if (options.crl) {
-    if (util.isArray(options.crl)) {
+    if (Array.isArray(options.crl)) {
       for (var i = 0, len = options.crl.length; i < len; i++) {
         c.context.addCRL(options.crl[i]);
       }
diff --git a/lib/_tls_legacy.js b/lib/_tls_legacy.js
index f5be838d05..4148085503 100644
--- a/lib/_tls_legacy.js
+++ b/lib/_tls_legacy.js
@@ -36,7 +36,7 @@ SlabBuffer.prototype.use = function use(context, fn, size) {
 
   var actualSize = this.remaining;
 
-  if (!util.isNull(size)) actualSize = Math.min(size, actualSize);
+  if (size !== null) actualSize = Math.min(size, actualSize);
 
   var bytes = fn.call(context, this.pool, this.offset, actualSize);
   if (bytes > 0) {
@@ -72,7 +72,7 @@ function CryptoStream(pair, options) {
   this._finished = false;
   this._opposite = null;
 
-  if (util.isNull(slabBuffer)) slabBuffer = new SlabBuffer();
+  if (slabBuffer === null) slabBuffer = new SlabBuffer();
   this._buffer = slabBuffer;
 
   this.once('finish', onCryptoStreamFinish);
@@ -142,7 +142,7 @@ CryptoStream.prototype.init = function init() {
 
 
 CryptoStream.prototype._write = function write(data, encoding, cb) {
-  assert(util.isNull(this._pending));
+  assert(this._pending === null);
 
   // Black-hole data
   if (!this.pair.ssl) return cb(null);
@@ -189,7 +189,7 @@ CryptoStream.prototype._write = function write(data, encoding, cb) {
 
       // Invoke callback only when all data read from opposite stream
       if (this._opposite._halfRead) {
-        assert(util.isNull(this._sslOutCb));
+        assert(this._sslOutCb === null);
         this._sslOutCb = cb;
       } else {
         cb(null);
@@ -288,8 +288,8 @@ CryptoStream.prototype._read = function read(size) {
   }
 
   // Try writing pending data
-  if (!util.isNull(this._pending)) this._writePending();
-  if (!util.isNull(this._opposite._pending)) this._opposite._writePending();
+  if (this._pending !== null) this._writePending();
+  if (this._opposite._pending !== null) this._opposite._writePending();
 
   if (bytesRead === 0) {
     // EOF when cleartext has finished and we have nothing to read
@@ -398,7 +398,7 @@ CryptoStream.prototype.end = function(chunk, encoding) {
   }
 
   // Write pending data first
-  if (!util.isNull(this._pending)) this._writePending();
+  if (this._pending !== null) this._writePending();
 
   this.writable = false;
 
diff --git a/lib/_tls_wrap.js b/lib/_tls_wrap.js
index 6ce7c9f6ba..c6bf31b980 100644
--- a/lib/_tls_wrap.js
+++ b/lib/_tls_wrap.js
@@ -447,7 +447,7 @@ TLSSocket.prototype.setServername = function(name) {
 };
 
 TLSSocket.prototype.setSession = function(session) {
-  if (util.isString(session))
+  if (typeof session === 'string')
     session = new Buffer(session, 'binary');
   this.ssl.setSession(session);
 };
@@ -554,10 +554,11 @@ TLSSocket.prototype.getCipher = function(err) {
 //
 function Server(/* [options], listener */) {
   var options, listener;
-  if (util.isObject(arguments[0])) {
+
+  if (arguments[0] !== null && typeof arguments[0] === 'object') {
     options = arguments[0];
     listener = arguments[1];
-  } else if (util.isFunction(arguments[0])) {
+  } else if (typeof arguments[0] === 'function') {
     options = {};
     listener = arguments[0];
   }
@@ -590,7 +591,7 @@ function Server(/* [options], listener */) {
 
   var timeout = options.handshakeTimeout || (120 * 1000);
 
-  if (!util.isNumber(timeout)) {
+  if (typeof timeout !== 'number') {
     throw new TypeError('handshakeTimeout must be a number');
   }
 
@@ -676,13 +677,13 @@ Server.prototype._setServerData = function(data) {
 
 
 Server.prototype.setOptions = function(options) {
-  if (util.isBoolean(options.requestCert)) {
+  if (typeof options.requestCert === 'boolean') {
     this.requestCert = options.requestCert;
   } else {
     this.requestCert = false;
   }
 
-  if (util.isBoolean(options.rejectUnauthorized)) {
+  if (typeof options.rejectUnauthorized === 'boolean') {
     this.rejectUnauthorized = options.rejectUnauthorized;
   } else {
     this.rejectUnauthorized = false;
@@ -696,7 +697,7 @@ Server.prototype.setOptions = function(options) {
   if (options.secureProtocol) this.secureProtocol = options.secureProtocol;
   if (options.crl) this.crl = options.crl;
   if (options.ciphers) this.ciphers = options.ciphers;
-  if (!util.isUndefined(options.ecdhCurve))
+  if (options.ecdhCurve !== undefined)
     this.ecdhCurve = options.ecdhCurve;
   if (options.dhparam) this.dhparam = options.dhparam;
   if (options.sessionTimeout) this.sessionTimeout = options.sessionTimeout;
@@ -734,7 +735,7 @@ function SNICallback(servername, callback) {
   var ctx;
 
   this.server._contexts.some(function(elem) {
-    if (!util.isNull(servername.match(elem[0]))) {
+    if (servername.match(elem[0]) !== null) {
       ctx = elem[1];
       return true;
     }
@@ -763,9 +764,9 @@ function normalizeConnectArgs(listArgs) {
   var options = args[0];
   var cb = args[1];
 
-  if (util.isObject(listArgs[1])) {
+  if (listArgs[1] !== null && typeof listArgs[1] === 'object') {
     options = util._extend(options, listArgs[1]);
-  } else if (util.isObject(listArgs[2])) {
+  } else if (listArgs[2] !== null && typeof listArgs[2] === 'object') {
     options = util._extend(options, listArgs[2]);
   }
 
diff --git a/lib/assert.js b/lib/assert.js
index a13cc50316..5cd75b220a 100644
--- a/lib/assert.js
+++ b/lib/assert.js
@@ -59,7 +59,7 @@ assert.AssertionError = function AssertionError(options) {
 util.inherits(assert.AssertionError, Error);
 
 function truncate(s, n) {
-  if (util.isString(s)) {
+  if (typeof s === 'string') {
     return s.length < n ? s : s.slice(0, n);
   } else {
     return s;
@@ -138,8 +138,7 @@ function _deepEqual(actual, expected) {
   // 7.1. All identical values are equivalent, as determined by ===.
   if (actual === expected) {
     return true;
-
-  } else if (util.isBuffer(actual) && util.isBuffer(expected)) {
+  } else if (actual instanceof Buffer && expected instanceof Buffer) {
     if (actual.length != expected.length) return false;
 
     for (var i = 0; i < actual.length; i++) {
@@ -165,7 +164,8 @@ function _deepEqual(actual, expected) {
 
   // 7.4. Other pairs that do not both pass typeof value == 'object',
   // equivalence is determined by ==.
-  } else if (!util.isObject(actual) && !util.isObject(expected)) {
+  } else if ((actual === null || typeof actual !== 'object') &&
+             (expected === null || typeof expected !== 'object')) {
     return actual == expected;
 
   // 7.5 For all other Object pairs, including Array objects, equivalence is
@@ -184,7 +184,7 @@ function isArguments(object) {
 }
 
 function objEquiv(a, b) {
-  if (util.isNullOrUndefined(a) || util.isNullOrUndefined(b))
+  if (a === null || a === undefined || b === null || b === undefined)
     return false;
   // an identical 'prototype' property.
   if (a.prototype !== b.prototype) return false;
@@ -270,11 +270,11 @@ function expectedException(actual, expected) {
 function _throws(shouldThrow, block, expected, message) {
   var actual;
 
-  if (!util.isFunction(block)) {
+  if (typeof block !== 'function') {
     throw new TypeError('block must be a function');
   }
 
-  if (util.isString(expected)) {
+  if (typeof expected === 'string') {
     message = expected;
     expected = null;
   }
diff --git a/lib/buffer.js b/lib/buffer.js
index 41de85e673..19208b36ab 100644
--- a/lib/buffer.js
+++ b/lib/buffer.js
@@ -27,20 +27,20 @@ createPool();
 
 
 function Buffer(subject, encoding) {
-  if (!util.isBuffer(this))
+  if (!(this instanceof Buffer))
     return new Buffer(subject, encoding);
 
-  if (util.isNumber(subject)) {
+  if (typeof subject === 'number') {
     this.length = +subject;
 
-  } else if (util.isString(subject)) {
-    if (!util.isString(encoding) || encoding.length === 0)
+  } else if (typeof subject === 'string') {
+    if (typeof encoding !== 'string' || encoding.length === 0)
       encoding = 'utf8';
     this.length = Buffer.byteLength(subject, encoding);
 
   // Handle Arrays, Buffers, Uint8Arrays or JSON.
-  } else if (util.isObject(subject)) {
-    if (subject.type === 'Buffer' && util.isArray(subject.data))
+  } else if (subject !== null && typeof subject === 'object') {
+    if (subject.type === 'Buffer' && Array.isArray(subject.data))
       subject = subject.data;
     this.length = +subject.length;
 
@@ -71,11 +71,11 @@ function Buffer(subject, encoding) {
     alloc(this, this.length);
   }
 
-  if (util.isNumber(subject)) {
+  if (typeof subject === 'number') {
     return;
   }
 
-  if (util.isString(subject)) {
+  if (typeof subject === 'string') {
     // In the case of base64 it's possible that the size of the buffer
     // allocated was slightly too large. In this case we need to rewrite
     // the length to the actual length written.
@@ -88,10 +88,10 @@ function Buffer(subject, encoding) {
       poolOffset -= (prevLen - len);
     }
 
-  } else if (util.isBuffer(subject)) {
+  } else if (subject instanceof Buffer) {
     subject.copy(this, 0, 0, this.length);
 
-  } else if (util.isNumber(subject.length) || util.isArray(subject)) {
+  } else if (typeof subject.length === 'number' || Array.isArray(subject)) {
     // Really crappy way to handle Uint8Arrays, but V8 doesn't give a simple
     // way to access the data from the C++ API.
     for (var i = 0; i < this.length; i++)
@@ -130,7 +130,7 @@ buffer.setupBufferJS(NativeBuffer, internal);
 // Static methods
 
 Buffer.isBuffer = function isBuffer(b) {
-  return util.isBuffer(b);
+  return b instanceof Buffer;
 };
 
 
@@ -165,10 +165,10 @@ Buffer.isEncoding = function(encoding) {
 
 
 Buffer.concat = function(list, length) {
-  if (!util.isArray(list))
+  if (!Array.isArray(list))
     throw new TypeError('Usage: Buffer.concat(list[, length])');
 
-  if (util.isUndefined(length)) {
+  if (length === undefined) {
     length = 0;
     for (var i = 0; i < list.length; i++)
       length += list[i].length;
@@ -223,7 +223,7 @@ Buffer.prototype.toString = function(encoding, start, end) {
   var loweredCase = false;
 
   start = start >>> 0;
-  end = util.isUndefined(end) || end === Infinity ? this.length : end >>> 0;
+  end = end === undefined || end === Infinity ? this.length : end >>> 0;
 
   if (!encoding) encoding = 'utf8';
   if (start < 0) start = 0;
@@ -341,13 +341,13 @@ const writeMsg = '.write(string, encoding, offset, length) is deprecated.' +
                  ' Use write(string[, offset[, length]][, encoding]) instead.';
 Buffer.prototype.write = function(string, offset, length, encoding) {
   // Buffer#write(string);
-  if (util.isUndefined(offset)) {
+  if (offset === undefined) {
     encoding = 'utf8';
     length = this.length;
     offset = 0;
 
   // Buffer#write(string, encoding)
-  } else if (util.isUndefined(length) && util.isString(offset)) {
+  } else if (length === undefined && typeof offset === 'string') {
     encoding = offset;
     length = this.length;
     offset = 0;
@@ -357,7 +357,7 @@ Buffer.prototype.write = function(string, offset, length, encoding) {
     offset = offset >>> 0;
     if (isFinite(length)) {
       length = length >>> 0;
-      if (util.isUndefined(encoding))
+      if (encoding === undefined)
         encoding = 'utf8';
     } else {
       encoding = length;
@@ -383,7 +383,7 @@ Buffer.prototype.write = function(string, offset, length, encoding) {
   }
 
   var remaining = this.length - offset;
-  if (util.isUndefined(length) || length > remaining)
+  if (length === undefined || length > remaining)
     length = remaining;
 
   encoding = !!encoding ? (encoding + '').toLowerCase() : 'utf8';
@@ -443,7 +443,7 @@ Buffer.prototype.toJSON = function() {
 Buffer.prototype.slice = function(start, end) {
   var len = this.length;
   start = ~~start;
-  end = util.isUndefined(end) ? len : ~~end;
+  end = end === undefined ? len : ~~end;
 
   if (start < 0) {
     start += len;
@@ -468,7 +468,7 @@ Buffer.prototype.slice = function(start, end) {
   sliceOnto(this, buf, start, end);
   buf.length = end - start;
   if (buf.length > 0)
-    buf.parent = util.isUndefined(this.parent) ? this : this.parent;
+    buf.parent = this.parent === undefined ? this : this.parent;
 
   return buf;
 };
diff --git a/lib/child_process.js b/lib/child_process.js
index 9226d127b3..a7587d0ac5 100644
--- a/lib/child_process.js
+++ b/lib/child_process.js
@@ -22,7 +22,7 @@ function handleWrapGetter(name, callback) {
 
   Object.defineProperty(handleWraps, name, {
     get: function() {
-      if (!util.isUndefined(cons)) return cons;
+      if (cons !== undefined) return cons;
       return cons = callback();
     }
   });
@@ -295,9 +295,9 @@ function getSocketList(type, slave, key) {
 const INTERNAL_PREFIX = 'NODE_';
 function handleMessage(target, message, handle) {
   var eventName = 'message';
-  if (!util.isNull(message) &&
-      util.isObject(message) &&
-      util.isString(message.cmd) &&
+  if (message !== null &&
+      typeof message === 'object' &&
+      typeof message.cmd === 'string' &&
       message.cmd.length > INTERNAL_PREFIX.length &&
       message.cmd.slice(0, INTERNAL_PREFIX.length) === INTERNAL_PREFIX) {
     eventName = 'internalMessage';
@@ -353,7 +353,7 @@ function setupChannel(target, channel) {
   target.on('internalMessage', function(message, handle) {
     // Once acknowledged - continue sending handles.
     if (message.cmd === 'NODE_HANDLE_ACK') {
-      assert(util.isArray(target._handleQueue));
+      assert(Array.isArray(target._handleQueue));
       var queue = target._handleQueue;
       target._handleQueue = null;
 
@@ -400,7 +400,7 @@ function setupChannel(target, channel) {
   target._send = function(message, handle, swallowErrors) {
     assert(this.connected || this._channel);
 
-    if (util.isUndefined(message))
+    if (message === undefined)
       throw new TypeError('message cannot be undefined');
 
     // package messages with a handle object
@@ -538,7 +538,7 @@ exports.fork = function(modulePath /*, args, options*/) {
 
   // Get options and args arguments.
   var options, args, execArgv;
-  if (util.isArray(arguments[1])) {
+  if (Array.isArray(arguments[1])) {
     args = arguments[1];
     options = util._extend({}, arguments[2]);
   } else {
@@ -583,7 +583,7 @@ exports._forkChild = function(fd) {
 function normalizeExecArgs(command /*, options, callback */) {
   var file, args, options, callback;
 
-  if (util.isFunction(arguments[1])) {
+  if (typeof arguments[1] === 'function') {
     options = undefined;
     callback = arguments[1];
   } else {
@@ -638,11 +638,11 @@ exports.execFile = function(file /* args, options, callback */) {
 
   // Parse the parameters.
 
-  if (util.isFunction(arguments[arguments.length - 1])) {
+  if (typeof arguments[arguments.length - 1] === 'function') {
     callback = arguments[arguments.length - 1];
   }
 
-  if (util.isArray(arguments[1])) {
+  if (Array.isArray(arguments[1])) {
     args = arguments[1];
     options = util._extend(options, arguments[2]);
   } else {
@@ -806,14 +806,14 @@ function _validateStdio(stdio, sync) {
       ipcFd;
 
   // Replace shortcut with an array
-  if (util.isString(stdio)) {
+  if (typeof stdio === 'string') {
     switch (stdio) {
       case 'ignore': stdio = ['ignore', 'ignore', 'ignore']; break;
       case 'pipe': stdio = ['pipe', 'pipe', 'pipe']; break;
       case 'inherit': stdio = [0, 1, 2]; break;
       default: throw new TypeError('Incorrect value of stdio option: ' + stdio);
     }
-  } else if (!util.isArray(stdio)) {
+  } else if (!Array.isArray(stdio)) {
     throw new TypeError('Incorrect value of stdio option: ' +
         util.inspect(stdio));
   }
@@ -837,13 +837,13 @@ function _validateStdio(stdio, sync) {
     }
 
     // Defaults
-    if (util.isNullOrUndefined(stdio)) {
+    if (stdio === null || stdio === undefined) {
       stdio = i < 3 ? 'pipe' : 'ignore';
     }
 
     if (stdio === null || stdio === 'ignore') {
       acc.push({type: 'ignore'});
-    } else if (stdio === 'pipe' || util.isNumber(stdio) && stdio < 0) {
+    } else if (stdio === 'pipe' || typeof stdio === 'number' && stdio < 0) {
       var a = {
         type: 'pipe',
         readable: i === 0,
@@ -855,7 +855,7 @@ function _validateStdio(stdio, sync) {
 
       acc.push(a);
     } else if (stdio === 'ipc') {
-      if (sync || !util.isUndefined(ipc)) {
+      if (sync || ipc !== undefined) {
         // Cleanup previously created pipes
         cleanup();
         if (!sync)
@@ -877,7 +877,7 @@ function _validateStdio(stdio, sync) {
         type: 'inherit',
         fd: i
       });
-    } else if (util.isNumber(stdio) || util.isNumber(stdio.fd)) {
+    } else if (typeof stdio === 'number' || typeof stdio.fd === 'number') {
       acc.push({
         type: 'fd',
         fd: stdio.fd || stdio
@@ -893,7 +893,7 @@ function _validateStdio(stdio, sync) {
         wrapType: getHandleWrapType(handle),
         handle: handle
       });
-    } else if (util.isBuffer(stdio) || util.isString(stdio)) {
+    } else if (stdio instanceof Buffer || typeof stdio === 'string') {
       if (!sync) {
         cleanup();
         throw new TypeError('Asynchronous forks do not support Buffer input: ' +
@@ -919,7 +919,8 @@ function normalizeSpawnArguments(file /*, args, options*/) {
   if (Array.isArray(arguments[1])) {
     args = arguments[1].slice(0);
     options = arguments[2];
-  } else if (arguments[1] !== undefined && !util.isObject(arguments[1])) {
+  } else if (arguments[1] !== undefined &&
+             (arguments[1] === null || typeof arguments[1] !== 'object')) {
     throw new TypeError('Incorrect value of args option');
   } else {
     args = [];
@@ -928,7 +929,7 @@ function normalizeSpawnArguments(file /*, args, options*/) {
 
   if (options === undefined)
     options = {};
-  else if (!util.isObject(options))
+  else if (options === null || typeof options !== 'object')
     throw new TypeError('options argument must be an object');
 
   options = util._extend({}, options);
@@ -1089,7 +1090,7 @@ ChildProcess.prototype.spawn = function(options) {
   ipcFd = stdio.ipcFd;
   stdio = options.stdio = stdio.stdio;
 
-  if (!util.isUndefined(ipc)) {
+  if (ipc !== undefined) {
     // Let child process know about opened IPC channel
     options.envPairs = options.envPairs || [];
     options.envPairs.push('NODE_CHANNEL_FD=' + ipcFd);
@@ -1150,19 +1151,19 @@ ChildProcess.prototype.spawn = function(options) {
     }
   });
 
-  this.stdin = stdio.length >= 1 && !util.isUndefined(stdio[0].socket) ?
+  this.stdin = stdio.length >= 1 && stdio[0].socket !== undefined ?
       stdio[0].socket : null;
-  this.stdout = stdio.length >= 2 && !util.isUndefined(stdio[1].socket) ?
+  this.stdout = stdio.length >= 2 && stdio[1].socket !== undefined ?
       stdio[1].socket : null;
-  this.stderr = stdio.length >= 3 && !util.isUndefined(stdio[2].socket) ?
+  this.stderr = stdio.length >= 3 && stdio[2].socket !== undefined ?
       stdio[2].socket : null;
 
   this.stdio = stdio.map(function(stdio) {
-    return util.isUndefined(stdio.socket) ? null : stdio.socket;
+    return stdio.socket === undefined ? null : stdio.socket;
   });
 
   // Add .send() method and start listening for IPC data
-  if (!util.isUndefined(ipc)) setupChannel(this, ipc);
+  if (ipc !== undefined) setupChannel(this, ipc);
 
   return err;
 };
@@ -1183,7 +1184,7 @@ ChildProcess.prototype.kill = function(sig) {
     signal = constants[sig];
   }
 
-  if (util.isUndefined(signal)) {
+  if (signal === undefined) {
     throw new Error('Unknown signal: ' + sig);
   }
 
@@ -1262,7 +1263,7 @@ function spawnSync(/*file, args, options*/) {
       var pipe = options.stdio[i] = util._extend({}, options.stdio[i]);
       if (Buffer.isBuffer(input))
         pipe.input = input;
-      else if (util.isString(input))
+      else if (typeof input === 'string')
         pipe.input = new Buffer(input, options.encoding);
       else
         throw new TypeError(util.format(
diff --git a/lib/cluster.js b/lib/cluster.js
index 9312fc28f2..14d0bc69b3 100644
--- a/lib/cluster.js
+++ b/lib/cluster.js
@@ -22,7 +22,7 @@ function Worker(options) {
 
   EventEmitter.call(this);
 
-  if (!util.isObject(options))
+  if (options === null || typeof options !== 'object')
     options = {};
 
   this.suicide = undefined;
@@ -68,7 +68,7 @@ function SharedHandle(key, address, port, addressType, backlog, fd) {
   else
     rval = net._createServerHandle(address, port, addressType, fd);
 
-  if (util.isNumber(rval))
+  if (typeof rval === 'number')
     this.errno = rval;
   else
     this.handle = rval;
@@ -135,7 +135,7 @@ RoundRobinHandle.prototype.add = function(worker, send) {
     self.handoff(worker);  // In case there are connections pending.
   }
 
-  if (util.isNull(this.server)) return done();
+  if (this.server === null) return done();
   // Still busy binding.
   this.server.once('listening', done);
   this.server.once('error', function(err) {
@@ -170,7 +170,7 @@ RoundRobinHandle.prototype.handoff = function(worker) {
     return;  // Worker is closing (or has closed) the server.
   }
   var handle = this.handles.shift();
-  if (util.isUndefined(handle)) {
+  if (handle === undefined) {
     this.free.push(worker);  // Add to ready queue again.
     return;
   }
@@ -203,7 +203,7 @@ function masterInit() {
     'rr': SCHED_RR
   }[process.env.NODE_CLUSTER_SCHED_POLICY];
 
-  if (util.isUndefined(schedulingPolicy)) {
+  if (schedulingPolicy === undefined) {
     // FIXME Round-robin doesn't perform well on Windows right now due to the
     // way IOCP is wired up. Bert is going to fix that, eventually.
     schedulingPolicy = (process.platform === 'win32') ? SCHED_NONE : SCHED_RR;
@@ -438,7 +438,7 @@ function masterInit() {
                 message.fd];
     var key = args.join(':');
     var handle = handles[key];
-    if (util.isUndefined(handle)) {
+    if (handle === undefined) {
       var constructor = RoundRobinHandle;
       // UDP is exempt from round-robin connection balancing for what should
       // be obvious reasons: it's connectionless. There is nothing to send to
@@ -562,7 +562,7 @@ function workerInit() {
       delete handles[key];
       return close.apply(this, arguments);
     };
-    assert(util.isUndefined(handles[key]));
+    assert(handles[key] === undefined);
     handles[key] = handle;
     cb(message.errno, handle);
   }
@@ -586,7 +586,7 @@ function workerInit() {
       // the ack by the master process in which we can still receive handles.
       // onconnection() below handles that by sending those handles back to
       // the master.
-      if (util.isUndefined(key)) return;
+      if (key === undefined) return;
       send({ act: 'close', key: key });
       delete handles[key];
       key = undefined;
@@ -607,7 +607,7 @@ function workerInit() {
     if (message.sockname) {
       handle.getsockname = getsockname;  // TCP handles only.
     }
-    assert(util.isUndefined(handles[key]));
+    assert(handles[key] === undefined);
     handles[key] = handle;
     cb(0, handle);
   }
@@ -616,7 +616,7 @@ function workerInit() {
   function onconnection(message, handle) {
     var key = message.key;
     var server = handles[key];
-    var accepted = !util.isUndefined(server);
+    var accepted = server !== undefined;
     send({ ack: message.seq, accepted: accepted });
     if (accepted) server.onconnection(0, handle);
   }
@@ -664,7 +664,7 @@ function internal(worker, cb) {
   return function(message, handle) {
     if (message.cmd !== 'NODE_CLUSTER') return;
     var fn = cb;
-    if (!util.isUndefined(message.ack)) {
+    if (message.ack !== undefined) {
       fn = callbacks[message.ack];
       delete callbacks[message.ack];
     }
diff --git a/lib/console.js b/lib/console.js
index 323d23374a..63e3c5e0bf 100644
--- a/lib/console.js
+++ b/lib/console.js
@@ -6,7 +6,7 @@ function Console(stdout, stderr) {
   if (!(this instanceof Console)) {
     return new Console(stdout, stderr);
   }
-  if (!stdout || !util.isFunction(stdout.write)) {
+  if (!stdout || typeof stdout.write !== 'function') {
     throw new TypeError('Console expects a writable stream instance');
   }
   if (!stderr) {
diff --git a/lib/crypto.js b/lib/crypto.js
index f8efffda48..42564c79e4 100644
--- a/lib/crypto.js
+++ b/lib/crypto.js
@@ -25,7 +25,7 @@ const DH_GENERATOR = 2;
 // to break them unnecessarily.
 function toBuf(str, encoding) {
   encoding = encoding || 'binary';
-  if (util.isString(str)) {
+  if (typeof str === 'string') {
     if (encoding === 'buffer')
       encoding = 'binary';
     str = new Buffer(str, encoding);
@@ -92,7 +92,7 @@ Hash.prototype._flush = function(callback) {
 
 Hash.prototype.update = function(data, encoding) {
   encoding = encoding || exports.DEFAULT_ENCODING;
-  if (encoding === 'buffer' && util.isString(data))
+  if (encoding === 'buffer' && typeof data === 'string')
     encoding = 'binary';
   this._handle.update(data, encoding);
   return this;
@@ -369,7 +369,7 @@ function DiffieHellman(sizeOrKey, keyEncoding, generator, genEncoding) {
   if (!(this instanceof DiffieHellman))
     return new DiffieHellman(sizeOrKey, keyEncoding, generator, genEncoding);
 
-  if (!util.isBuffer(sizeOrKey) &&
+  if (!(sizeOrKey instanceof Buffer) &&
       typeof sizeOrKey !== 'number' &&
       typeof sizeOrKey !== 'string')
     throw new TypeError('First argument should be number, string or Buffer');
@@ -513,7 +513,7 @@ DiffieHellman.prototype.setPrivateKey = function(key, encoding) {
 
 
 function ECDH(curve) {
-  if (!util.isString(curve))
+  if (typeof curve !== 'string')
     throw new TypeError('curve should be a string');
 
   this._handle = new binding.ECDH(curve);
@@ -566,12 +566,12 @@ exports.pbkdf2 = function(password,
                           keylen,
                           digest,
                           callback) {
-  if (util.isFunction(digest)) {
+  if (typeof digest === 'function') {
     callback = digest;
     digest = undefined;
   }
 
-  if (!util.isFunction(callback))
+  if (typeof callback !== 'function')
     throw new Error('No callback provided to pbkdf2');
 
   return pbkdf2(password, salt, iterations, keylen, digest, callback);
@@ -632,10 +632,10 @@ Certificate.prototype.exportChallenge = function(object, encoding) {
 
 
 exports.setEngine = function setEngine(id, flags) {
-  if (!util.isString(id))
+  if (typeof id !== 'string')
     throw new TypeError('id should be a string');
 
-  if (flags && !util.isNumber(flags))
+  if (flags && typeof flags !== 'number')
     throw new TypeError('flags should be a number, if present');
   flags = flags >>> 0;
 
diff --git a/lib/dgram.js b/lib/dgram.js
index 19cf1c4ea0..a919e16497 100644
--- a/lib/dgram.js
+++ b/lib/dgram.js
@@ -61,7 +61,7 @@ function newHandle(type) {
 
 exports._createSocketHandle = function(address, port, addressType, fd) {
   // Opening an existing fd is not supported for UDP handles.
-  assert(!util.isNumber(fd) || fd < 0);
+  assert(typeof fd !== 'number' || fd < 0);
 
   var handle = newHandle(addressType);
 
@@ -97,7 +97,7 @@ function Socket(type, listener) {
   // If true - UV_UDP_REUSEADDR flag will be set
   this._reuseAddr = options && options.reuseAddr;
 
-  if (util.isFunction(listener))
+  if (typeof listener === 'function')
     this.on('message', listener);
 }
 util.inherits(Socket, events.EventEmitter);
@@ -143,7 +143,7 @@ Socket.prototype.bind = function(port /*, address, callback*/) {
 
   this._bindState = BIND_STATE_BINDING;
 
-  if (util.isFunction(arguments[arguments.length - 1]))
+  if (typeof arguments[arguments.length - 1] === 'function')
     self.once('listening', arguments[arguments.length - 1]);
 
   const UDP = process.binding('udp_wrap').UDP;
@@ -156,12 +156,12 @@ Socket.prototype.bind = function(port /*, address, callback*/) {
   var address;
   var exclusive;
 
-  if (util.isObject(port)) {
+  if (port !== null && typeof port === 'object') {
     address = port.address || '';
     exclusive = !!port.exclusive;
     port = port.port;
   } else {
-    address = util.isFunction(arguments[1]) ? '' : arguments[1];
+    address = typeof arguments[1] === 'function' ? '' : arguments[1];
     exclusive = false;
   }
 
@@ -225,10 +225,10 @@ Socket.prototype.sendto = function(buffer,
                                    port,
                                    address,
                                    callback) {
-  if (!util.isNumber(offset) || !util.isNumber(length))
+  if (typeof offset !== 'number' || typeof length !== 'number')
     throw new Error('send takes offset and length as args 2 and 3');
 
-  if (!util.isString(address))
+  if (typeof address !== 'string')
     throw new Error(this.type + ' sockets must send to port, address');
 
   this.send(buffer, offset, length, port, address, callback);
@@ -243,10 +243,10 @@ Socket.prototype.send = function(buffer,
                                  callback) {
   var self = this;
 
-  if (util.isString(buffer))
+  if (typeof buffer === 'string')
     buffer = new Buffer(buffer);
 
-  if (!util.isBuffer(buffer))
+  if (!(buffer instanceof Buffer))
     throw new TypeError('First argument must be a buffer or string.');
 
   offset = offset | 0;
@@ -272,7 +272,7 @@ Socket.prototype.send = function(buffer,
 
   // Normalize callback so it's either a function or undefined but not anything
   // else.
-  if (!util.isFunction(callback))
+  if (typeof callback !== 'function')
     callback = undefined;
 
   self._healthCheck();
@@ -378,7 +378,7 @@ Socket.prototype.setBroadcast = function(arg) {
 
 
 Socket.prototype.setTTL = function(arg) {
-  if (!util.isNumber(arg)) {
+  if (typeof arg !== 'number') {
     throw new TypeError('Argument must be a number');
   }
 
@@ -392,7 +392,7 @@ Socket.prototype.setTTL = function(arg) {
 
 
 Socket.prototype.setMulticastTTL = function(arg) {
-  if (!util.isNumber(arg)) {
+  if (typeof arg !== 'number') {
     throw new TypeError('Argument must be a number');
   }
 
diff --git a/lib/dns.js b/lib/dns.js
index 8cfa7f58c5..e084fb5e5d 100644
--- a/lib/dns.js
+++ b/lib/dns.js
@@ -53,7 +53,7 @@ function errnoException(err, syscall, hostname) {
 //   callback.immediately = true;
 // }
 function makeAsync(callback) {
-  if (!util.isFunction(callback)) {
+  if (typeof callback !== 'function') {
     return callback;
   }
   return function asyncCallback() {
@@ -96,7 +96,7 @@ exports.lookup = function lookup(hostname, options, callback) {
     family = 0;
   } else if (typeof callback !== 'function') {
     throw TypeError('invalid arguments: callback must be passed');
-  } else if (util.isObject(options)) {
+  } else if (options !== null && typeof options === 'object') {
     hints = options.hints >>> 0;
     family = options.family >>> 0;
 
@@ -192,9 +192,9 @@ function resolver(bindingName) {
   var binding = cares[bindingName];
 
   return function query(name, callback) {
-    if (!util.isString(name)) {
+    if (typeof name !== 'string') {
       throw new Error('Name must be a string');
-    } else if (!util.isFunction(callback)) {
+    } else if (typeof callback !== 'function') {
       throw new Error('Callback must be a function');
     }
 
@@ -228,17 +228,17 @@ exports.reverse = resolveMap.PTR = resolver('getHostByAddr');
 
 exports.resolve = function(hostname, type_, callback_) {
   var resolver, callback;
-  if (util.isString(type_)) {
+  if (typeof type_ === 'string') {
     resolver = resolveMap[type_];
     callback = callback_;
-  } else if (util.isFunction(type_)) {
+  } else if (typeof type_ === 'function') {
     resolver = exports.resolve4;
     callback = type_;
   } else {
     throw new Error('Type must be a string');
   }
 
-  if (util.isFunction(resolver)) {
+  if (typeof resolver === 'function') {
     return resolver(hostname, callback);
   } else {
     throw new Error('Unknown type "' + type_ + '"');
diff --git a/lib/events.js b/lib/events.js
index 672131deed..cece093d38 100644
--- a/lib/events.js
+++ b/lib/events.js
@@ -1,7 +1,6 @@
 'use strict';
 
 var domain;
-const util = require('util');
 
 function EventEmitter() {
   EventEmitter.init.call(this);
@@ -40,14 +39,14 @@ EventEmitter.init = function() {
 // Obviously not all Emitters should be limited to 10. This function allows
 // that to be increased. Set to zero for unlimited.
 EventEmitter.prototype.setMaxListeners = function setMaxListeners(n) {
-  if (!util.isNumber(n) || n < 0 || isNaN(n))
+  if (typeof n !== 'number' || n < 0 || isNaN(n))
     throw TypeError('n must be a positive number');
   this._maxListeners = n;
   return this;
 };
 
 function $getMaxListeners(that) {
-  if (util.isUndefined(that._maxListeners))
+  if (that._maxListeners === undefined)
     return EventEmitter.defaultMaxListeners;
   return that._maxListeners;
 }
@@ -82,13 +81,13 @@ EventEmitter.prototype.emit = function emit(type) {
 
   handler = this._events[type];
 
-  if (util.isUndefined(handler))
+  if (handler === undefined)
     return false;
 
   if (this.domain && this !== process)
     this.domain.enter();
 
-  if (util.isFunction(handler)) {
+  if (typeof handler === 'function') {
     switch (arguments.length) {
       // fast cases
       case 1:
@@ -108,7 +107,7 @@ EventEmitter.prototype.emit = function emit(type) {
           args[i - 1] = arguments[i];
         handler.apply(this, args);
     }
-  } else if (util.isObject(handler)) {
+  } else if (handler !== null && typeof handler === 'object') {
     len = arguments.length;
     args = new Array(len - 1);
     for (i = 1; i < len; i++)
@@ -129,7 +128,7 @@ EventEmitter.prototype.emit = function emit(type) {
 EventEmitter.prototype.addListener = function addListener(type, listener) {
   var m;
 
-  if (!util.isFunction(listener))
+  if (typeof listener !== 'function')
     throw TypeError('listener must be a function');
 
   if (!this._events)
@@ -139,13 +138,13 @@ EventEmitter.prototype.addListener = function addListener(type, listener) {
   // adding it to the listeners, first emit "newListener".
   if (this._events.newListener)
     this.emit('newListener', type,
-              util.isFunction(listener.listener) ?
+              typeof listener.listener === 'function' ?
               listener.listener : listener);
 
   if (!this._events[type])
     // Optimize the case of one listener. Don't need the extra array object.
     this._events[type] = listener;
-  else if (util.isObject(this._events[type]))
+  else if (typeof this._events[type] === 'object')
     // If we've already got an array, just append.
     this._events[type].push(listener);
   else
@@ -153,7 +152,8 @@ EventEmitter.prototype.addListener = function addListener(type, listener) {
     this._events[type] = [this._events[type], listener];
 
   // Check for listener leak
-  if (util.isObject(this._events[type]) && !this._events[type].warned) {
+  if (this._events[type] !== null && typeof this._events[type] === 'object' &&
+      !this._events[type].warned) {
     var m = $getMaxListeners(this);
     if (m && m > 0 && this._events[type].length > m) {
       this._events[type].warned = true;
@@ -171,7 +171,7 @@ EventEmitter.prototype.addListener = function addListener(type, listener) {
 EventEmitter.prototype.on = EventEmitter.prototype.addListener;
 
 EventEmitter.prototype.once = function once(type, listener) {
-  if (!util.isFunction(listener))
+  if (typeof listener !== 'function')
     throw TypeError('listener must be a function');
 
   var fired = false;
@@ -196,7 +196,7 @@ EventEmitter.prototype.removeListener =
     function removeListener(type, listener) {
       var list, position, length, i;
 
-      if (!util.isFunction(listener))
+      if (typeof listener !== 'function')
         throw TypeError('listener must be a function');
 
       if (!this._events || !this._events[type])
@@ -207,12 +207,13 @@ EventEmitter.prototype.removeListener =
       position = -1;
 
       if (list === listener ||
-          (util.isFunction(list.listener) && list.listener === listener)) {
+          (typeof list.listener === 'function' &&
+          list.listener === listener)) {
         delete this._events[type];
         if (this._events.removeListener)
           this.emit('removeListener', type, listener);
 
-      } else if (util.isObject(list)) {
+      } else if (list !== null && typeof list === 'object') {
         for (i = length; i-- > 0;) {
           if (list[i] === listener ||
               (list[i].listener && list[i].listener === listener)) {
@@ -267,7 +268,7 @@ EventEmitter.prototype.removeAllListeners =
 
       listeners = this._events[type];
 
-      if (util.isFunction(listeners)) {
+      if (typeof listeners === 'function') {
         this.removeListener(type, listeners);
       } else if (Array.isArray(listeners)) {
         // LIFO order
@@ -283,7 +284,7 @@ EventEmitter.prototype.listeners = function listeners(type) {
   var ret;
   if (!this._events || !this._events[type])
     ret = [];
-  else if (util.isFunction(this._events[type]))
+  else if (typeof this._events[type] === 'function')
     ret = [this._events[type]];
   else
     ret = this._events[type].slice();
@@ -294,7 +295,7 @@ EventEmitter.listenerCount = function(emitter, type) {
   var ret;
   if (!emitter._events || !emitter._events[type])
     ret = 0;
-  else if (util.isFunction(emitter._events[type]))
+  else if (typeof emitter._events[type] === 'function')
     ret = 1;
   else
     ret = emitter._events[type].length;
diff --git a/lib/fs.js b/lib/fs.js
index d7c82f9dbc..d967d22f59 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -57,14 +57,14 @@ function rethrow() {
 }
 
 function maybeCallback(cb) {
-  return util.isFunction(cb) ? cb : rethrow();
+  return typeof cb === 'function' ? cb : rethrow();
 }
 
 // Ensure that callbacks run in the global context. Only use this function
 // for callbacks that are passed to the binding layer, callbacks that are
 // invoked from JS already run in the proper scope.
 function makeCallback(cb) {
-  if (!util.isFunction(cb)) {
+  if (typeof cb !== 'function') {
     return rethrow();
   }
 
@@ -218,11 +218,11 @@ fs.existsSync = function(path) {
 fs.readFile = function(path, options, callback_) {
   var callback = maybeCallback(arguments[arguments.length - 1]);
 
-  if (util.isFunction(options) || !options) {
+  if (!options || typeof options === 'function') {
     options = { encoding: null, flag: 'r' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, flag: 'r' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -317,9 +317,9 @@ fs.readFile = function(path, options, callback_) {
 fs.readFileSync = function(path, options) {
   if (!options) {
     options = { encoding: null, flag: 'r' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, flag: 'r' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -395,7 +395,7 @@ fs.readFileSync = function(path, options) {
 // Used by binding.open and friends
 function stringToFlags(flag) {
   // Only mess with strings
-  if (!util.isString(flag)) {
+  if (typeof flag !== 'string') {
     return flag;
   }
 
@@ -448,9 +448,9 @@ fs.closeSync = function(fd) {
 };
 
 function modeNum(m, def) {
-  if (util.isNumber(m))
+  if (typeof m === 'number')
     return m;
-  if (util.isString(m))
+  if (typeof m === 'string')
     return parseInt(m, 8);
   if (def)
     return modeNum(def);
@@ -479,7 +479,7 @@ fs.openSync = function(path, flags, mode) {
 };
 
 fs.read = function(fd, buffer, offset, length, position, callback) {
-  if (!util.isBuffer(buffer)) {
+  if (!(buffer instanceof Buffer)) {
     // legacy string interface (fd, length, position, encoding, callback)
     var cb = arguments[4],
         encoding = arguments[3];
@@ -513,7 +513,7 @@ fs.read = function(fd, buffer, offset, length, position, callback) {
 
 fs.readSync = function(fd, buffer, offset, length, position) {
   var legacy = false;
-  if (!util.isBuffer(buffer)) {
+  if (!(buffer instanceof Buffer)) {
     // legacy string interface (fd, length, position, encoding, callback)
     legacy = true;
     var encoding = arguments[3];
@@ -551,9 +551,9 @@ fs.write = function(fd, buffer, offset, length, position, callback) {
     callback(err, written || 0, buffer);
   }
 
-  if (util.isBuffer(buffer)) {
+  if (buffer instanceof Buffer) {
     // if no position is passed then assume null
-    if (util.isFunction(position)) {
+    if (typeof position === 'function') {
       callback = position;
       position = null;
     }
@@ -563,10 +563,10 @@ fs.write = function(fd, buffer, offset, length, position, callback) {
     return binding.writeBuffer(fd, buffer, offset, length, position, req);
   }
 
-  if (util.isString(buffer))
+  if (typeof buffer === 'string')
     buffer += '';
-  if (!util.isFunction(position)) {
-    if (util.isFunction(offset)) {
+  if (typeof position !== 'function') {
+    if (typeof offset === 'function') {
       position = offset;
       offset = null;
     } else {
@@ -585,14 +585,14 @@ fs.write = function(fd, buffer, offset, length, position, callback) {
 // OR
 //  fs.writeSync(fd, string[, position[, encoding]]);
 fs.writeSync = function(fd, buffer, offset, length, position) {
-  if (util.isBuffer(buffer)) {
-    if (util.isUndefined(position))
+  if (buffer instanceof Buffer) {
+    if (position === undefined)
       position = null;
     return binding.writeBuffer(fd, buffer, offset, length, position);
   }
-  if (!util.isString(buffer))
+  if (typeof buffer !== 'string')
     buffer += '';
-  if (util.isUndefined(offset))
+  if (offset === undefined)
     offset = null;
   return binding.writeString(fd, buffer, offset, length, position);
 };
@@ -616,15 +616,15 @@ fs.renameSync = function(oldPath, newPath) {
 };
 
 fs.truncate = function(path, len, callback) {
-  if (util.isNumber(path)) {
+  if (typeof path === 'number') {
     var req = new FSReqWrap();
     req.oncomplete = callback;
     return fs.ftruncate(path, len, req);
   }
-  if (util.isFunction(len)) {
+  if (typeof len === 'function') {
     callback = len;
     len = 0;
-  } else if (util.isUndefined(len)) {
+  } else if (len === undefined) {
     len = 0;
   }
 
@@ -642,11 +642,11 @@ fs.truncate = function(path, len, callback) {
 };
 
 fs.truncateSync = function(path, len) {
-  if (util.isNumber(path)) {
+  if (typeof path === 'number') {
     // legacy
     return fs.ftruncateSync(path, len);
   }
-  if (util.isUndefined(len)) {
+  if (len === undefined) {
     len = 0;
   }
   // allow error to be thrown, but still close fd.
@@ -660,10 +660,10 @@ fs.truncateSync = function(path, len) {
 };
 
 fs.ftruncate = function(fd, len, callback) {
-  if (util.isFunction(len)) {
+  if (typeof len === 'function') {
     callback = len;
     len = 0;
-  } else if (util.isUndefined(len)) {
+  } else if (len === undefined) {
     len = 0;
   }
   var req = new FSReqWrap();
@@ -672,7 +672,7 @@ fs.ftruncate = function(fd, len, callback) {
 };
 
 fs.ftruncateSync = function(fd, len) {
-  if (util.isUndefined(len)) {
+  if (len === undefined) {
     len = 0;
   }
   return binding.ftruncate(fd, len);
@@ -712,7 +712,7 @@ fs.fsyncSync = function(fd) {
 };
 
 fs.mkdir = function(path, mode, callback) {
-  if (util.isFunction(mode)) callback = mode;
+  if (typeof mode === 'function') callback = mode;
   callback = makeCallback(callback);
   if (!nullCheck(path, callback)) return;
   var req = new FSReqWrap();
@@ -806,7 +806,7 @@ function preprocessSymlinkDestination(path, type, linkPath) {
 }
 
 fs.symlink = function(destination, path, type_, callback) {
-  var type = (util.isString(type_) ? type_ : null);
+  var type = (typeof type_ === 'string' ? type_ : null);
   var callback = makeCallback(arguments[arguments.length - 1]);
 
   if (!nullCheck(destination, callback)) return;
@@ -822,7 +822,7 @@ fs.symlink = function(destination, path, type_, callback) {
 };
 
 fs.symlinkSync = function(destination, path, type) {
-  type = (util.isString(type) ? type : null);
+  type = (typeof type === 'string' ? type : null);
 
   nullCheck(destination);
   nullCheck(path);
@@ -973,7 +973,7 @@ fs.chownSync = function(path, uid, gid) {
 
 // converts Date or number to a fractional UNIX timestamp
 function toUnixTimestamp(time) {
-  if (util.isNumber(time)) {
+  if (typeof time === 'number') {
     return time;
   }
   if (util.isDate(time)) {
@@ -1043,11 +1043,11 @@ function writeAll(fd, buffer, offset, length, position, callback) {
 fs.writeFile = function(path, data, options, callback) {
   var callback = maybeCallback(arguments[arguments.length - 1]);
 
-  if (util.isFunction(options) || !options) {
+  if (!options || typeof options === 'function') {
     options = { encoding: 'utf8', mode: 0o666, flag: 'w' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, mode: 0o666, flag: 'w' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -1058,7 +1058,7 @@ fs.writeFile = function(path, data, options, callback) {
     if (openErr) {
       if (callback) callback(openErr);
     } else {
-      var buffer = util.isBuffer(data) ? data : new Buffer('' + data,
+      var buffer = (data instanceof Buffer) ? data : new Buffer('' + data,
           options.encoding || 'utf8');
       var position = /a/.test(flag) ? null : 0;
       writeAll(fd, buffer, 0, buffer.length, position, callback);
@@ -1069,9 +1069,9 @@ fs.writeFile = function(path, data, options, callback) {
 fs.writeFileSync = function(path, data, options) {
   if (!options) {
     options = { encoding: 'utf8', mode: 0o666, flag: 'w' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, mode: 0o666, flag: 'w' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -1079,7 +1079,7 @@ fs.writeFileSync = function(path, data, options) {
 
   var flag = options.flag || 'w';
   var fd = fs.openSync(path, flag, options.mode);
-  if (!util.isBuffer(data)) {
+  if (!(data instanceof Buffer)) {
     data = new Buffer('' + data, options.encoding || 'utf8');
   }
   var written = 0;
@@ -1098,11 +1098,11 @@ fs.writeFileSync = function(path, data, options) {
 fs.appendFile = function(path, data, options, callback_) {
   var callback = maybeCallback(arguments[arguments.length - 1]);
 
-  if (util.isFunction(options) || !options) {
+  if (!options || typeof options === 'function') {
     options = { encoding: 'utf8', mode: 0o666, flag: 'a' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, mode: 0o666, flag: 'a' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -1114,9 +1114,9 @@ fs.appendFile = function(path, data, options, callback_) {
 fs.appendFileSync = function(path, data, options) {
   if (!options) {
     options = { encoding: 'utf8', mode: 0o666, flag: 'a' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, mode: 0o666, flag: 'a' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
   if (!options.flag)
@@ -1165,7 +1165,7 @@ fs.watch = function(filename) {
   var options;
   var listener;
 
-  if (util.isObject(arguments[1])) {
+  if (arguments[1] !== null && typeof arguments[1] === 'object') {
     options = arguments[1];
     listener = arguments[2];
   } else {
@@ -1173,8 +1173,8 @@ fs.watch = function(filename) {
     listener = arguments[1];
   }
 
-  if (util.isUndefined(options.persistent)) options.persistent = true;
-  if (util.isUndefined(options.recursive)) options.recursive = false;
+  if (options.persistent === undefined) options.persistent = true;
+  if (options.recursive === undefined) options.recursive = false;
 
   watcher = new FSWatcher();
   watcher.start(filename, options.persistent, options.recursive);
@@ -1247,7 +1247,7 @@ fs.watchFile = function(filename) {
     persistent: true
   };
 
-  if (util.isObject(arguments[1])) {
+  if (arguments[1] !== null && typeof arguments[1] === 'object') {
     options = util._extend(options, arguments[1]);
     listener = arguments[2];
   } else {
@@ -1275,7 +1275,7 @@ fs.unwatchFile = function(filename, listener) {
 
   var stat = statWatchers[filename];
 
-  if (util.isFunction(listener)) {
+  if (typeof listener === 'function') {
     stat.removeListener('change', listener);
   } else {
     stat.removeAllListeners('change');
@@ -1378,7 +1378,7 @@ fs.realpathSync = function realpathSync(p, cache) {
           linkTarget = seenLinks[id];
         }
       }
-      if (util.isNull(linkTarget)) {
+      if (linkTarget === null) {
         fs.statSync(base);
         linkTarget = fs.readlinkSync(base);
       }
@@ -1400,7 +1400,7 @@ fs.realpathSync = function realpathSync(p, cache) {
 
 
 fs.realpath = function realpath(p, cache, cb) {
-  if (!util.isFunction(cb)) {
+  if (typeof cb !== 'function') {
     cb = maybeCallback(cache);
     cache = null;
   }
@@ -1561,13 +1561,13 @@ function ReadStream(path, options) {
       options.autoClose : true;
   this.pos = undefined;
 
-  if (!util.isUndefined(this.start)) {
-    if (!util.isNumber(this.start)) {
+  if (this.start !== undefined) {
+    if (typeof this.start !== 'number') {
       throw TypeError('start must be a Number');
     }
-    if (util.isUndefined(this.end)) {
+    if (this.end === undefined) {
       this.end = Infinity;
-    } else if (!util.isNumber(this.end)) {
+    } else if (typeof this.end !== 'number') {
       throw TypeError('end must be a Number');
     }
 
@@ -1578,7 +1578,7 @@ function ReadStream(path, options) {
     this.pos = this.start;
   }
 
-  if (!util.isNumber(this.fd))
+  if (typeof this.fd !== 'number')
     this.open();
 
   this.on('end', function() {
@@ -1609,7 +1609,7 @@ ReadStream.prototype.open = function() {
 };
 
 ReadStream.prototype._read = function(n) {
-  if (!util.isNumber(this.fd))
+  if (typeof this.fd !== 'number')
     return this.once('open', function() {
       this._read(n);
     });
@@ -1630,7 +1630,7 @@ ReadStream.prototype._read = function(n) {
   var toRead = Math.min(pool.length - pool.used, n);
   var start = pool.used;
 
-  if (!util.isUndefined(this.pos))
+  if (this.pos !== undefined)
     toRead = Math.min(this.end - this.pos + 1, toRead);
 
   // already read everything we were supposed to read!
@@ -1643,7 +1643,7 @@ ReadStream.prototype._read = function(n) {
   fs.read(this.fd, pool, pool.used, toRead, this.pos, onread);
 
   // move the pool positions, and internal position for reading.
-  if (!util.isUndefined(this.pos))
+  if (this.pos !== undefined)
     this.pos += toRead;
   pool.used += toRead;
 
@@ -1676,8 +1676,8 @@ ReadStream.prototype.close = function(cb) {
   var self = this;
   if (cb)
     this.once('close', cb);
-  if (this.closed || !util.isNumber(this.fd)) {
-    if (!util.isNumber(this.fd)) {
+  if (this.closed || typeof this.fd !== 'number') {
+    if (typeof this.fd !== 'number') {
       this.once('open', close);
       return;
     }
@@ -1725,8 +1725,8 @@ function WriteStream(path, options) {
   this.pos = undefined;
   this.bytesWritten = 0;
 
-  if (!util.isUndefined(this.start)) {
-    if (!util.isNumber(this.start)) {
+  if (this.start !== undefined) {
+    if (typeof this.start !== 'number') {
       throw TypeError('start must be a Number');
     }
     if (this.start < 0) {
@@ -1736,7 +1736,7 @@ function WriteStream(path, options) {
     this.pos = this.start;
   }
 
-  if (!util.isNumber(this.fd))
+  if (typeof this.fd !== 'number')
     this.open();
 
   // dispose on finish.
@@ -1761,10 +1761,10 @@ WriteStream.prototype.open = function() {
 
 
 WriteStream.prototype._write = function(data, encoding, cb) {
-  if (!util.isBuffer(data))
+  if (!(data instanceof Buffer))
     return this.emit('error', new Error('Invalid data'));
 
-  if (!util.isNumber(this.fd))
+  if (typeof this.fd !== 'number')
     return this.once('open', function() {
       this._write(data, encoding, cb);
     });
@@ -1779,7 +1779,7 @@ WriteStream.prototype._write = function(data, encoding, cb) {
     cb();
   });
 
-  if (!util.isUndefined(this.pos))
+  if (this.pos !== undefined)
     this.pos += data.length;
 };
 
@@ -1817,10 +1817,10 @@ SyncWriteStream.prototype.write = function(data, arg1, arg2) {
 
   // parse arguments
   if (arg1) {
-    if (util.isString(arg1)) {
+    if (typeof arg1 === 'string') {
       encoding = arg1;
       cb = arg2;
-    } else if (util.isFunction(arg1)) {
+    } else if (typeof arg1 === 'function') {
       cb = arg1;
     } else {
       throw new Error('bad arg');
@@ -1829,7 +1829,7 @@ SyncWriteStream.prototype.write = function(data, arg1, arg2) {
   assertEncoding(encoding);
 
   // Change strings to buffers. SLOW
-  if (util.isString(data)) {
+  if (typeof data === 'string') {
     data = new Buffer(data, encoding);
   }
 
diff --git a/lib/https.js b/lib/https.js
index acbbe0b6c2..4ee15b2537 100644
--- a/lib/https.js
+++ b/lib/https.js
@@ -4,7 +4,7 @@ const tls = require('tls');
 const url = require('url');
 const http = require('http');
 const util = require('util');
-const inherits = require('util').inherits;
+const inherits = util.inherits;
 const debug = util.debuglog('https');
 
 function Server(opts, requestListener) {
@@ -41,21 +41,21 @@ exports.createServer = function(opts, requestListener) {
 // HTTPS agents.
 
 function createConnection(port, host, options) {
-  if (util.isObject(port)) {
+  if (port !== null && typeof port === 'object') {
     options = port;
-  } else if (util.isObject(host)) {
+  } else if (host !== null && typeof host === 'object') {
     options = host;
-  } else if (util.isObject(options)) {
+  } else if (options !== null && typeof options === 'object') {
     options = options;
   } else {
     options = {};
   }
 
-  if (util.isNumber(port)) {
+  if (typeof port === 'number') {
     options.port = port;
   }
 
-  if (util.isString(host)) {
+  if (typeof host === 'string') {
     options.host = host;
   }
 
@@ -96,7 +96,7 @@ Agent.prototype.getName = function(options) {
     name += options.pfx;
 
   name += ':';
-  if (!util.isUndefined(options.rejectUnauthorized))
+  if (options.rejectUnauthorized !== undefined)
     name += options.rejectUnauthorized;
 
   return name;
@@ -108,7 +108,7 @@ exports.globalAgent = globalAgent;
 exports.Agent = Agent;
 
 exports.request = function(options, cb) {
-  if (util.isString(options)) {
+  if (typeof options === 'string') {
     options = url.parse(options);
   } else {
     options = util._extend({}, options);
diff --git a/lib/module.js b/lib/module.js
index 07c32b7eee..3f42975a63 100644
--- a/lib/module.js
+++ b/lib/module.js
@@ -347,7 +347,7 @@ Module.prototype.load = function(filename) {
 // `exports` property.
 Module.prototype.require = function(path) {
   assert(path, 'missing path');
-  assert(util.isString(path), 'path must be a string');
+  assert(typeof path === 'string', 'path must be a string');
   return Module._load(path, this);
 };
 
diff --git a/lib/net.js b/lib/net.js
index 13a3ca424b..1cd8b6da35 100644
--- a/lib/net.js
+++ b/lib/net.js
@@ -45,7 +45,7 @@ function createHandle(fd) {
 const debug = util.debuglog('net');
 
 function isPipeName(s) {
-  return util.isString(s) && toNumber(s) === false;
+  return typeof s === 'string' && toNumber(s) === false;
 }
 
 exports.createServer = function() {
@@ -77,7 +77,7 @@ exports.connect = exports.createConnection = function() {
 function normalizeConnectArgs(args) {
   var options = {};
 
-  if (util.isObject(args[0])) {
+  if (args[0] !== null && typeof args[0] === 'object') {
     // connect(options, [cb])
     options = args[0];
   } else if (isPipeName(args[0])) {
@@ -86,13 +86,13 @@ function normalizeConnectArgs(args) {
   } else {
     // connect(port, [host], [cb])
     options.port = args[0];
-    if (util.isString(args[1])) {
+    if (typeof args[1] === 'string') {
       options.host = args[1];
     }
   }
 
   var cb = args[args.length - 1];
-  return util.isFunction(cb) ? [options, cb] : [options];
+  return typeof cb === 'function' ? [options, cb] : [options];
 }
 exports._normalizeConnectArgs = normalizeConnectArgs;
 
@@ -122,16 +122,16 @@ function Socket(options) {
   this._handle = null;
   this._host = null;
 
-  if (util.isNumber(options))
+  if (typeof options === 'number')
     options = { fd: options }; // Legacy interface.
-  else if (util.isUndefined(options))
+  else if (options === undefined)
     options = {};
 
   stream.Duplex.call(this, options);
 
   if (options.handle) {
     this._handle = options.handle; // private
-  } else if (!util.isUndefined(options.fd)) {
+  } else if (options.fd !== undefined) {
     this._handle = createHandle(options.fd);
     this._handle.open(options.fd);
     if ((options.fd == 1 || options.fd == 2) &&
@@ -261,7 +261,7 @@ function onSocketEnd() {
 // of the other side sending a FIN.  The standard 'write after end'
 // is overly vague, and makes it seem like the user's code is to blame.
 function writeAfterFIN(chunk, encoding, cb) {
-  if (util.isFunction(encoding)) {
+  if (typeof encoding === 'function') {
     cb = encoding;
     encoding = null;
   }
@@ -271,7 +271,7 @@ function writeAfterFIN(chunk, encoding, cb) {
   var self = this;
   // TODO: defer error events consistently everywhere, not just the cb
   self.emit('error', er);
-  if (util.isFunction(cb)) {
+  if (typeof cb === 'function') {
     process.nextTick(function() {
       cb(er);
     });
@@ -324,7 +324,7 @@ Socket.prototype._onTimeout = function() {
 Socket.prototype.setNoDelay = function(enable) {
   // backwards compatibility: assume true when `enable` is omitted
   if (this._handle && this._handle.setNoDelay)
-    this._handle.setNoDelay(util.isUndefined(enable) ? true : !!enable);
+    this._handle.setNoDelay(enable === undefined ? true : !!enable);
 };
 
 
@@ -599,7 +599,7 @@ Socket.prototype.__defineGetter__('localPort', function() {
 
 
 Socket.prototype.write = function(chunk, encoding, cb) {
-  if (!util.isString(chunk) && !util.isBuffer(chunk))
+  if (typeof chunk !== 'string' && !(chunk instanceof Buffer))
     throw new TypeError('invalid data');
   return stream.Duplex.prototype.write.apply(this, arguments);
 };
@@ -647,7 +647,7 @@ Socket.prototype._writeGeneric = function(writev, data, encoding, cb) {
     if (err === 0) req._chunks = chunks;
   } else {
     var enc;
-    if (util.isBuffer(data)) {
+    if (data instanceof Buffer) {
       req.buffer = data;  // Keep reference alive.
       enc = 'buffer';
     } else {
@@ -713,14 +713,14 @@ Socket.prototype.__defineGetter__('bytesWritten', function() {
       encoding = this._pendingEncoding;
 
   state.getBuffer().forEach(function(el) {
-    if (util.isBuffer(el.chunk))
+    if (el.chunk instanceof Buffer)
       bytes += el.chunk.length;
     else
       bytes += Buffer.byteLength(el.chunk, el.encoding);
   });
 
   if (data) {
-    if (util.isBuffer(data))
+    if (data instanceof Buffer)
       bytes += data.length;
     else
       bytes += Buffer.byteLength(data, encoding);
@@ -830,7 +830,7 @@ Socket.prototype.connect = function(options, cb) {
   if (this.write !== Socket.prototype.write)
     this.write = Socket.prototype.write;
 
-  if (!util.isObject(options)) {
+  if (options === null || typeof options !== 'object') {
     // Old API:
     // connect(port, [host], [cb])
     // connect(path, [cb]);
@@ -859,7 +859,7 @@ Socket.prototype.connect = function(options, cb) {
     initSocketHandle(this);
   }
 
-  if (util.isFunction(cb)) {
+  if (typeof cb === 'function') {
     self.once('connect', cb);
   }
 
@@ -885,7 +885,7 @@ Socket.prototype.connect = function(options, cb) {
     if (localAddress && !exports.isIP(localAddress))
       throw new TypeError('localAddress must be a valid IP: ' + localAddress);
 
-    if (localPort && !util.isNumber(localPort))
+    if (localPort && typeof localPort !== 'number')
       throw new TypeError('localPort should be a number: ' + localPort);
 
     if (port <= 0 || port > 65535)
@@ -998,13 +998,13 @@ function Server(/* [ options, ] listener */) {
 
   var options;
 
-  if (util.isFunction(arguments[0])) {
+  if (typeof arguments[0] === 'function') {
     options = {};
     self.on('connection', arguments[0]);
   } else {
     options = arguments[0] || {};
 
-    if (util.isFunction(arguments[1])) {
+    if (typeof arguments[1] === 'function') {
       self.on('connection', arguments[1]);
     }
   }
@@ -1052,7 +1052,7 @@ var createServerHandle = exports._createServerHandle =
   var handle;
 
   var isTCP = false;
-  if (util.isNumber(fd) && fd >= 0) {
+  if (typeof fd === 'number' && fd >= 0) {
     try {
       handle = createHandle(fd);
     }
@@ -1117,10 +1117,10 @@ Server.prototype._listen2 = function(address, port, addressType, backlog, fd) {
 
     var rval = null;
 
-    if (!address && !util.isNumber(fd)) {
+    if (!address && typeof fd !== 'number') {
       rval = createServerHandle('::', port, 6, fd);
 
-      if (util.isNumber(rval)) {
+      if (typeof rval === 'number') {
         rval = null;
         address = '0.0.0.0';
         addressType = 4;
@@ -1133,7 +1133,7 @@ Server.prototype._listen2 = function(address, port, addressType, backlog, fd) {
     if (rval === null)
       rval = createServerHandle(address, port, addressType, fd);
 
-    if (util.isNumber(rval)) {
+    if (typeof rval === 'number') {
       var error = exceptionWithHostPort(rval, 'listen', address, port);
       process.nextTick(function() {
         self.emit('error', error);
@@ -1212,7 +1212,7 @@ Server.prototype.listen = function() {
   var self = this;
 
   var lastArg = arguments[arguments.length - 1];
-  if (util.isFunction(lastArg)) {
+  if (typeof lastArg === 'function') {
     self.once('listening', lastArg);
   }
 
@@ -1224,24 +1224,24 @@ Server.prototype.listen = function() {
 
   const TCP = process.binding('tcp_wrap').TCP;
 
-  if (arguments.length === 0 || util.isFunction(arguments[0])) {
+  if (arguments.length === 0 || typeof arguments[0] === 'function') {
     // Bind to a random port.
     listen(self, null, 0, null, backlog);
-  } else if (util.isObject(arguments[0])) {
+  } else if (arguments[0] !== null && typeof arguments[0] === 'object') {
     var h = arguments[0];
     h = h._handle || h.handle || h;
 
     if (h instanceof TCP) {
       self._handle = h;
       listen(self, null, -1, -1, backlog);
-    } else if (util.isNumber(h.fd) && h.fd >= 0) {
+    } else if (typeof h.fd === 'number' && h.fd >= 0) {
       listen(self, null, null, null, backlog, h.fd);
     } else {
       // The first argument is a configuration object
       if (h.backlog)
         backlog = h.backlog;
 
-      if (util.isNumber(h.port)) {
+      if (typeof h.port === 'number') {
         if (h.host)
           listenAfterLookup(h.port, h.host, backlog, h.exclusive);
         else
@@ -1258,9 +1258,9 @@ Server.prototype.listen = function() {
     var pipeName = self._pipeName = arguments[0];
     listen(self, pipeName, -1, -1, backlog);
 
-  } else if (util.isUndefined(arguments[1]) ||
-             util.isFunction(arguments[1]) ||
-             util.isNumber(arguments[1])) {
+  } else if (arguments[1] === undefined ||
+             typeof arguments[1] === 'function' ||
+             typeof arguments[1] === 'number') {
     // The first argument is the port, no IP given.
     listen(self, null, port, 4, backlog);
 
@@ -1456,11 +1456,11 @@ if (process.platform === 'win32') {
   var simultaneousAccepts;
 
   exports._setSimultaneousAccepts = function(handle) {
-    if (util.isUndefined(handle)) {
+    if (handle === undefined) {
       return;
     }
 
-    if (util.isUndefined(simultaneousAccepts)) {
+    if (simultaneousAccepts === undefined) {
       simultaneousAccepts = (process.env.NODE_MANY_ACCEPTS &&
                              process.env.NODE_MANY_ACCEPTS !== '0');
     }
diff --git a/lib/path.js b/lib/path.js
index f1acb06b45..6259c5fd1d 100644
--- a/lib/path.js
+++ b/lib/path.js
@@ -1,8 +1,6 @@
 'use strict';
 
 const isWindows = process.platform === 'win32';
-const util = require('util');
-
 
 // resolves . and .. elements in a path array with directory names there
 // must be no slashes or device names (c:\) in the array
@@ -87,7 +85,7 @@ win32.resolve = function() {
     }
 
     // Skip empty and invalid entries
-    if (!util.isString(path)) {
+    if (typeof path !== 'string') {
       throw new TypeError('Arguments to path.resolve must be strings');
     } else if (!path) {
       continue;
@@ -176,7 +174,7 @@ win32.isAbsolute = function(path) {
 
 win32.join = function() {
   function f(p) {
-    if (!util.isString(p)) {
+    if (typeof p !== 'string') {
       throw new TypeError('Arguments to path.join must be strings');
     }
     return p;
@@ -265,7 +263,7 @@ win32.relative = function(from, to) {
 
 win32._makeLong = function(path) {
   // Note: this will *probably* throw somewhere.
-  if (!util.isString(path))
+  if (typeof path !== 'string')
     return path;
 
   if (!path) {
@@ -323,7 +321,7 @@ win32.extname = function(path) {
 
 
 win32.format = function(pathObject) {
-  if (!util.isObject(pathObject)) {
+  if (pathObject === null || typeof pathObject !== 'object') {
     throw new TypeError(
         "Parameter 'pathObject' must be an object, not " + typeof pathObject
     );
@@ -331,7 +329,7 @@ win32.format = function(pathObject) {
 
   var root = pathObject.root || '';
 
-  if (!util.isString(root)) {
+  if (typeof root !== 'string') {
     throw new TypeError(
         "'pathObject.root' must be a string or undefined, not " +
         typeof pathObject.root
@@ -353,7 +351,7 @@ win32.format = function(pathObject) {
 
 
 win32.parse = function(pathString) {
-  if (!util.isString(pathString)) {
+  if (typeof pathString !== 'string') {
     throw new TypeError(
         "Parameter 'pathString' must be a string, not " + typeof pathString
     );
@@ -398,7 +396,7 @@ posix.resolve = function() {
     var path = (i >= 0) ? arguments[i] : process.cwd();
 
     // Skip empty and invalid entries
-    if (!util.isString(path)) {
+    if (typeof path !== 'string') {
       throw new TypeError('Arguments to path.resolve must be strings');
     } else if (!path) {
       continue;
@@ -447,7 +445,7 @@ posix.join = function() {
   var path = '';
   for (var i = 0; i < arguments.length; i++) {
     var segment = arguments[i];
-    if (!util.isString(segment)) {
+    if (typeof segment !== 'string') {
       throw new TypeError('Arguments to path.join must be strings');
     }
     if (segment) {
@@ -546,7 +544,7 @@ posix.extname = function(path) {
 
 
 posix.format = function(pathObject) {
-  if (!util.isObject(pathObject)) {
+  if (pathObject === null || typeof pathObject !== 'object') {
     throw new TypeError(
         "Parameter 'pathObject' must be an object, not " + typeof pathObject
     );
@@ -554,7 +552,7 @@ posix.format = function(pathObject) {
 
   var root = pathObject.root || '';
 
-  if (!util.isString(root)) {
+  if (typeof root !== 'string') {
     throw new TypeError(
         "'pathObject.root' must be a string or undefined, not " +
         typeof pathObject.root
@@ -568,7 +566,7 @@ posix.format = function(pathObject) {
 
 
 posix.parse = function(pathString) {
-  if (!util.isString(pathString)) {
+  if (typeof pathString !== 'string') {
     throw new TypeError(
         "Parameter 'pathString' must be a string, not " + typeof pathString
     );
diff --git a/lib/querystring.js b/lib/querystring.js
index 5270bc6f1a..b574978bc7 100644
--- a/lib/querystring.js
+++ b/lib/querystring.js
@@ -3,8 +3,6 @@
 'use strict';
 
 const QueryString = exports;
-const util = require('util');
-
 
 // If obj.hasOwnProperty has been overridden, then calling
 // obj.hasOwnProperty(prop) will break.
@@ -100,11 +98,13 @@ QueryString.escape = function(str) {
 };
 
 var stringifyPrimitive = function(v) {
-  if (util.isString(v))
+  let type = typeof v;
+
+  if (type === 'string')
     return v;
-  if (util.isBoolean(v))
+  if (type === 'boolean')
     return v ? 'true' : 'false';
-  if (util.isNumber(v))
+  if (type === 'number')
     return isFinite(v) ? v : '';
   return '';
 };
@@ -119,7 +119,7 @@ QueryString.stringify = QueryString.encode = function(obj, sep, eq, options) {
     encode = options.encodeURIComponent;
   }
 
-  if (util.isObject(obj)) {
+  if (obj !== null && typeof obj === 'object') {
     var keys = Object.keys(obj);
     var fields = [];
 
@@ -128,7 +128,7 @@ QueryString.stringify = QueryString.encode = function(obj, sep, eq, options) {
       var v = obj[k];
       var ks = encode(stringifyPrimitive(k)) + eq;
 
-      if (util.isArray(v)) {
+      if (Array.isArray(v)) {
         for (var j = 0; j < v.length; j++)
           fields.push(ks + encode(stringifyPrimitive(v[j])));
       } else {
@@ -146,7 +146,7 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq, options) {
   eq = eq || '=';
   var obj = {};
 
-  if (!util.isString(qs) || qs.length === 0) {
+  if (typeof qs !== 'string' || qs.length === 0) {
     return obj;
   }
 
@@ -154,7 +154,7 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq, options) {
   qs = qs.split(sep);
 
   var maxKeys = 1000;
-  if (options && util.isNumber(options.maxKeys)) {
+  if (options && typeof options.maxKeys === 'number') {
     maxKeys = options.maxKeys;
   }
 
@@ -192,7 +192,7 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq, options) {
 
     if (!hasOwnProperty(obj, k)) {
       obj[k] = v;
-    } else if (util.isArray(obj[k])) {
+    } else if (Array.isArray(obj[k])) {
       obj[k].push(v);
     } else {
       obj[k] = [obj[k], v];
diff --git a/lib/readline.js b/lib/readline.js
index 1a159c9a1b..04e3f97aac 100644
--- a/lib/readline.js
+++ b/lib/readline.js
@@ -43,13 +43,13 @@ function Interface(input, output, completer, terminal) {
 
   completer = completer || function() { return []; };
 
-  if (!util.isFunction(completer)) {
+  if (typeof completer !== 'function') {
     throw new TypeError('Argument \'completer\' must be a function');
   }
 
   // backwards compat; check the isTTY prop of the output stream
   //  when `terminal` was not specified
-  if (util.isUndefined(terminal) && !util.isNullOrUndefined(output)) {
+  if (terminal === undefined && !(output === null || output === undefined)) {
     terminal = !!output.isTTY;
   }
 
@@ -72,14 +72,15 @@ function Interface(input, output, completer, terminal) {
   }
 
   function onend() {
-    if (util.isString(self._line_buffer) && self._line_buffer.length > 0) {
+    if (typeof self._line_buffer === 'string' &&
+        self._line_buffer.length > 0) {
       self.emit('line', self._line_buffer);
     }
     self.close();
   }
 
   function ontermend() {
-    if (util.isString(self.line) && self.line.length > 0) {
+    if (typeof self.line === 'string' && self.line.length > 0) {
       self.emit('line', self.line);
     }
     self.close();
@@ -123,13 +124,13 @@ function Interface(input, output, completer, terminal) {
     this.history = [];
     this.historyIndex = -1;
 
-    if (!util.isNullOrUndefined(output))
+    if (output !== null && output !== undefined)
       output.on('resize', onresize);
 
     self.once('close', function() {
       input.removeListener('keypress', onkeypress);
       input.removeListener('end', ontermend);
-      if (!util.isNullOrUndefined(output)) {
+      if (output !== null && output !== undefined) {
         output.removeListener('resize', onresize);
       }
     });
@@ -153,7 +154,7 @@ Interface.prototype.setPrompt = function(prompt) {
 
 
 Interface.prototype._setRawMode = function(mode) {
-  if (util.isFunction(this.input.setRawMode)) {
+  if (typeof this.input.setRawMode === 'function') {
     return this.input.setRawMode(mode);
   }
 };
@@ -171,7 +172,7 @@ Interface.prototype.prompt = function(preserveCursor) {
 
 
 Interface.prototype.question = function(query, cb) {
-  if (util.isFunction(cb)) {
+  if (typeof cb === 'function') {
     if (this._questionCallback) {
       this.prompt();
     } else {
@@ -196,10 +197,10 @@ Interface.prototype._onLine = function(line) {
 };
 
 Interface.prototype._writeToOutput = function _writeToOutput(stringToWrite) {
-  if (!util.isString(stringToWrite))
+  if (typeof stringToWrite !== 'string')
     throw new TypeError('stringToWrite must be a string');
 
-  if (!util.isNullOrUndefined(this.output))
+  if (this.output !== null && this.output !== undefined)
     this.output.write(stringToWrite);
 };
 
@@ -296,7 +297,7 @@ Interface.prototype.write = function(d, key) {
 // \r\n, \n, or \r followed by something other than \n
 const lineEnding = /\r?\n|\r(?!\n)/;
 Interface.prototype._normalWrite = function(b) {
-  if (util.isUndefined(b)) {
+  if (b === undefined) {
     return;
   }
   var string = this._decoder.write(b);
@@ -854,7 +855,7 @@ Interface.prototype._ttyWrite = function(s, key) {
         break;
 
       default:
-        if (util.isBuffer(s))
+        if (s instanceof Buffer)
           s = s.toString('utf-8');
 
         if (s) {
@@ -952,8 +953,8 @@ const escapeCodeReAnywhere = new RegExp([
 ].join('|'));
 
 function emitKeys(stream, s) {
-  if (util.isBuffer(s)) {
-    if (s[0] > 127 && util.isUndefined(s[1])) {
+  if (s instanceof Buffer) {
+    if (s[0] > 127 && s[1] === undefined) {
       s[0] -= 128;
       s = '\x1b' + s.toString(stream.encoding || 'utf-8');
     } else {
@@ -1143,7 +1144,7 @@ function emitKeys(stream, s) {
     }
 
     // Don't emit a key if no name was found
-    if (util.isUndefined(key.name)) {
+    if (key.name === undefined) {
       key = undefined;
     }
 
@@ -1163,16 +1164,16 @@ function emitKeys(stream, s) {
  */
 
 function cursorTo(stream, x, y) {
-  if (util.isNullOrUndefined(stream))
+  if (stream === null || stream === undefined)
     return;
 
-  if (!util.isNumber(x) && !util.isNumber(y))
+  if (typeof x !== 'number' && typeof y !== 'number')
     return;
 
-  if (!util.isNumber(x))
+  if (typeof x !== 'number')
     throw new Error("Can't set cursor row without also setting it's column");
 
-  if (!util.isNumber(y)) {
+  if (typeof y !== 'number') {
     stream.write('\x1b[' + (x + 1) + 'G');
   } else {
     stream.write('\x1b[' + (y + 1) + ';' + (x + 1) + 'H');
@@ -1186,7 +1187,7 @@ exports.cursorTo = cursorTo;
  */
 
 function moveCursor(stream, dx, dy) {
-  if (util.isNullOrUndefined(stream))
+  if (stream === null || stream === undefined)
     return;
 
   if (dx < 0) {
@@ -1212,7 +1213,7 @@ exports.moveCursor = moveCursor;
  */
 
 function clearLine(stream, dir) {
-  if (util.isNullOrUndefined(stream))
+  if (stream === null || stream === undefined)
     return;
 
   if (dir < 0) {
@@ -1234,7 +1235,7 @@ exports.clearLine = clearLine;
  */
 
 function clearScreenDown(stream) {
-  if (util.isNullOrUndefined(stream))
+  if (stream === null || stream === undefined)
     return;
 
   stream.write('\x1b[0J');
diff --git a/lib/repl.js b/lib/repl.js
index 64d99b0001..0a63f325c6 100644
--- a/lib/repl.js
+++ b/lib/repl.js
@@ -63,7 +63,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
   }
 
   var options, input, output, dom;
-  if (util.isObject(prompt)) {
+  if (prompt !== null && typeof prompt === 'object') {
     // an options object was given
     options = prompt;
     stream = options.stream || options.socket;
@@ -74,7 +74,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
     ignoreUndefined = options.ignoreUndefined;
     prompt = options.prompt;
     dom = options.domain;
-  } else if (!util.isString(prompt)) {
+  } else if (typeof prompt !== 'string') {
     throw new Error('An options Object, or a prompt String are required');
   } else {
     options = {};
@@ -176,7 +176,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
     options.terminal
   ]);
 
-  self.setPrompt(!util.isUndefined(prompt) ? prompt : '> ');
+  self.setPrompt(prompt !== undefined ? prompt : '> ');
 
   this.commands = {};
   defineDefaultCommands(this);
@@ -184,7 +184,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
   // figure out which "writer" function to use
   self.writer = options.writer || exports.writer;
 
-  if (util.isUndefined(options.useColors)) {
+  if (options.useColors === undefined) {
     options.useColors = self.terminal;
   }
   self.useColors = !!options.useColors;
@@ -303,7 +303,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
       self.bufferedCommand = '';
 
       // If we got any output - print it (if no error)
-      if (!e && (!self.ignoreUndefined || !util.isUndefined(ret))) {
+      if (!e && (!self.ignoreUndefined || ret !== undefined)) {
         self.context._ = ret;
         self.outputStream.write(self.writer(ret) + '\n');
       }
@@ -431,7 +431,7 @@ const simpleExpressionRE =
 // getter code.
 REPLServer.prototype.complete = function(line, callback) {
   // There may be local variables to evaluate, try a nested REPL
-  if (!util.isUndefined(this.bufferedCommand) && this.bufferedCommand.length) {
+  if (this.bufferedCommand !== undefined && this.bufferedCommand.length) {
     // Get a new array of inputed lines
     var tmp = this.lines.slice();
     // Kill off all function declarations to push all local variables into
@@ -573,7 +573,7 @@ REPLServer.prototype.complete = function(line, callback) {
           this.eval('.scope', this.context, 'repl', function(err, globals) {
             if (err || !globals) {
               addStandardGlobals(completionGroups, filter);
-            } else if (util.isArray(globals[0])) {
+            } else if (Array.isArray(globals[0])) {
               // Add grouped globals
               globals.forEach(function(group) {
                 completionGroups.push(group);
@@ -590,19 +590,19 @@ REPLServer.prototype.complete = function(line, callback) {
           // if (e) console.log(e);
 
           if (obj != null) {
-            if (util.isObject(obj) || util.isFunction(obj)) {
+            if (typeof obj === 'object' || typeof obj === 'function') {
               memberGroups.push(Object.getOwnPropertyNames(obj));
             }
             // works for non-objects
             try {
               var sentinel = 5;
               var p;
-              if (util.isObject(obj) || util.isFunction(obj)) {
+              if (typeof obj === 'object' || typeof obj === 'function') {
                 p = Object.getPrototypeOf(obj);
               } else {
                 p = obj.constructor ? obj.constructor.prototype : null;
               }
-              while (!util.isNull(p)) {
+              while (p !== null) {
                 memberGroups.push(Object.getOwnPropertyNames(p));
                 p = Object.getPrototypeOf(p);
                 // Circular refs possible? Let's guard against that.
@@ -701,9 +701,9 @@ REPLServer.prototype.parseREPLKeyword = function(keyword, rest) {
 
 
 REPLServer.prototype.defineCommand = function(keyword, cmd) {
-  if (util.isFunction(cmd)) {
+  if (typeof cmd === 'function') {
     cmd = {action: cmd};
-  } else if (!util.isFunction(cmd.action)) {
+  } else if (typeof cmd.action !== 'function') {
     throw new Error('bad argument, action must be a function');
   }
   this.commands[keyword] = cmd;
diff --git a/lib/smalloc.js b/lib/smalloc.js
index beccba71a4..b6eb21fbc3 100644
--- a/lib/smalloc.js
+++ b/lib/smalloc.js
@@ -40,10 +40,10 @@ Object.defineProperty(exports, 'Types', {
 function alloc(n, obj, type) {
   n = n >>> 0;
 
-  if (util.isUndefined(obj))
+  if (obj === undefined)
     obj = {};
 
-  if (util.isNumber(obj)) {
+  if (typeof obj === 'number') {
     type = obj >>> 0;
     obj = {};
   } else if (util.isPrimitive(obj)) {
@@ -53,7 +53,7 @@ function alloc(n, obj, type) {
   // 1 == v8::kExternalUint8Array, 9 == v8::kExternalUint8ClampedArray
   if (type < 1 || type > 9)
     throw new TypeError('unknown external array type: ' + type);
-  if (util.isArray(obj))
+  if (Array.isArray(obj))
     throw new TypeError('Arrays are not supported');
   if (n > kMaxLength)
     throw new RangeError('n > kMaxLength');
@@ -65,7 +65,7 @@ function alloc(n, obj, type) {
 function dispose(obj) {
   if (util.isPrimitive(obj))
     throw new TypeError('obj must be an Object');
-  if (util.isBuffer(obj))
+  if (obj instanceof Buffer)
     throw new TypeError('obj cannot be a Buffer');
   if (smalloc.isTypedArray(obj))
     throw new TypeError('obj cannot be a typed array');
diff --git a/lib/stream.js b/lib/stream.js
index bbd6bc5aae..dd381d9737 100644
--- a/lib/stream.js
+++ b/lib/stream.js
@@ -65,7 +65,7 @@ Stream.prototype.pipe = function(dest, options) {
     if (didOnEnd) return;
     didOnEnd = true;
 
-    if (util.isFunction(dest.destroy)) dest.destroy();
+    if (typeof dest.destroy === 'function') dest.destroy();
   }
 
   // don't leave dangling pipes when there are errors.
diff --git a/lib/tls.js b/lib/tls.js
index d52adbabc8..0e13516add 100644
--- a/lib/tls.js
+++ b/lib/tls.js
@@ -36,7 +36,7 @@ exports.getCiphers = function() {
 // ("\x06spdy/2\x08http/1.1\x08http/1.0")
 exports.convertNPNProtocols = function convertNPNProtocols(NPNProtocols, out) {
   // If NPNProtocols is Array - translate it into buffer
-  if (util.isArray(NPNProtocols)) {
+  if (Array.isArray(NPNProtocols)) {
     var buff = new Buffer(NPNProtocols.reduce(function(p, c) {
       return p + 1 + Buffer.byteLength(c);
     }, 0));
@@ -53,7 +53,7 @@ exports.convertNPNProtocols = function convertNPNProtocols(NPNProtocols, out) {
   }
 
   // If it's already a Buffer - store it
-  if (util.isBuffer(NPNProtocols)) {
+  if (NPNProtocols instanceof Buffer) {
     out.NPNProtocols = NPNProtocols;
   }
 };
@@ -160,7 +160,7 @@ exports.checkServerIdentity = function checkServerIdentity(host, cert) {
     // RFC6125
     if (matchCN) {
       var commonNames = cert.subject.CN;
-      if (util.isArray(commonNames)) {
+      if (Array.isArray(commonNames)) {
         for (var i = 0, k = commonNames.length; i < k; ++i) {
           dnsNames.push(regexpify(commonNames[i], true));
         }
@@ -208,7 +208,7 @@ exports.parseCertString = function parseCertString(s) {
       var key = parts[i].slice(0, sepIndex);
       var value = parts[i].slice(sepIndex + 1);
       if (key in out) {
-        if (!util.isArray(out[key])) {
+        if (!Array.isArray(out[key])) {
           out[key] = [out[key]];
         }
         out[key].push(value);
diff --git a/lib/tty.js b/lib/tty.js
index 84731a5e75..1241d52535 100644
--- a/lib/tty.js
+++ b/lib/tty.js
@@ -1,11 +1,10 @@
 'use strict';
 
-const inherits = require('util').inherits;
+const util = require('util');
 const net = require('net');
 const TTY = process.binding('tty_wrap').TTY;
 const isTTY = process.binding('tty_wrap').isTTY;
-const util = require('util');
-
+const inherits = util.inherits;
 const errnoException = util._errnoException;
 
 
diff --git a/lib/url.js b/lib/url.js
index 50126ae9bf..65012cd7c4 100644
--- a/lib/url.js
+++ b/lib/url.js
@@ -1,7 +1,6 @@
 'use strict';
 
 const punycode = require('punycode');
-const util = require('util');
 
 exports.parse = urlParse;
 exports.resolve = urlResolve;
@@ -79,7 +78,7 @@ const slashedProtocol = {
 const querystring = require('querystring');
 
 function urlParse(url, parseQueryString, slashesDenoteHost) {
-  if (url && util.isObject(url) && url instanceof Url) return url;
+  if (url instanceof Url) return url;
 
   var u = new Url;
   u.parse(url, parseQueryString, slashesDenoteHost);
@@ -87,7 +86,7 @@ function urlParse(url, parseQueryString, slashesDenoteHost) {
 }
 
 Url.prototype.parse = function(url, parseQueryString, slashesDenoteHost) {
-  if (!util.isString(url)) {
+  if (typeof url !== 'string') {
     throw new TypeError("Parameter 'url' must be a string, not " + typeof url);
   }
 
@@ -353,7 +352,7 @@ function urlFormat(obj) {
   // If it's an obj, this is a no-op.
   // this way, you can call url_format() on strings
   // to clean up potentially wonky urls.
-  if (util.isString(obj)) obj = urlParse(obj);
+  if (typeof obj === 'string') obj = urlParse(obj);
   if (!(obj instanceof Url)) return Url.prototype.format.call(obj);
   return obj.format();
 }
@@ -383,8 +382,8 @@ Url.prototype.format = function() {
     }
   }
 
-  if (this.query &&
-      util.isObject(this.query) &&
+  if (this.query !== null &&
+      typeof this.query === 'object' &&
       Object.keys(this.query).length) {
     query = querystring.stringify(this.query);
   }
@@ -428,7 +427,7 @@ function urlResolveObject(source, relative) {
 }
 
 Url.prototype.resolveObject = function(relative) {
-  if (util.isString(relative)) {
+  if (typeof relative === 'string') {
     var rel = new Url();
     rel.parse(relative, false, true);
     relative = rel;
@@ -574,7 +573,7 @@ Url.prototype.resolveObject = function(relative) {
     srcPath = srcPath.concat(relPath);
     result.search = relative.search;
     result.query = relative.query;
-  } else if (!util.isNullOrUndefined(relative.search)) {
+  } else if (relative.search !== null && relative.search !== undefined) {
     // just pull out the search.
     // like href='?foo'.
     // Put this after the other two cases because it simplifies the booleans
@@ -593,7 +592,7 @@ Url.prototype.resolveObject = function(relative) {
     result.search = relative.search;
     result.query = relative.query;
     //to support http.request
-    if (!util.isNull(result.pathname) || !util.isNull(result.search)) {
+    if (result.pathname !== null || result.search !== null) {
       result.path = (result.pathname ? result.pathname : '') +
                     (result.search ? result.search : '');
     }
@@ -687,7 +686,7 @@ Url.prototype.resolveObject = function(relative) {
   }
 
   //to support request.http
-  if (!util.isNull(result.pathname) || !util.isNull(result.search)) {
+  if (result.pathname !== null || result.search !== null) {
     result.path = (result.pathname ? result.pathname : '') +
                   (result.search ? result.search : '');
   }
diff --git a/lib/util.js b/lib/util.js
index 7c3e03c24e..f84f528ce3 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -2,7 +2,7 @@
 
 const formatRegExp = /%[sdj%]/g;
 exports.format = function(f) {
-  if (!isString(f)) {
+  if (typeof f !== 'string') {
     var objects = [];
     for (var i = 0; i < arguments.length; i++) {
       objects.push(inspect(arguments[i]));
@@ -30,7 +30,7 @@ exports.format = function(f) {
     }
   });
   for (var x = args[i]; i < len; x = args[++i]) {
-    if (isNull(x) || !isObject(x)) {
+    if (x === null || typeof x !== 'object') {
       str += ' ' + x;
     } else {
       str += ' ' + inspect(x);
@@ -45,7 +45,7 @@ exports.format = function(f) {
 // If --no-deprecation is set, then it is a no-op.
 exports.deprecate = function(fn, msg) {
   // Allow for deprecating things in the process of starting up.
-  if (isUndefined(global.process)) {
+  if (global.process === undefined) {
     return function() {
       return exports.deprecate(fn, msg).apply(this, arguments);
     };
@@ -77,7 +77,7 @@ exports.deprecate = function(fn, msg) {
 var debugs = {};
 var debugEnviron;
 exports.debuglog = function(set) {
-  if (isUndefined(debugEnviron))
+  if (debugEnviron === undefined)
     debugEnviron = process.env.NODE_DEBUG || '';
   set = set.toUpperCase();
   if (!debugs[set]) {
@@ -112,7 +112,7 @@ function inspect(obj, opts) {
   // legacy...
   if (arguments.length >= 3) ctx.depth = arguments[2];
   if (arguments.length >= 4) ctx.colors = arguments[3];
-  if (isBoolean(opts)) {
+  if (typeof opts === 'boolean') {
     // legacy...
     ctx.showHidden = opts;
   } else if (opts) {
@@ -120,10 +120,10 @@ function inspect(obj, opts) {
     exports._extend(ctx, opts);
   }
   // set default options
-  if (isUndefined(ctx.showHidden)) ctx.showHidden = false;
-  if (isUndefined(ctx.depth)) ctx.depth = 2;
-  if (isUndefined(ctx.colors)) ctx.colors = false;
-  if (isUndefined(ctx.customInspect)) ctx.customInspect = true;
+  if (ctx.showHidden === undefined) ctx.showHidden = false;
+  if (ctx.depth === undefined) ctx.depth = 2;
+  if (ctx.colors === undefined) ctx.colors = false;
+  if (ctx.customInspect === undefined) ctx.customInspect = true;
   if (ctx.colors) ctx.stylize = stylizeWithColor;
   return formatValue(ctx, obj, ctx.depth);
 }
@@ -195,13 +195,13 @@ function formatValue(ctx, value, recurseTimes) {
   // Check that value is an object with an inspect function on it
   if (ctx.customInspect &&
       value &&
-      isFunction(value.inspect) &&
+      typeof value.inspect === 'function' &&
       // Filter out the util module, it's inspect function is special
       value.inspect !== exports.inspect &&
       // Also filter out any prototype objects using the circular check.
       !(value.constructor && value.constructor.prototype === value)) {
     var ret = value.inspect(recurseTimes, ctx);
-    if (!isString(ret)) {
+    if (typeof ret !== 'string') {
       ret = formatValue(ctx, ret, recurseTimes);
     }
     return ret;
@@ -236,7 +236,7 @@ function formatValue(ctx, value, recurseTimes) {
     // ignore...
   }
 
-  if (isString(raw)) {
+  if (typeof raw === 'string') {
     // for boxed Strings, we have to remove the 0-n indexed entries,
     // since they just noisey up the output and are redundant
     keys = keys.filter(function(key) {
@@ -246,7 +246,7 @@ function formatValue(ctx, value, recurseTimes) {
 
   // Some type of object without properties can be shortcutted.
   if (keys.length === 0) {
-    if (isFunction(value)) {
+    if (typeof value === 'function') {
       var name = value.name ? ': ' + value.name : '';
       return ctx.stylize('[Function' + name + ']', 'special');
     }
@@ -260,15 +260,15 @@ function formatValue(ctx, value, recurseTimes) {
       return formatError(value);
     }
     // now check the `raw` value to handle boxed primitives
-    if (isString(raw)) {
+    if (typeof raw === 'string') {
       formatted = formatPrimitiveNoColor(ctx, raw);
       return ctx.stylize('[String: ' + formatted + ']', 'string');
     }
-    if (isNumber(raw)) {
+    if (typeof raw === 'number') {
       formatted = formatPrimitiveNoColor(ctx, raw);
       return ctx.stylize('[Number: ' + formatted + ']', 'number');
     }
-    if (isBoolean(raw)) {
+    if (typeof raw === 'boolean') {
       formatted = formatPrimitiveNoColor(ctx, raw);
       return ctx.stylize('[Boolean: ' + formatted + ']', 'boolean');
     }
@@ -277,13 +277,13 @@ function formatValue(ctx, value, recurseTimes) {
   var base = '', array = false, braces = ['{', '}'];
 
   // Make Array say that they are Array
-  if (isArray(value)) {
+  if (Array.isArray(value)) {
     array = true;
     braces = ['[', ']'];
   }
 
   // Make functions say that they are functions
-  if (isFunction(value)) {
+  if (typeof value === 'function') {
     var n = value.name ? ': ' + value.name : '';
     base = ' [Function' + n + ']';
   }
@@ -304,19 +304,19 @@ function formatValue(ctx, value, recurseTimes) {
   }
 
   // Make boxed primitive Strings look like such
-  if (isString(raw)) {
+  if (typeof raw === 'string') {
     formatted = formatPrimitiveNoColor(ctx, raw);
     base = ' ' + '[String: ' + formatted + ']';
   }
 
   // Make boxed primitive Numbers look like such
-  if (isNumber(raw)) {
+  if (typeof raw === 'number') {
     formatted = formatPrimitiveNoColor(ctx, raw);
     base = ' ' + '[Number: ' + formatted + ']';
   }
 
   // Make boxed primitive Booleans look like such
-  if (isBoolean(raw)) {
+  if (typeof raw === 'boolean') {
     formatted = formatPrimitiveNoColor(ctx, raw);
     base = ' ' + '[Boolean: ' + formatted + ']';
   }
@@ -351,28 +351,32 @@ function formatValue(ctx, value, recurseTimes) {
 
 
 function formatPrimitive(ctx, value) {
-  if (isUndefined(value))
+  if (value === undefined)
     return ctx.stylize('undefined', 'undefined');
-  if (isString(value)) {
+
+  // For some reason typeof null is "object", so special case here.
+  if (value === null)
+    return ctx.stylize('null', 'null');
+
+  var type = typeof value;
+
+  if (type === 'string') {
     var simple = '\'' + JSON.stringify(value).replace(/^"|"$/g, '')
         .replace(/'/g, "\\'")
         .replace(/\\"/g, '"') + '\'';
     return ctx.stylize(simple, 'string');
   }
-  if (isNumber(value)) {
+  if (type === 'number') {
     // Format -0 as '-0'. Strict equality won't distinguish 0 from -0,
     // so instead we use the fact that 1 / -0 < 0 whereas 1 / 0 > 0 .
     if (value === 0 && 1 / value < 0)
       return ctx.stylize('-0', 'number');
     return ctx.stylize('' + value, 'number');
   }
-  if (isBoolean(value))
+  if (type === 'boolean')
     return ctx.stylize('' + value, 'boolean');
-  // For some reason typeof null is "object", so special case here.
-  if (isNull(value))
-    return ctx.stylize('null', 'null');
   // es6 symbol primitive
-  if (isSymbol(value))
+  if (type === 'symbol')
     return ctx.stylize(value.toString(), 'symbol');
 }
 
@@ -402,7 +406,7 @@ function formatArray(ctx, value, recurseTimes, visibleKeys, keys) {
     }
   }
   keys.forEach(function(key) {
-    if (isSymbol(key) || !key.match(/^\d+$/)) {
+    if (typeof key === 'symbol' || !key.match(/^\d+$/)) {
       output.push(formatProperty(ctx, value, recurseTimes, visibleKeys,
           key, true));
     }
@@ -426,7 +430,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
     }
   }
   if (!hasOwnProperty(visibleKeys, key)) {
-    if (isSymbol(key)) {
+    if (typeof key === 'symbol') {
       name = '[' + ctx.stylize(key.toString(), 'symbol') + ']';
     } else {
       name = '[' + key + ']';
@@ -434,7 +438,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
   }
   if (!str) {
     if (ctx.seen.indexOf(desc.value) < 0) {
-      if (isNull(recurseTimes)) {
+      if (recurseTimes === null) {
         str = formatValue(ctx, desc.value, null);
       } else {
         str = formatValue(ctx, desc.value, recurseTimes - 1);
@@ -454,7 +458,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
       str = ctx.stylize('[Circular]', 'special');
     }
   }
-  if (isUndefined(name)) {
+  if (name === undefined) {
     if (array && key.match(/^\d+$/)) {
       return str;
     }
@@ -508,7 +512,7 @@ function isNull(arg) {
 exports.isNull = isNull;
 
 function isNullOrUndefined(arg) {
-  return arg == null;
+  return arg === null || arg === undefined;
 }
 exports.isNullOrUndefined = isNullOrUndefined;
 
@@ -528,27 +532,29 @@ function isSymbol(arg) {
 exports.isSymbol = isSymbol;
 
 function isUndefined(arg) {
-  return arg === void 0;
+  return arg === undefined;
 }
 exports.isUndefined = isUndefined;
 
 function isRegExp(re) {
-  return isObject(re) && objectToString(re) === '[object RegExp]';
+  return re !== null && typeof re === 'object' &&
+         objectToString(re) === '[object RegExp]';
 }
 exports.isRegExp = isRegExp;
 
 function isObject(arg) {
-  return typeof arg === 'object' && arg !== null;
+  return arg !== null && typeof arg === 'object';
 }
 exports.isObject = isObject;
 
 function isDate(d) {
-  return isObject(d) && objectToString(d) === '[object Date]';
+  return d !== null && typeof d === 'object' &&
+         objectToString(d) === '[object Date]';
 }
 exports.isDate = isDate;
 
 function isError(e) {
-  return isObject(e) &&
+  return e !== null && typeof e === 'object' &&
       (objectToString(e) === '[object Error]' || e instanceof Error);
 }
 exports.isError = isError;
@@ -629,7 +635,7 @@ exports.inherits = function(ctor, superCtor) {
 
 exports._extend = function(origin, add) {
   // Don't do anything if add isn't an object
-  if (!add || !isObject(add)) return origin;
+  if (add === null || typeof add !== 'object') return origin;
 
   var keys = Object.keys(add);
   var i = keys.length;
@@ -724,7 +730,7 @@ exports.pump = exports.deprecate(function(readStream, writeStream, callback) {
 
 var uv;
 exports._errnoException = function(err, syscall, original) {
-  if (isUndefined(uv)) uv = process.binding('uv');
+  if (uv === undefined) uv = process.binding('uv');
   var errname = uv.errname(err);
   var message = syscall + ' ' + errname;
   if (original)
diff --git a/lib/vm.js b/lib/vm.js
index 8a190fa9ed..b4a2b99990 100644
--- a/lib/vm.js
+++ b/lib/vm.js
@@ -2,7 +2,6 @@
 
 const binding = process.binding('contextify');
 const Script = binding.ContextifyScript;
-const util = require('util');
 
 // The binding provides a few useful primitives:
 // - ContextifyScript(code, { filename = "evalmachine.anonymous",
@@ -26,7 +25,7 @@ exports.createScript = function(code, options) {
 };
 
 exports.createContext = function(sandbox) {
-  if (util.isUndefined(sandbox)) {
+  if (sandbox === undefined) {
     sandbox = {};
   } else if (binding.isContext(sandbox)) {
     return sandbox;
diff --git a/lib/zlib.js b/lib/zlib.js
index 53e8eb66fd..14ccf8f959 100644
--- a/lib/zlib.js
+++ b/lib/zlib.js
@@ -92,7 +92,7 @@ exports.createUnzip = function(o) {
 // Convenience methods.
 // compress/decompress a string or buffer in one step.
 exports.deflate = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -104,7 +104,7 @@ exports.deflateSync = function(buffer, opts) {
 };
 
 exports.gzip = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -116,7 +116,7 @@ exports.gzipSync = function(buffer, opts) {
 };
 
 exports.deflateRaw = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -128,7 +128,7 @@ exports.deflateRawSync = function(buffer, opts) {
 };
 
 exports.unzip = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -140,7 +140,7 @@ exports.unzipSync = function(buffer, opts) {
 };
 
 exports.inflate = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -152,7 +152,7 @@ exports.inflateSync = function(buffer, opts) {
 };
 
 exports.gunzip = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -164,7 +164,7 @@ exports.gunzipSync = function(buffer, opts) {
 };
 
 exports.inflateRaw = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -209,9 +209,9 @@ function zlibBuffer(engine, buffer, callback) {
 }
 
 function zlibBufferSync(engine, buffer) {
-  if (util.isString(buffer))
+  if (typeof buffer === 'string')
     buffer = new Buffer(buffer);
-  if (!util.isBuffer(buffer))
+  if (!(buffer instanceof Buffer))
     throw new TypeError('Not a string or buffer');
 
   var flushFlag = binding.Z_FINISH;
@@ -327,7 +327,7 @@ function Zlib(opts, mode) {
   }
 
   if (opts.dictionary) {
-    if (!util.isBuffer(opts.dictionary)) {
+    if (!(opts.dictionary instanceof Buffer)) {
       throw new Error('Invalid dictionary: it should be a Buffer instance');
     }
   }
@@ -349,10 +349,10 @@ function Zlib(opts, mode) {
   };
 
   var level = exports.Z_DEFAULT_COMPRESSION;
-  if (util.isNumber(opts.level)) level = opts.level;
+  if (typeof opts.level === 'number') level = opts.level;
 
   var strategy = exports.Z_DEFAULT_STRATEGY;
-  if (util.isNumber(opts.strategy)) strategy = opts.strategy;
+  if (typeof opts.strategy === 'number') strategy = opts.strategy;
 
   this._handle.init(opts.windowBits || exports.Z_DEFAULT_WINDOWBITS,
                     level,
@@ -414,7 +414,7 @@ Zlib.prototype._flush = function(callback) {
 Zlib.prototype.flush = function(kind, callback) {
   var ws = this._writableState;
 
-  if (util.isFunction(kind) || (util.isUndefined(kind) && !callback)) {
+  if (typeof kind === 'function' || (kind === undefined && !callback)) {
     callback = kind;
     kind = binding.Z_FULL_FLUSH;
   }
@@ -459,7 +459,7 @@ Zlib.prototype._transform = function(chunk, encoding, cb) {
   var ending = ws.ending || ws.ended;
   var last = ending && (!chunk || ws.length === chunk.length);
 
-  if (!util.isNull(chunk) && !util.isBuffer(chunk))
+  if (chunk !== null && !(chunk instanceof Buffer))
     return cb(new Error('invalid input'));
 
   if (this._closed)
@@ -490,7 +490,7 @@ Zlib.prototype._processChunk = function(chunk, flushFlag, cb) {
 
   var self = this;
 
-  var async = util.isFunction(cb);
+  var async = typeof cb === 'function';
 
   if (!async) {
     var buffers = [];