1 files changed, 146 insertions, 4 deletions

diff --git a/lib/internal/errors.js b/lib/internal/errors.js
index 6e6852f380..6bbb5d3548 100644
--- a/lib/internal/errors.js
+++ b/lib/internal/errors.js
@@ -132,22 +132,164 @@ class SystemError extends makeNodeError(Error) {
   }
 }
 
+function createErrDiff(actual, expected, operator) {
+  var other = '';
+  var res = '';
+  var lastPos = 0;
+  var end = '';
+  var skipped = false;
+  const actualLines = util
+    .inspect(actual, { compact: false }).split('\n');
+  const expectedLines = util
+    .inspect(expected, { compact: false }).split('\n');
+  const msg = `Input A expected to ${operator} input B:\n` +
+        '\u001b[32m+ expected\u001b[39m \u001b[31m- actual\u001b[39m';
+  const skippedMsg = ' ... Lines skipped';
+
+  // Remove all ending lines that match (this optimizes the output for
+  // readability by reducing the number of total changed lines).
+  var a = actualLines[actualLines.length - 1];
+  var b = expectedLines[expectedLines.length - 1];
+  var i = 0;
+  while (a === b) {
+    if (i++ < 2) {
+      end = `\n  ${a}${end}`;
+    } else {
+      other = a;
+    }
+    actualLines.pop();
+    expectedLines.pop();
+    a = actualLines[actualLines.length - 1];
+    b = expectedLines[expectedLines.length - 1];
+  }
+  if (i > 3) {
+    end = `\n...${end}`;
+    skipped = true;
+  }
+  if (other !== '') {
+    end = `\n  ${other}${end}`;
+    other = '';
+  }
+
+  const maxLines = Math.max(actualLines.length, expectedLines.length);
+  var printedLines = 0;
+  for (i = 0; i < maxLines; i++) {
+    // Only extra expected lines exist
+    const cur = i - lastPos;
+    if (actualLines.length < i + 1) {
+      if (cur > 1 && i > 2) {
+        if (cur > 4) {
+          res += '\n...';
+          skipped = true;
+        } else if (cur > 3) {
+          res += `\n  ${expectedLines[i - 2]}`;
+          printedLines++;
+        }
+        res += `\n  ${expectedLines[i - 1]}`;
+        printedLines++;
+      }
+      lastPos = i;
+      other += `\n\u001b[32m+\u001b[39m ${expectedLines[i]}`;
+      printedLines++;
+    // Only extra actual lines exist
+    } else if (expectedLines.length < i + 1) {
+      if (cur > 1 && i > 2) {
+        if (cur > 4) {
+          res += '\n...';
+          skipped = true;
+        } else if (cur > 3) {
+          res += `\n  ${actualLines[i - 2]}`;
+          printedLines++;
+        }
+        res += `\n  ${actualLines[i - 1]}`;
+        printedLines++;
+      }
+      lastPos = i;
+      res += `\n\u001b[31m-\u001b[39m ${actualLines[i]}`;
+      printedLines++;
+    // Lines diverge
+    } else if (actualLines[i] !== expectedLines[i]) {
+      if (cur > 1 && i > 2) {
+        if (cur > 4) {
+          res += '\n...';
+          skipped = true;
+        } else if (cur > 3) {
+          res += `\n  ${actualLines[i - 2]}`;
+          printedLines++;
+        }
+        res += `\n  ${actualLines[i - 1]}`;
+        printedLines++;
+      }
+      lastPos = i;
+      res += `\n\u001b[31m-\u001b[39m ${actualLines[i]}`;
+      other += `\n\u001b[32m+\u001b[39m ${expectedLines[i]}`;
+      printedLines += 2;
+    // Lines are identical
+    } else {
+      res += other;
+      other = '';
+      if (cur === 1 || i === 0) {
+        res += `\n  ${actualLines[i]}`;
+        printedLines++;
+      }
+    }
+    // Inspected object to big (Show ~20 rows max)
+    if (printedLines > 20 && i < maxLines - 2) {
+      return `${msg}${skippedMsg}\n${res}\n...${other}\n...`;
+    }
+  }
+  return `${msg}${skipped ? skippedMsg : ''}\n${res}${other}${end}`;
+}
+
 class AssertionError extends Error {
   constructor(options) {
     if (typeof options !== 'object' || options === null) {
       throw new exports.TypeError('ERR_INVALID_ARG_TYPE', 'options', 'Object');
     }
-    var { actual, expected, message, operator, stackStartFn } = options;
+    var {
+      actual,
+      expected,
+      message,
+      operator,
+      stackStartFn,
+      errorDiff = 0
+    } = options;
+
     if (message != null) {
       super(message);
     } else {
+      if (util === null) util = require('util');
+
       if (actual && actual.stack && actual instanceof Error)
         actual = `${actual.name}: ${actual.message}`;
       if (expected && expected.stack && expected instanceof Error)
         expected = `${expected.name}: ${expected.message}`;
-      if (util === null) util = require('util');
-      super(`${util.inspect(actual).slice(0, 128)} ` +
-        `${operator} ${util.inspect(expected).slice(0, 128)}`);
+
+      if (errorDiff === 0) {
+        let res = util.inspect(actual);
+        let other = util.inspect(expected);
+        if (res.length > 128)
+          res = `${res.slice(0, 125)}...`;
+        if (other.length > 128)
+          other = `${other.slice(0, 125)}...`;
+        super(`${res} ${operator} ${other}`);
+      } else if (errorDiff === 1) {
+        // In case the objects are equal but the operator requires unequal, show
+        // the first object and say A equals B
+        const res = util
+          .inspect(actual, { compact: false }).split('\n');
+
+        if (res.length > 20) {
+          res[19] = '...';
+          while (res.length > 20) {
+            res.pop();
+          }
+        }
+        // Only print a single object.
+        super(`Identical input passed to ${operator}:\n${res.join('\n')}`);
+      } else {
+        super(createErrDiff(actual, expected, operator));
+      }
     }
 
     this.generatedMessage = !message;