Implemented diffJson.

It takes two objects, serializes them as canonical JSON, then does a line-based diff that ignores differences in trailing commas.

See discussion here: mochajs/mocha#1182

diff.js

@@ -127,8 +127,11 @@ var JsDiff = (function() {
         while (newPos+1 < newLen && oldPos+1 < oldLen && this.equals(newString[newPos+1], oldString[oldPos+1])) {
           newPos++;
           oldPos++;
-
-          this.pushComponent(basePath.components, newString[newPos], undefined, undefined);
+          var value = newString[newPos];
+          if (this.useLongestToken && oldString[oldPos].length > value.length) {
+            value = oldString[oldPos];
+          }
+          this.pushComponent(basePath.components, value, undefined, undefined);
         }
         basePath.newPos = newPos;
         return oldPos;
@@ -183,6 +186,52 @@ var JsDiff = (function() {
     return retLines;
   };
 
+  JsonDiff = new Diff();
+  JsonDiff.useLongestToken = true;
+  JsonDiff.tokenize = LineDiff.tokenize;
+  JsonDiff.equals = function(left, right) {
+    return LineDiff.equals(left.replace(/,([\r\n])/g, '$1'), right.replace(/,([\r\n])/g, '$1'));
+  };
+
+  function canonicalize(obj, stack) {
+    stack = stack || [];
+
+    var i;
+
+    for (var i = 0 ; i < stack.length ; i += 1) {
+      if (stack[i] === obj) {
+        return obj;
+      }
+    }
+
+    var canonicalizedObj;
+
+    if ('[object Array]' == {}.toString.call(obj)) {
+      stack.push(obj);
+      canonicalizedObj = new Array(obj.length);
+      for (i = 0 ; i < obj.length ; i += 1) {
+        canonicalizedObj[i] = canonicalize(obj[i], stack);
+      }
+      stack.pop();
+    } else if (typeof obj === 'object' && obj !== null) {
+      stack.push(obj);
+      canonicalizedObj = {};
+      var sortedKeys = [];
+      for (var key in obj) {
+        sortedKeys.push(key);
+      }
+      sortedKeys.sort();
+      for (i = 0 ; i < sortedKeys.length ; i += 1) {
+        var key = sortedKeys[i];
+        canonicalizedObj[key] = canonicalize(obj[key], stack);
+      }
+      stack.pop();
+    } else {
+      canonicalizedObj = obj;
+    }
+    return canonicalizedObj;
+  }
+
   return {
     Diff: Diff,
 
@@ -191,6 +240,13 @@ var JsDiff = (function() {
     diffWordsWithSpace: function(oldStr, newStr) { return WordWithSpaceDiff.diff(oldStr, newStr); },
     diffLines: function(oldStr, newStr) { return LineDiff.diff(oldStr, newStr); },
 
+    diffJson: function(oldObj, newObj) {
+      return JsonDiff.diff(
+        JSON.stringify(canonicalize(oldObj), undefined, "  "),
+        JSON.stringify(canonicalize(newObj), undefined, "  ")
+      );
+    },
+
     diffCss: function(oldStr, newStr) { return CssDiff.diff(oldStr, newStr); },
 
     createPatch: function(fileName, oldStr, newStr, oldHeader, newHeader) {

test/diffTest.js

@@ -112,6 +112,22 @@ describe('#diffLines', function() {
   });
 });
 
+describe('#diffJson', function () {
+  it('should ignore trailing comma on the previous line when the property has been removed', function() {
+    var diffResult = diff.diffJson(
+      {a: 123, b: 456, c: 789},
+      {a: 123, b: 456});
+    diff.convertChangesToXML(diffResult).should.equal('{\n  &quot;a&quot;: 123,\n  &quot;b&quot;: 456,\n<del>  &quot;c&quot;: 789\n</del>}');
+  });
+
+  it('should ignore the missing trailing comma on the last line when a property has been added after it', function() {
+    var diffResult = diff.diffJson(
+      {a: 123, b: 456},
+      {a: 123, b: 456, c: 789});
+    diff.convertChangesToXML(diffResult).should.equal('{\n  &quot;a&quot;: 123,\n  &quot;b&quot;: 456,\n<ins>  &quot;c&quot;: 789\n</ins>}');
+  });
+});
+
 describe('convertToDMP', function() {
   it('should output diff-match-patch format', function() {
     var diffResult = diff.diffWords('New Value  ', 'New  ValueMoreData ');