Reimplemented _.isEqual tests and patch in a fresh branch.

test/objects.js

@@ -295,6 +295,58 @@ $(document).ready(function() {
     ok(_.isEqual(isEqualObjClone, isEqualObj), 'Commutative equality is implemented for objects with custom `isEqual` methods');
     ok(!_.isEqual(isEqualObj, {}), 'Objects that do not implement equivalent `isEqual` methods are not equal');
     ok(!_.isEqual({}, isEqualObj), 'Commutative equality is implemented for objects with different `isEqual` methods');
+
+    // Custom `isEqual` methods - comparing different types
+    LocalizedString = (function() {
+      function LocalizedString(id) { this.id = id; this.string = (this.id===10)? 'Bonjour': ''; }
+      LocalizedString.prototype.isEqual = function(that) {
+        if (_.isString(that)) return this.string == that;
+        else if (that instanceof LocalizedString) return this.id == that.id;
+        return false;
+      };
+      return LocalizedString;
+    })();
+    var localized_string1 = new LocalizedString(10), localized_string2 = new LocalizedString(10), localized_string3 = new LocalizedString(11);
+    ok(_.isEqual(localized_string1, localized_string2), 'comparing same typed instances with same ids');
+    ok(!_.isEqual(localized_string1, localized_string3), 'comparing same typed instances with different ids');
+    ok(_.isEqual(localized_string1, 'Bonjour'), 'comparing different typed instances with same values');
+    ok(_.isEqual('Bonjour', localized_string1), 'comparing different typed instances with same values');
+    ok(!_.isEqual('Bonjour', localized_string3), 'comparing two localized strings with different ids');
+    ok(!_.isEqual(localized_string1, 'Au revoir'), 'comparing different typed instances with different values');
+    ok(!_.isEqual('Au revoir', localized_string1), 'comparing different typed instances with different values');
+
+    // Custom `isEqual` methods - comparing with serialized data
+    Date.prototype.toJSON = function() {
+      return {
+        _type:'Date',
+        year:this.getUTCFullYear(),
+        month:this.getUTCMonth(),
+        day:this.getUTCDate(),
+        hours:this.getUTCHours(),
+        minutes:this.getUTCMinutes(),
+        seconds:this.getUTCSeconds()
+      };
+    };
+    Date.prototype.isEqual = function(that) {
+      var this_date_components = this.toJSON();
+      var that_date_components = (that instanceof Date) ? that.toJSON() : that;
+      delete this_date_components['_type']; delete that_date_components['_type']
+      return _.isEqual(this_date_components, that_date_components);
+    };
+
+    var date = new Date();
+    var date_json = {
+      _type:'Date',
+      year:date.getUTCFullYear(),
+      month:date.getUTCMonth(),
+      day:date.getUTCDate(),
+      hours:date.getUTCHours(),
+      minutes:date.getUTCMinutes(),
+      seconds:date.getUTCSeconds()
+    };
+
+    ok(_.isEqual(date_json, date), 'serialized date matches date');
+    ok(_.isEqual(date, date_json), 'date matches serialized date');
   });
 
   test("objects: isEmpty", function() {

underscore.js

@@ -635,7 +635,15 @@
     // See the Harmony `egal` proposal: http://wiki.ecmascript.org/doku.php?id=harmony:egal.
     if (a === b) return a !== 0 || 1 / a == 1 / b;
     // A strict comparison is necessary because `null == undefined`.
-    if (a == null) return a === b;
+    if ((a == null) || (b == null)) return a === b;
+    // Either one is undefined
+    if ((a === void 0) || (b === void 0)) return false;
+    // Unwrap any wrapped objects.
+    if (a._chain) a = a._wrapped;
+    if (b._chain) b = b._wrapped;
+    // One of them implements an isEqual()
+    if (a.isEqual) return a.isEqual(b);
+    if (b.isEqual) return b.isEqual(a);
     // Compare object types.
     var typeA = typeof a;
     if (typeA != typeof b) return false;
@@ -667,9 +675,6 @@
     }
     // Ensure that both values are objects.
     if (typeA != 'object') return false;
-    // Unwrap any wrapped objects.
-    if (a._chain) a = a._wrapped;
-    if (b._chain) b = b._wrapped;
     // Invoke a custom `isEqual` method if one is provided.
     if (_.isFunction(a.isEqual)) return a.isEqual(b);
     // Assume equality for cyclic structures. The algorithm for detecting cyclic structures is