Two symbols are equal by valueOf(). Closes jashkenas#2450

test/objects.js

@@ -565,6 +565,14 @@
     assert.equal(_.isEqual({a: 0}, {a: -0}), false);
     assert.equal(_.isEqual([NaN], [NaN]), true);
     assert.equal(_.isEqual({a: NaN}, {a: NaN}), true);
+
+    if (typeof Symbol !== 'undefined') {
+      var symbol = Symbol('x');
+      assert.strictEqual(_.isEqual(symbol, symbol), true, 'A symbol is equal to itself');
+      assert.strictEqual(_.isEqual(symbol, Object(symbol)), true, 'Even when wrapped in Object()');
+      assert.strictEqual(_.isEqual(symbol, null), false, 'Different types are not equal');
+    }
+
   });
 
   QUnit.test('isEmpty', function(assert) {

underscore.js

@@ -20,6 +20,7 @@
 
   // Save bytes in the minified (but not gzipped) version:
   var ArrayProto = Array.prototype, ObjProto = Object.prototype;
+  var SymbolProto = typeof Symbol !== 'undefined' ? Symbol.prototype : null;
 
   // Create quick reference variables for speed access to core prototypes.
   var push = ArrayProto.push,
@@ -1195,6 +1196,8 @@
         // millisecond representations. Note that invalid dates with millisecond representations
         // of `NaN` are not equivalent.
         return +a === +b;
+      case '[object Symbol]':
+        return SymbolProto.valueOf.call(a) === SymbolProto.valueOf.call(b);
     }
 
     var areArrays = className === '[object Array]';