For-of calls [Symbol.iterator]() on RHS to get iterator

src/array-iterator.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Array = global.Array;
 
 
 var arrayIteratorObjectSymbol = GLOBAL_PRIVATE("ArrayIterator#object");
 var arrayIteratorNextIndexSymbol = GLOBAL_PRIVATE("ArrayIterator#next");
 var arrayIterationKindSymbol = GLOBAL_PRIVATE("ArrayIterator#kind");
 
 
 function ArrayIterator() {}
 
 
+// TODO(wingo): Update section numbers when ES6 has stabilized.  The
+// section numbers below are already out of date as of the May 2014
+// draft.
+
+
 // 15.4.5.1 CreateArrayIterator Abstract Operation
 function CreateArrayIterator(array, kind) {
   var object = ToObject(array);
   var iterator = new ArrayIterator;
   SET_PRIVATE(iterator, arrayIteratorObjectSymbol, object);
   SET_PRIVATE(iterator, arrayIteratorNextIndexSymbol, 0);
   SET_PRIVATE(iterator, arrayIterationKindSymbol, kind);
   return iterator;
 }
 
 
 // 15.19.4.3.4 CreateItrResultObject
 function CreateIteratorResultObject(value, done) {
   return {value: value, done: done};
 }
 
 
+// 22.1.5.2.2 %ArrayIteratorPrototype%[@@iterator]
+function ArrayIteratorIterator() {
+    return this;
+}
+
+
 // 15.4.5.2.2 ArrayIterator.prototype.next( )
 function ArrayIteratorNext() {
   var iterator = ToObject(this);
 
   if (!HAS_PRIVATE(iterator, arrayIteratorObjectSymbol)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['Array Iterator.prototype.next']);
   }
 
   var array = GET_PRIVATE(iterator, arrayIteratorObjectSymbol);
@@ skipping 43 matching lines @@
 
 function SetUpArrayIterator() {
   %CheckIsBootstrapping();
 
   %FunctionSetPrototype(ArrayIterator, new $Object());
   %FunctionSetInstanceClassName(ArrayIterator, 'Array Iterator');
 
   InstallFunctions(ArrayIterator.prototype, DONT_ENUM, $Array(
     'next', ArrayIteratorNext
   ));
+  %FunctionSetName(ArrayIteratorIterator, '[Symbol.iterator]');
+  %SetProperty(ArrayIterator.prototype, symbolIterator, ArrayIteratorIterator,
+      DONT_ENUM | DONT_DELETE | READ_ONLY);
 }
 SetUpArrayIterator();
 
 
 function ExtendArrayPrototype() {
   %CheckIsBootstrapping();
 
   InstallFunctions($Array.prototype, DONT_ENUM, $Array(
     'entries', ArrayEntries,
     'values', ArrayValues,
     'keys', ArrayKeys
   ));
 }
 ExtendArrayPrototype();