Array.prototype.{reduce, reduceRight}: Wrong order of operations when determining initial value.

src/array.js

 // Copyright 2012 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 declarations have been made
 // in runtime.js:
 // var $Array = global.Array;
 
@@ skipping 1380 matching lines @@
   var array = ToObject(this);
   var length = ToUint32(array.length);
 
   if (!IS_SPEC_FUNCTION(callback)) {
     throw MakeTypeError('called_non_callable', [callback]);
   }
 
   var i = 0;
   find_initial: if (%_ArgumentsLength() < 2) {
     for (; i < length; i++) {
-      current = array[i];
-      if (!IS_UNDEFINED(current) || i in array) {
-        i++;
+      if (i in array) {
+        current = array[i++];
         break find_initial;
       }
     }
     throw MakeTypeError('reduce_no_initial', []);
   }
 
   var receiver = %GetDefaultReceiver(callback);
   var stepping = DEBUG_IS_ACTIVE && %DebugCallbackSupportsStepping(callback);
   for (; i < length; i++) {
     if (i in array) {
@@ skipping 14 matching lines @@
   var array = ToObject(this);
   var length = ToUint32(array.length);
 
   if (!IS_SPEC_FUNCTION(callback)) {
     throw MakeTypeError('called_non_callable', [callback]);
   }
 
   var i = length - 1;
   find_initial: if (%_ArgumentsLength() < 2) {
     for (; i >= 0; i--) {
-      current = array[i];
-      if (!IS_UNDEFINED(current) || i in array) {
-        i--;
+      if (i in array) {
+        current = array[i--];
         break find_initial;
       }
     }
     throw MakeTypeError('reduce_no_initial', []);
   }
 
   var receiver = %GetDefaultReceiver(callback);
   var stepping = DEBUG_IS_ACTIVE && %DebugCallbackSupportsStepping(callback);
   for (; i >= 0; i--) {
     if (i in array) {
@@ skipping 96 matching lines @@
   ));
 
   SetUpLockedPrototype(InternalPackedArray, $Array(), $Array(
     "join", getFunction("join", ArrayJoin),
     "pop", getFunction("pop", ArrayPop),
     "push", getFunction("push", ArrayPush)
   ));
 }
 
 SetUpArray();