gistfile1.diff

Index: src/array.js
===================================================================
--- src/array.js	(revision 12923)
+++ src/array.js	(working copy)
@@ -1239,8 +1239,8 @@
   var accumulator = new InternalArray(length);
   if (%DebugCallbackSupportsStepping(f)) {
     for (var i = 0; i < length; i++) {
-      if (i in array) {
-        var element = array[i];
+      var element = array[i];
+      if (!IS_UNDEFINED(element)) {
         // Prepare break slots for debugger step in.
         %DebugPrepareStepInIfStepping(f);
         accumulator[i] = %_CallFunction(receiver, element, i, array, f);
@@ -1249,8 +1249,8 @@
   } else {
     // This is a duplicate of the previous loop sans debug stepping.
     for (var i = 0; i < length; i++) {
-      if (i in array) {
-        var element = array[i];
+      var element = array[i];
+      if (!IS_UNDEFINED(element)) {
         accumulator[i] = %_CallFunction(receiver, element, i, array, f);
       }
     }