Version 2.5.8...

src/array.js

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
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   return Join(this, this.length, ',', ConvertToLocaleString);
 }
 
 
 function ArrayJoin(separator) {
   if (IS_UNDEFINED(separator)) {
     separator = ',';
   } else if (!IS_STRING(separator)) {
     separator = ToString(separator);
   }
+
+  var result = %_FastAsciiArrayJoin(this, separator);
+  if (typeof result != "undefined") return result;
+
   var length = TO_UINT32(this.length);
   return Join(this, length, separator, ConvertToString);
 }
 
 
 // Removes the last element from the array and returns it. See
 // ECMA-262, section 15.4.4.6.
 function ArrayPop() {
   var n = TO_UINT32(this.length);
   if (n == 0) {
@@ skipping 787 matching lines @@
     "lastIndexOf", getFunction("lastIndexOf", ArrayLastIndexOf, 1),
     "reduce", getFunction("reduce", ArrayReduce, 1),
     "reduceRight", getFunction("reduceRight", ArrayReduceRight, 1)
   ));
 
   %FinishArrayPrototypeSetup($Array.prototype);
 }
 
 
 SetupArray();