Simplify Join and speedup joining arrays of numbers.

src/array.js

 // Copyright 2010 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
@@ skipping 99 matching lines @@
 
   // Attempt to convert the elements.
   try {
     if (UseSparseVariant(array, length, is_array) && (separator.length == 0)) {
       return SparseJoin(array, length, convert);
     }
 
     // Fast case for one-element arrays.
     if (length == 1) {
       var e = array[0];
-      if (!IS_UNDEFINED(e) || (0 in array)) {
-        if (IS_STRING(e)) return e;
-        return convert(e);
-      }
-      return '';
+      if (IS_STRING(e)) return e;
+      return convert(e);
     }
 
     // Construct an array for the elements.
     var elements = new $Array(length);
-    var elements_length = 0;
 
     // We pull the empty separator check outside the loop for speed!
     if (separator.length == 0) {
+      var elements_length = 0;
       for (var i = 0; i < length; i++) {
         var e = array[i];
         if (!IS_UNDEFINED(e)) {
           if (!IS_STRING(e)) e = convert(e);
           elements[elements_length++] = e;
         }
       }
       elements.length = elements_length;
       var result = %_FastAsciiArrayJoin(elements, '');
       if (!IS_UNDEFINED(result)) return result;
       return %StringBuilderConcat(elements, elements_length, '');
     }
-    // Non-empty separator.
-    for (var i = 0; i < length; i++) {
-      var e = array[i];
-      if (!IS_UNDEFINED(e)) {
+    // Non-empty separator case.
+    // If the first element is a number then use the heuristic that the 
+    // remaining elements are also likely to be numbers.
+    if (!IS_NUMBER(array[0])) {
+      for (var i = 0; i < length; i++) {
+        var e = array[i];
         if (!IS_STRING(e)) e = convert(e);
         elements[i] = e;
-      } else {
-        elements[i] = '';
       }
-    }
+    } else { 
+      for (var i = 0; i < length; i++) {
+        var e = array[i];
+        if (IS_NUMBER(e)) elements[i] = %_NumberToString(e);
+        else {
+          if (!IS_STRING(e)) e = convert(e);
+          elements[i] = e;
+        }
+      }
+    }   
     var result = %_FastAsciiArrayJoin(elements, separator);
     if (!IS_UNDEFINED(result)) return result;   
 
     var length2 = (length << 1) - 1;
     var j = length2;
     var i = length;
     elements[--j] = elements[--i];
     while (i > 0) {
       elements[--j] = separator;
       elements[--j] = elements[--i];
@@ skipping 1056 matching lines @@
     "lastIndexOf", getFunction("lastIndexOf", ArrayLastIndexOf, 1),
     "reduce", getFunction("reduce", ArrayReduce, 1),
     "reduceRight", getFunction("reduceRight", ArrayReduceRight, 1)
   ));
 
   %FinishArrayPrototypeSetup($Array.prototype);
 }
 
 
 SetupArray();