Optimized Array.prototype.{lastIndexOf,indexOf} by special casing comparison to undefined.

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
@@ skipping 976 matching lines @@
   var length = this.length;
   if (index == null) {
     index = 0;
   } else {
     index = TO_INTEGER(index);
     // If index is negative, index from the end of the array.
     if (index < 0) index = length + index;
     // If index is still negative, search the entire array.
     if (index < 0) index = 0;
   }
+  if (!IS_UNDEFINED(element)) {
+    for (var i = index; i < length; i++) {
+      if (this[i] === element) return i;
+    }
+    return -1;
+  }
   // Lookup through the array.
   for (var i = index; i < length; i++) {
-    var current = this[i];
-    if (!IS_UNDEFINED(current) || i in this) {
-      if (current === element) return i;
+    if (IS_UNDEFINED(this[i]) && i in this) {
+      return i;
     }
   }
   return -1;
 }
 
 
 function ArrayLastIndexOf(element, index) {
   var length = this.length;
   if (index == null) {
     index = length - 1;
   } else {
     index = TO_INTEGER(index);
     // If index is negative, index from end of the array.
     if (index < 0) index = length + index;
     // If index is still negative, do not search the array.
     if (index < 0) index = -1;
     else if (index >= length) index = length - 1;
   }
   // Lookup through the array.
+  if (!IS_UNDEFINED(element)) {
+    for (var i = index; i >= 0; i--) {
+      if (this[i] === element) return i;
+    }
+    return -1;
+  }
   for (var i = index; i >= 0; i--) {
-    var current = this[i];
-    if (!IS_UNDEFINED(current) || i in this) {
-      if (current === element) return i;
+    if (IS_UNDEFINED(this[i]) && i in this) {
+      return i;
     }
   }
   return -1;
 }
 
 
 function ArrayReduce(callback, current) {
   if (!IS_FUNCTION(callback)) {
     throw MakeTypeError('called_non_callable', [callback]);
   }
@@ skipping 104 matching lines @@
     "lastIndexOf", getFunction("lastIndexOf", ArrayLastIndexOf, 1),
     "reduce", getFunction("reduce", ArrayReduce, 1),
     "reduceRight", getFunction("reduceRight", ArrayReduceRight, 1)
   ));
     
   %FinishArrayPrototypeSetup($Array.prototype);
 }
 
 
 SetupArray();