JS implementation of isFinite.

src/v8natives.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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 // ECMA 262 - 15.1.4
 function GlobalIsNaN(number) {
   var n = ToNumber(number);
   return NUMBER_IS_NAN(n);
 }
 
 
 // ECMA 262 - 15.1.5
 function GlobalIsFinite(number) {
-  return %NumberIsFinite(ToNumber(number));
+  if (!IS_NUMBER(number)) number = ToNumber(number);
+
+  // NaN - NaN == NaN, Infinity - Infinity == NaN, -Infinity - -Infinity == NaN.
+  return %_IsSmi(number) || number - number == 0;
 }
 
 
 // ECMA-262 - 15.1.2.2
 function GlobalParseInt(string, radix) {
   if (IS_UNDEFINED(radix)) {
     // Some people use parseInt instead of Math.floor.  This
     // optimization makes parseInt on a Smi 12 times faster (60ns
     // vs 800ns).  The following optimization makes parseInt on a
     // non-Smi number 9 times faster (230ns vs 2070ns).  Together
@@ skipping 902 matching lines @@
 
 // ----------------------------------------------------------------------------
 
 function SetupFunction() {
   InstallFunctions($Function.prototype, DONT_ENUM, $Array(
     "toString", FunctionToString
   ));
 }
 
 SetupFunction();