Reimplement Maps and Sets in JS

src/runtime.js

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This files contains runtime support implemented in JavaScript.
 
 // CAUTION: Some of the functions specified in this file are called
 // directly from compiled code. These are the functions with names in
 // ALL CAPS. The compiled code passes the first argument in 'this'.
 
@@ skipping 595 matching lines @@
     // x is +0 and y is -0 or vice versa.
     if (x === 0 && y === 0 && %_IsMinusZero(x) != %_IsMinusZero(y)) {
       return false;
     }
   }
   return x === y;
 }
 
 // ES6, section 7.2.4
 function SameValueZero(x, y) {
-  if (typeof x != typeof y) return false;
-  if (IS_NUMBER(x)) {
-    if (NUMBER_IS_NAN(x) && NUMBER_IS_NAN(y)) return true;
-  }
-  return x === y;
+  if (x === y) return true;
+  return IS_NUMBER(x) && NUMBER_IS_NAN(x) && IS_NUMBER(y) && NUMBER_IS_NAN(y);
 }
+%SetInlineBuiltinFlag(SameValueZero);
 
 
 /* ---------------------------------
    - - -   U t i l i t i e s   - - -
    ---------------------------------
 */
 
 // Returns if the given x is a primitive value - not an object or a
 // function.
 function IsPrimitive(x) {
@@ skipping 55 matching lines @@
   return i;
 }
 
 
 // NOTE: Setting the prototype for Array must take place as early as
 // possible due to code generation for array literals.  When
 // generating code for a array literal a boilerplate array is created
 // that is cloned when running the code.  It is essential that the
 // boilerplate gets the right prototype.
 %FunctionSetPrototype($Array, new $Array(0));