es6 WeakMap.js

/*
  Strong implementation must check that key is object, 
  hide weakMapN_ from enumeration, make it externally readonly and non deletable(Object.defineProperty, if presents),
  but its bad for performance.
*/

(function($G)
{
  var nextStoreKeyId = 0;
  
  $G.WeakMap = function()
  {
    this.storeKeyName_ = 'weakMap' + nextStoreKeyId++ + '_';
  };

  $G.WeakMap.prototype._set = function(key, value)
  {
    key[this.storeKeyName_] = value;
  };

  $G.WeakMap.prototype._get = function(key)
  {
    return key[this.storeKeyName_];
  };

  $G.WeakMap.prototype._delete = function(key)
  {
    return delete key[this.storeKeyName_];
  };

  $G.WeakMap.prototype._has = function(key)
  {
    return this.storeKeyName_ in key;
  };
})(this);