Javascript private variables + Object.create (reference to closure variables)

Question

I was wondering how I could make private variables in javascript through clojure. But still have them cloned when using Object.create.

var point = {};
(function(){
  var x, y;
  x = 0;
  y = 0;
Object.defineProperties(point, {
    "x": {
      set: function (value) {
        x = value;
      },
      get: function() {
        return x;
      }
    },
    "y": {
      set: function (value) {
        y = value;
      },
      get: function () {
        return y;
      }
    }
  });
}());

var p1 = Object.create(point);
p1.x = 100;
console.log(p1.x); // = 100
var p2 = Object.create(point);
p2.x = 200;
console.log(p2.x); //= 200
console.log(p1.x); //= 200

I got this technique from http://ejohn.org/blog/ecmascript-5-objects-and-properties/ but it got this limitation that the closure variables is the same on all Objects. I know this behaviour on javascript is supposed but how can I create true private variables?

this code looks auto generated, was it generated by clojure code? — Raynos, Apr 08 '12 at 22:38
As an alternative without private variables could be http://jsfiddle.net/heera/G9E9m/ — The Alpha, Apr 08 '12 at 23:34

score 3 · Accepted Answer · answered Apr 08 '12 at 22:55

3

I know this behaviour on javascript is supposed but how can I create true private variables?

You can't, there is no private in ES5. You can use ES6 private names if you want.

You can emulate ES6 private names with ES6 WeakMaps which can be shimmed in ES5. This is an expensive and ugly emulation, that's not worth the cost.

answered Apr 08 '12 at 22:55

Raynos

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I think, as an alternative without private variables could be http://jsfiddle.net/heera/G9E9m/ – The Alpha Apr 08 '12 at 23:37

rominf · Answer 2 · 2013-05-05T19:19:40.470

When you need to add private variables to just one object which was created with Object.create you can make this:

var parent = { x: 0 }
var son = Object.create(parent)
son.init_private = function()
{
    var private = 0;
    this.print_and_increment_private = function()       
    {
        print(private++);
    } 
}
son.init_private()
// now we can reach parent.x, son.x, son.print_and_increment_private but not son.private

If you want you can even avoid unnecessary public function init_private like this:

(function()                                                                                                                                                                                                    
{                                                                                                                                                                                                              
    var private = 0;
    this.print_and_increment = function()
    {
        print(private++);
    }
}
).call(son)

Bad thing is that you could not append private members with several calls. The good thing is that this method is quite intuitive in my opinion.

This code was tested with Rhino 1.7 release 3 2013 01 27

Javascript private variables + Object.create (reference to closure variables)

2 Answers2