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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Object instance's local variable is not separate
Date: Sat, 07 Jun 2014 13:48:14 +0200
Organization: PointedEars Software (PES)
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JJ <duh@nah.meh> wrote:
^^^^^^^^^^^^^^^^
<http://www.interhack.net/pubs/munging-harmful/>.  Please fix.

> I wanted to create a class that closely mimic the primitive type clases
> such as Number, Boolean, Array, etc.

There are no classes.

<http://ecma-international.org/ecma-262/5.1/#sec-4.2.1>

> I think they use a local variable to store the value instead of a
> property. I got a sample code from MDN here:
> 
> <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Closures#Emulating_private_methods_with_closures>
> 
> To start with, I change it to below since it needs to be prototypal and be
> an instance of the class.
> 
> function Num() {
>   var val = 0;
>   Num.prototype.changeTo= function(n) {
>     val = n;
>   };
>   Num.prototype.value= function() {
>     return val;
>   };
> }

Your change has b0rked the functionality of the original code.
 
> But the local variable seems no longer separate into each object instance
> as tested below. Why is that?
>  
> n1 = new Num();
> n2 = new Num();

After this constructor call, “val” in Num.prototype.value is bound to the 
local variable in the context of *that* call.

> n2.changeTo(1);

This changes *that* variable.

> console.log(n1.value() + ", " + n2.value()); //1, 1 = fail (should be 0,

And both calls return the value of *that* variable, because both objects 
have the *same* object referred to by Num.prototype next in their prototype 
chain.

A property can only have one value, in this case one reference to a Function 
instance.  Property values are resolved at runtime, through the prototype 
chain if necessary.

> 1) console.log(n1 instanceof Num); //true = ok
> console.log(Object.getPrototypeOf(n1).constructor===Num); //true = ok

It has to be

  function Num ()
  {
    var val = 0;

    this.changeTo = function(n) {
      val = n;
    };

    this.value = function() {
      return val;
    };
  }

if objects should not share their property value, and should be something 
similar to

  function Num (value)
  {
    var _value = 0;

    this.setValue = function (n) {
      _value = +n;
    };

    this.getValue = function () {
      return _value;
    };

    if (typeof Object.defineProperty == "function")
    {
      Object.defineProperty(this, "value", {
        set: this.setValue
        get: this.getValue,
      });
    }

    if (arguments.length > 0)
    {
      this.setValue(value);
    }
  }

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