Create only one instance of constructor function?

Question

I have a part of plugin which uses private variables and exposes public method :

function  myWorld()
    {
      var myPrivate=1;
      this.do1=function (){alert(myPrivate);} ;
    }

var a=new  myWorld();

a.do1() //1
alert(a.myPrivate); //undefined (as it should be)

But I want to prevent doing this again : new myWorld();

The only option I know is with object literal :

JSBIN 2

var myWorld=
    {
      myPrivate:1,
      do1:function (){alert(this.myPrivate);} 
    }


alert(myWorld.myPrivate); //1 ( ouch....)
myWorld.do1() //1

Question

How can encapsulate private fields and still prevent uses from myWorld to be instanced >1 times ?

jantimon · Accepted Answer · 2013-03-07 15:26:02Z

2

Closures are a great tool to define the scope:

  var myWorld= (function(){
    var myPrivate = 1;
    return {
      do1:function (){alert(myPrivate);} 
    }
  }());


   myWorld.do1();

You might want to check out the free Learning JavaScript Design Patterns book

answered Mar 7, 2013 at 15:26

jantimon

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Comments

Niet the Dark Absol · Accepted Answer · 2013-03-07 15:25:57Z

2

Try something along these lines:

(function(){
  var instanciated = false;
  window.myWorld = function() {
    if( instanciated) throw new Error("myWorld can only be instanciated once!");
    instanciated = true;
    var myPrivate = 1;
    this.do1 = function(){alert(myPrivate);};
  }
})();

answered Mar 7, 2013 at 15:25

Niet the Dark Absol

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Comments

SLaks · Accepted Answer · 2013-03-07 15:26:14Z

1

You can hide the private variable inside an IIFE:

var myWorld = (function() { 
    var myPrivate = 1;
    return { ... };
}());

answered Mar 7, 2013 at 15:26

SLaks

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Comments

the system · Accepted Answer · 2013-03-07 15:29:21Z

1

var a = new function myWorld()
{
  var myPrivate=1;
  this.do1=function (){alert(myPrivate);} ;
}

This makes myWorld available only inside the function. If you don't event want it accessable there, then remove the name.

answered Mar 7, 2013 at 15:29

the system

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Comments

Corey · Accepted Answer · 2013-03-07 15:56:07Z

1

You could use a singleton pattern to maintain one instance of the object. Something like:

(function (global) {
    var _inst;

    global.myWorld = function () {
        if (_inst) throw new Error("A myWorld instance already exists. Please use myWorld.getInstance()");
        _inst = this;
    };

    global.myWorld.prototype = {
        do1: function() {
            console.log("do1");
        }
    };

    global.myWorld.getInstance = function() {
        if (_inst) return _inst;
        return new myWorld();
    };
}(window));

var world = new myWorld();
var world2 = myWorld.getInstance();
console.log(world === world2); // true
var world3 = new myWorld(); // throws Error

answered Mar 7, 2013 at 15:56

Corey

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