Comments on: Object-Oriented JavaScript Tip: Implementing the Singleton Pattern

By: Ans

Ans — Tue, 26 Jul 2011 16:36:49 +0000

Nice work! Thanks for sharing.

By: Rafael

Rafael — Tue, 26 Jul 2011 16:14:29 +0000

I found a way to do it…

var Teste = (function()
{
var instance = null;
var TesteClass = function()
{
this.texto = “texto”;
};

return new function()
{
this.getInstance = function()
{
if (instance == null)
instance = new TesteClass();

return instance;
};
};
})();
var t = Teste.getInstance();
console.log(“t: ” + t.texto);
t.texto = “Testando”;
var x = Teste.getInstance();
console.log(“x: ” + x.texto);

It works, and I can’t call “new Teste()”, it returns an error.

By: Ans

Ans — Tue, 26 Jul 2011 14:56:19 +0000

Hi Rafael,

Off the top of my head you could have this:

var instance; var Teste = new function Teste() { if (!instance) instance = this;


			Teste.getInstance = function()

			{

				return instance;

			};
			this.getInstance = function()

			{

				return instance;

			};
			this.texto = "Texto";

return Teste; };

Which would work with the rest of your code. It’s not so self-contained though.

By: Rafael

Rafael — Tue, 26 Jul 2011 14:47:36 +0000

I understood, I just though that this could be fixed in the function, thx for the help.

Explaning, I’m trying to find a Singleton that won’t have multiple instances, even if the “user” try to hack it.

By: Ans

Ans — Tue, 26 Jul 2011 14:43:12 +0000

Hi Rafael, This code doesn't prevent you from creating multiple instances of the singleton if you want to, it just provides a pattern to avoid doing that. In your code you have: var x = Teste.getInstance(); ...and... var t = new Teste(); Which means var x and var t are now two different instances, making your comparison equal false. Also, you have the code: var y = t.getInstance(); ...which won't do, as you are trying to access a method getInstance() on the Teste object assigned to t. The object t does not have that method as the getInstance() method is a static method attached to the class Teste, not an instance of that class. So to fix this, you'd need to eliminate var t and change var y to: var y = Teste.getInstance();

By: Rafael

Rafael — Tue, 26 Jul 2011 14:23:47 +0000

I found a bug on it….

var Teste = new function Teste()
{
var instance = this;
Teste.getInstance = function()
{
return instance;
};

this.texto = “Texto”;

return Teste;
};

var x = Teste.getInstance();
x.texto = “Testando”;
var t = new Teste();
var y = t.getInstance();
console.log( x == y );

It will return false…..
x.texto = Testando
y.texto = Teste

Can I somehow fix it?

By: Jeff Papineau

Jeff Papineau — Thu, 03 Feb 2011 18:23:56 +0000

I like being able to include private vars, I prefer the named object signature for singletons sometimes.

By: simon

simon — Mon, 24 Jan 2011 11:43:38 +0000

“no need for a class-like constructor for an object.
object literals is all you need.”

Douglas CrockFord,
senior JavaScript architect

By: fred

fred — Sun, 28 Mar 2010 16:01:59 +0000

Why not just using

Singleton = function() {
// private vars and functions
return {
// public vars and functions
 
}();

When the code run, the Singleton is automatically instantied an could not be instantied a second time
(except if used with var Second = new Singleton.constructor())

}();

By: Anthony

Anthony — Sun, 28 Mar 2010 08:15:59 +0000

It bothers me that (Class.Singleton() !== (new Class())) so I figured out how to stop instantiation of the class without disrupting the way it works… here it is:

var Test = new function Test()
{
	if ( Test.initialized )
		throw new Error('You may not instantiate Test');
 
	var instance = this;
 
	Test.Singleton = function() { return instance; };
	return Test;
};
Test.initialized = true;
Test.prototype.data = 'some data';
 
var obj = Test.Singleton();
alert(obj.data);
obj.data = 'test';
alert(obj.data);
 
var other = Test.Singleton();
alert(other.data);
 
var fail = new Test;
alert(fail.data);