How to make a JavaScript singleton with a constructor without using return?

Question

I currently know two ways to construct singletons in JavaScript. First:

var singleton = {
 publicVariable: "I'm public",
 publicMethod: function() {}
};

It is perfect except that it does not have a constructor where I could run initialization code.

Second:

(function() {

var privateVariable = "I'm private";
var privateFunction = function() {}

return {
 publicVariable: "I'm public",
 publicMethod: function () {}
}

})();

The first version does not have private properties nor does it have a constructor, but it is faster and simpler. The second version is more complex, ugly, but has a constructor and private properties.

I'm not in a need for private properties, I just want to have a constructor. Is there something I am missing or are the two approaches above the only ones I've got?

The second approach is not "ugly"; it's a natural part of the language. Being able to create functions with functions is one of Javascript's most powerful features. — Pointy, Oct 10 '10 at 12:00

score 9 · Accepted Answer · answered Oct 10 '10 at 11:56

9

function Singleton() {
  if ( Singleton.instance )
    return Singleton.instance;
  Singleton.instance = this;
  this.prop1 = 5;
  this.method = function() {};
}

answered Oct 10 '10 at 11:56

gblazex

49,155
12
98
91

I think this solution is not clear, because a using of this constructor is the next: `new Singleton()`. This means a developer expects to get a new instance each time he/she writes this code. I.e. `new Singleton() == new Singleton()` returns `true` but must return `false`. – pto3 May 15 '17 at 07:54
It seems so strange to me that this works, but it is also confirmed by https://stackoverflow.com/questions/1646698/what-is-the-new-keyword-in-javascript#3658673 - I wish he had a reference about the constructor. – Aerik Mar 22 '18 at 01:06

score 3 · Answer 2 · answered Apr 21 '12 at 14:23

Here is my solution with closures:

function Singleton() {

    Singleton.getInstance = (function(_this) {
        return function() { return _this; };
    })(this);
}

Test:

var foo = new Singleton();
var bar = Singleton.getInstance();
foo === bar; // true

score 3 · Answer 3 · answered Oct 22 '13 at 04:07

If you are just looking for a place to initialise your singleton, how about this?

var singleton = {
    'pubvar': null,
    'init': function() {
        this.pubvar = 'I am public!';
        return this;
    }
}.init();

console.assert(singleton.pubvar === 'I am public!');

Simple and elegant.

score 1 · Answer 4 · answered Oct 10 '10 at 12:18

var singleton = new function() {  // <<----Notice the new here
  //constructorcode....

  this.publicproperty ="blabla";
}

This is basically the same as creating a function, then instantly assiging a new instace of it to the variable singleton. Like var singleton = new SingletonObject();

I highly advice against using singletons this way in javscript though because of the execution order is based on where in the file you place the object and not on your own logic.

score 1 · Answer 5 · answered Dec 12 '11 at 08:34

What about this?

var Singleton = (function() {
    var instance;

    // this is actual constructor with params
    return function(cfg) {
        if (typeof instance == 'undefined') {
            instance = this;
            this.cfg = cfg;
        }
        return instance;
    };
})();

var a = new Singleton('a');
var b = new Singleton('b');

//a === b; <-- true
//a.cfg <-- 'a'
//b.cfg <-- 'a'

score 0 · Answer 6 · answered Mar 31 '20 at 18:15

I make it an actual Singleton with static functions and no this like so:

class S {
    //"constructor"
    static init() {
        //Note: Since it's a singleton, there's no "this" instance.
        //Instead, you store variables directly on the class.
        S.myVar = 7;
    }

    static myOtherFunc() {
        alert(S.myVar);
    }
}
//Immediately call init() to make it the "constructor".
//Alternatively, you can call init() elsewhere if you'd
//like to initialize it at a particular time.
S.init();

//Later:
S.myOtherFunc();
S.myVar = 10;

How to make a JavaScript singleton with a constructor without using return?

6 Answers6