Why Javascript Singleton is written like this?

Question

Everywhere I saw an implementation of singleton in javascript in a such manner:

var Singleton = (function () {
    var instance;

    function createInstance() {
        var object = new Object("I am the instance");
        return object;
    }

    return {
        getInstance: function () {
            if (!instance) {
                instance = createInstance();
            }
            return instance;
        }
    };
})();

Here we can see that it returns a function which we must call in order to get an object. Question: Why we can't just write like this:

var Singleton = (function () {

    function privateMethod() {
        ...
    }

    return {
        publicMethod1: function() { },
        publicMethod2: function() { },
    };
})();

Somebody will say that reason is for inheritance support. However, I don't see a difference. Anyway Singleton is an object now.

A Singleton should be created only once and further return only a reference of the created Singleton. Your Singleton is created everytime its called. — Stefan Brinkmann
– Stefan Brinkmann, Commented May 16, 2016 at 12:57
this is wrong, his singleton's implementation is an auto evaluated function, which will be executed only once. — Regis Portalez
– Regis Portalez, Commented May 16, 2016 at 13:16
@Regis Portalez: Your right! Then the difference is, that the first Singleton is only created when it's realy needed and may boost a bit performance depending on how difficult it is to create that Singleton and if its rarely needed. — Stefan Brinkmann
– Stefan Brinkmann, Commented May 16, 2016 at 13:24
that's indeed a good reason. Do you mind if I add it to my answer? If you provide another, I'll upvote it — Regis Portalez
– Regis Portalez, Commented May 16, 2016 at 13:26

Regis Portalez · Accepted Answer · 2016-05-16 13:29:40Z

Interesting question, and I'll try a guess.

I see a few reasons:

first is pointed by @Steffomio in the comments.
First version will be instanciated only at the first usage, while second will be instanciated immediately, which could lead to CPU pressure on page load (often an unwanted effect)

second is here:

 if (!instance) {
        instance = createInstance();
    }
    return instance;

part of the code.

With your version of the code (simple closure evaluation), it could fail if the singleton's script is embedded several times in the webpage.
The first time the singletong will be created, then some line of code modifies it, then another recreates the singleton, and prior modifications are lost.

third reason is that many languages can't express auto evaluated functions as easily as in JavaScript. If you look at how to implement a singleton in C#, you'll find a similar pattern, which is probably often translated as is in JavaScript.
final reason is inheritance support, as you stated yourself.

Ralf D'hooge · Accepted Answer · 2016-05-16 13:07:01Z

0

You can use the module revealing pattern like this.

var Singleton = Singleton || function () {

    var privateMethod1 = function() {
        ...
    }

    var privateMethod2 = function() {
        ...
    }


    return {
        publicMethod1: privateMethod1,
        publicMethod2: privateMethod2 
    };
}();

The way closure works, this method will be loaded only once in your application and it will keep a reference to your methods.

You don't need to make a instance of Singleton (new Singleton --> not needed)

You can call it directly Singleton.publicMethod1()

answered May 16, 2016 at 13:07

Ralf D'hooge

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