Singleton and Prototyping in Javascript

Question

I was just looking into ways of defining singletons in JavaScript, and after looking into a few samples I thought about the following way which I haven't found anywhere:

function MyClass() {
   if (MyClass._sharedInstance === undefined) {
      // actual constructor code

      MyClass._sharedInstance = this;
   }
   return MyClass._sharedInstance;
}

MyClass._sharedInstance = undefined;

Seems to work fine... but the fact that I couldn't find this anywhere makes me suspicious. Is there any reason not to do a singleton that way, especially considering the many little pitfalls of JavaScript. I know that by simply doing a var MyClass = { /* MyClass Stuff */ } I can achieve a singleton with less code, but for the sake of consistency I would like to define all my classes using prototypes.

What I don't understand is what problem this solves. How will it be used? — Travis J
– Travis J, Commented Jun 11, 2014 at 23:23
"for the sake of consistency I would like to define all my classes using prototypes" - but for a singleton object you don't need a class, and no prototype to share anything! — Bergi
– Bergi, Commented Jun 11, 2014 at 23:28
As I said, the only reason I'd do this is for consistency. @Bergi you are probably right, I am fairly new to OOP in JavaScript so a lot of my thinking is likely to still be influenced by "real" OOP languages. In my head, a singleton is just a special kind of class, so it makes sense to define it as such. Seems to be different in JS, though... — BlackWolf
– BlackWolf, Commented Jun 12, 2014 at 8:22
I'd say that you are too much influenced by "class-only" languages, and you should start with some real OOP languages :-) — Bergi
– Bergi, Commented Jun 12, 2014 at 12:22

Community · Accepted Answer · 2017-05-23 12:13:07Z

1

The proposed solution does not seem to "solve" anything¹; as such, I recommend a standard variation of a "simple module pattern" using an idiomatic IIFE:

MySingleton = (function () {
    // This function is only executed once
    // and whatever is returned is the singleton.
    return {
       // Expose members here
    };
})();

Depending on specific need a new'ed object could also be returned, making this pattern flexible without the need to introduce a separate "singleton" field and guard, e.g.

MySingleton = (function () {
    // Contrived, but valid - any object returned
    // functions as the singleton.
    function MyClass {}
    return new MyClass();
})();

¹If anything, it may lead to some "WTF?" moments when new MyClass() returns a shared/singleton object which is not "normal" behavior.

edited May 23, 2017 at 12:13

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answered Jun 11, 2014 at 23:27

user2864740

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2 Comments

BlackWolf Over a year ago

You are right that it does not solve anything besides allowing for a consistent way to define "classes" in JavaScript using prototyping. You are also probably right about the WTF-moment, but the same thing could be said about singletons in Java/C#/..., couldn't it? Anyway, I will go with the standard behaviour as you suggested and not use the prototyping solution.

Bergi Over a year ago

@user2864740: Notice that your second example (with the local MyClass, returning an instance from the IEFE) is equivalent to the new (function MyClass(){…}) approach and has the same issues.

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