singleton in javascript

1var SingletonFactory = (function(){
2    function SingletonClass() {
3        //do stuff
4    }
5    var instance;
6    return {
7        getInstance: function(){
8            if (instance == null) {
9                instance = new SingletonClass();
10                // Hide the constructor so the returned object can't be new'd...
11                instance.constructor = null;
12            }
13            return instance;
14        }
15   };
16})();
17

1I think the easiest way is to declare a simple object literal:
2
3var myInstance = {
4  method1: function () {
5    // ...
6  },
7  method2: function () {
8    // ...
9  }
10};
11If you want private members on your singleton instance, you can do something like this:
12
13var myInstance = (function() {
14  var privateVar = '';
15
16  function privateMethod () {
17    // ...
18  }
19
20  return { // public interface
21    publicMethod1: function () {
22      // All private members are accessible here
23    },
24    publicMethod2: function () {
25    }
26  };
27})();
28This has been called the module pattern, and it basically allows you to encapsulate private members on an object, by taking advantage of the use of closures.
29
30If you want to prevent the modification of the singleton object, you can freeze it, using the ES5 Object.freeze method.
31
32That will make the object immutable, preventing any modification to the its structure and values.
33
34If you are using ES6, you can represent a singleton using ES Modules very easily, and you can even hold private state by declaring variables at the module scope:
35
36// my-singleton.js
37const somePrivateState = []
38
39function privateFn () {
40  // ...
41}
42
43export default {
44  method1() {
45    // ...
46  },
47  method2() {
48    // ...
49  }
50}
51Then you can simply import the singleton object to use it:
52
53import myInstance from './my-singleton.js'
54// ...