JavaScript Singletons

constructor-as-function.js

var myWorld = (function () {
    var instance;
    
    function myWorld() {
        if (instance != null) return instance;
        if (!(this instanceof myWorld)) return new myWorld();
        instance = this;
        
        var myPrivate = 1;
        this.do1 = function () {
            alert(myPrivate);
        };
    }
    
    return myWorld;
})();

var a = myWorld();
var b = myWorld();

console.log(a === b);

readonly-static-property.js

var myWorld = (function () {
    var instance = new myWorld();
    
    function myWorld() {
        if (instance != null) {
            throw new Error('`myWorld` is a singleton. Use `myWorld.instance`.');
        }
        
        var myPrivate = 1;
        this.do1 = function () {
            alert(myPrivate);
        };
    }
    
    Object.defineProperty(myWorld, 'instance', {
        get: function () {
            return instance;
        },
        enumerable: true
    });
    
    return myWorld;
})();

try {
    console.log('Should never see this:', new myWorld());
} catch (e) {
    console.log('Error as expected:', e.message);
}

var a = myWorld.instance;
var b = myWorld.instance;

console.log(a === b);

single-instance-constructor.js

var myWorld = (function () {
    var instance;
    
    function myWorld() {
        if (instance != null) return instance;
        instance = this;
        
        var myPrivate = 1;
        this.do1 = function () {
            alert(myPrivate);
        };
    }
    
    return myWorld;
})();

var a = new myWorld();
var b = new myWorld();

console.log(a === b);

Singletons.md

From http://stackoverflow.com/a/15196301/:

p.s. i'll be glad to hear you solution to my other question stackoverflow.com/questions/15274927/…. ( maybe you have a better solution). Royi Namir 2013-03-08 07:03:32Z

The answers do include good examples of both eagerly-loaded/instant (e.g., jantimon, SLaks) and lazy-loaded singletons (e.g., Kolink).

Though, for a few variations on lazy-loaded: :)

It is possible to return the instance (single-instance-constructor.js) each time, but this will probably be confusing unless it's not actually new (constructor-as-function.js). :)

A possible ES5+ variant would be to use a get-only "static" property (readonly-static-property.js).

Nice 😎🤞