Question

I’m reading Javascript Patterns recently.And I don’t understand the code below when it talks about singleton pattern:

function Universe(){
    var instance;
    Universe=function Universe(){
        return instance;
    };
    Universe.prototype=this;

    //the new Universe below,refers to which one?The original one,
    //or the one:function(){return this;} ??
    instance=new Universe();

    instance.constructor=Universe;

    instance.bang="Big";

    return instance;
}
Universe.prototype.nothing=true;
var uni=new Universe();
Universe.prototype.everything=true;
var uni2=new Universe();

uni===uni2;//true

Answer 1

Not much going on here. The main focus should be on the constructor, it is returning an instantiated Universe for you. So anyone that calls it will have a reference to the same instance. Notice how the constructor is pointing to the Universe function.

I wouldn’t use this pattern, as the new keyword implies a new instance is being created, and it seems a little too esoteric for my taste. In JS you can perfectly just have an object literal, often used with a namespace pattern:

(function(ns, window, undefined) {
    ns.singleton = {
        bang: 'Big'
    };

    window.ns = ns;
})(ns || {}, window);

console.log(window.ns.singleton.bang === 'Big');

Granted, this isn’t a true singleton, but it does not need to be instantiated, and anyone who uses it will have the same values.

For more singleton implementations, see Javascript: best Singleton pattern