ES6 Singleton example. Use: import Singleton from 'Singleton'; let instance = Singleton.instance;

Singleton.js

'use strict';

/**
 * Created by Alexander Litvinov
 * Email: alexander@codeordie.ru
 * May be freely distributed under the MIT license
 */

let singleton = Symbol();
let singletonEnforcer = Symbol();

class Singleton {

    /**
     * @param enforcer
     */
    constructor(enforcer) {
        if (enforcer !== singletonEnforcer) {
            throw "Cannot construct singleton"
        }
    }

    /**
     * @returns Singleton
     */
    static get instance() {
        if (!this[singleton]) {
            this[singleton] = new Singleton(singletonEnforcer);
        }
        return this[singleton];
    }

}

export default Singleton;

Couldn't one get around this by calling new Singleton(singletonEnforcer). A function would, however, have to be injected into the Singleton class to gain access to singletonEnforcer. Chrome does not yet supported ES6 modules, so I can't actually test this out yet :(.

I have come up with the following function that I've been using to create singletons. There is a way around this method too. See if you can figure it out ;).

I use this function

function makeSingleton(instance){
  let className = instance.__proto__.constructor.name
  eval(className + "= function(){throw `${className} is a Singleton. Use ${className}.getInstance() instead`}")
  eval(className + ".getInstance = function(){return instance}")
}

Any class can then be made into a singleton by calling makeSingleton(this) in the constructor.

Also

//this function could be called outside of a class to make a class a singleton, however, the class's constructor cannot take in any arguments for this to work.
function makeSingleton2(theClass){
  const instance = new theClass();
  eval(theClass.name + "= () => instance") //Chrome doesn't support arrow function yet, but I believe I used it properly.
}

@mrme44 You should use babel man ! Work with ES6 now anywhere !

@schankam, I'll have to check Babel out. I tried using some other ES6 transpiler once; I don't remember what it was called, but it didn't work out so well.

@schankam, @mrme44 thanks for yours comments! I used this with Babel. Works fine!

If I understand CommonJS + the browser implementations correctly, the output of a module is cached, so export default new MyClass() will result in something that behaves as a singleton (only a single instance of this class will ever exist per process/client depending on env it's running in).

@StevenLangbroek export default new MyClass() pattern is using in ReactJS dispacher which is a single component for a ReactJS app. I think, you are correct about it.
Refer: https://gist.github.com/milankarunarathne/c565aa5970987aeca88b