Singleton pattern in ECMAScript 6

singleton_es6.js

let _singleton = Symbol();

class Singleton {

    constructor(singletonToken) {
        if (_singleton !== singletonToken)
            throw new Error('Cannot instantiate directly.');
    }

    static get instance() {
        if(!this[_singleton])
            this[_singleton] = new Singleton(_singleton);

        return this[_singleton]
    }

}

export default Singleton;

Why singletonToken?

@dpavlica Because in this way the singleton cannot be created with the new operator, unless you have access to the singleton symbol.