Singleton pattern with JS ES5 and ES6

singleton.js

function Singleton() {
    if (!(this instanceof Singleton))
        return new Singleton;
    if (Singleton.hasOwnProperty('singleton'))
        return Singleton.singleton;
    Object.defineProperty(Singleton, 'singleton',{
        value: this,
        enumerable:false,
        writable:false,
        configurable:false
    });
}

var instance1 = Singleton(), instance2 = new Singleton, instance3 = new Singleton;

console.log(instance1 == instance2); // true
console.log(instance2 == instance3); // true
console.log(instance1 == Singleton.singleton); // true
console.log(instance2 == Singleton.singleton); // true
console.log(instance3 == Singleton.singleton); // true

//es6
class Singleton{
    constructor() {
        if (Singleton.hasOwnProperty('singleton'))
            return Singleton.singleton;
        Object.defineProperty(Singleton, 'singleton',{
            value: this,
            enumerable:false,
            writable:false,
            configurable:false
        });
    }
}

const instance1 = new Singleton, instance2 = new Singleton;
console.log(instance1 == instance2); // true
console.log(instance1 == Singleton.singleton); // true
console.log(instance2 == Singleton.singleton); // true