function MyConstructor(foo, bar){

  if (!(this instanceof MyConstructor)){

    var obj = Object.create(MyConstructor.prototype);

    return MyConstructor.apply(obj, arguments);

  }

  this.foo = foo;

  this.bar = bar;

  return this;

}

​

​

var instance1 = MyConstructor('foo1', 'bar1');

var instance2 = new MyConstructor('foo2', 'bar2');

​

​

console.log([instance1.foo, instance1.bar]);

console.log([instance2.foo, instance2.bar]);

​

instance1.foo = 'whatever';

​

console.log([instance1.foo, instance1.bar]);

console.log([instance2.foo, instance2.bar]);

​

console.log(instance1 instanceof MyConstructor);

console.log(instance2 instanceof MyConstructor);