this part here is supposed to solve the problem with people forgetting new:

    if (!(this instanceof User)) {
       return new User(name, lastname);
    }

I'm pointing out that this doesn't work if you extend the class.

The point is - don't forget `new` because tricks like this will not save you, in fact this is arguably worse - you think you're getting a `Person` but you get a `User` with no immediate errors, at least if you forget `new` then you'll get an error straight away in strict mode.