The self keyword's ES2015 alternative

This post was originally published on my blog on 2015-08-28.

It's quite a common use case that you need more types of constructors
for your class, like copy constructors, or creating an instance from
a plain object. With static methods, you can easily do this.

However, if you extend your class with these alternative "constructors",
you probably don't want to redefine each in your child class just to
use it's own constructor, not the parent's.

In this case, you need a keyword like PHP's self, which refers
to the class (or the constructor in ES5) of the instance.

Unfortunately ES2015 doesn't have a keyword like this. However, due to
the nature of Javascript, you can use the this keyword to access it.
As the this keyword is not bound in static methods (thank you
BabelJS@slack for confirming it), as long as call it as a method
of the child class, this will refer to it.

Here's an example of having a class which can be instantiated with a
regular constructor or from a plain object:

class A {
    constructor(a1, a2) {
        this.a1 = a1;
        this.a2 = a1;
    }

    static fromVO(obj) {
        // for unknown reasons, we have to define the constructor
        // explicitly, otherwise it doesn't show up in the debugger         
        Object.defineProperty( Object.assign(Object.create(this.prototype), obj)
                             , 'constructor'
                             , { value: this }
                             });
    }
}

class B extends A {
    constructor(a1, a2, b1) {
        super(a1, a2);
        this.b1 = b1;
    }
}

B.fromVO({ a1: 5
         , a2: 6
         , b1: 7
         }) instanceof B; // === true

Of course, if you just want to call the constructor, a new this() will do
the trick in your static method.

An interesting bit is that even B.prototype.constructor.name is "B",
the Chrome debugger still prints it as Object { ... }. After setting the
constructor on the instance explicitly, the correct B { ... } form shows up.
Apperently you don't need to do this if you're the using the new keyword.