how to create a method which creates new instance from class?

Question

I have Bmw class extended from Car class I created before, now I need to create a createNewBmwCar method inside Bmw class which will create new instance from Bmw class...

 class Bmw extends Car { constructor(options) { this._model = options.model; this._year = options.year; this._price = options.price; } static createNewBmwCar() { // return FIXME: let newCar = new Bmw(); } }

Answer 1

You will need to instantiate Bmw in this static method:

 class Car {}; class Bmw extends Car { constructor(options) { super(options); this._model = options.model; this._year = options.year; this._price = options.price; } static createNewBmwCar(options) { return new Bmw({ model: "BMW", year: options.year, price: options.price }); } } let myBMW = Bmw.createNewBmwCar({ year: 2020, price: "1$" }); console.log(myBMW);

Answer 2

The problem isn't the static method, it's the constructor. Derived constructors must call super() , and must do so before they can use this .

If you fix that, and add the necessary parameter(s) to createNewBmwCar , it works (but keep reading):

 class Car { } class Bmw extends Car { constructor(options) { super(); // <========= this._model = options.model; this._year = options.year; this._price = options.price; } static createNewBmwCar(options) { let newCar = new Bmw(options); return newCar; } } const beemer = Bmw.createNewBmwCar({ model: "fancy", year: "recent", price: "lots", }); console.log(beemer._model); // "fancy"

You do have a second option that may be more flexible: You can use this instead of Bmw in the static method, since it will refer to the constructor you call the static method on:

static createNewBmwCar(options) {
    let newCar = new this(options);
    //               ^^^^
    return newCar;
}

 class Car { } class Bmw extends Car { constructor(options) { super(); // <========= this._model = options.model; this._year = options.year; this._price = options.price; } static createNewBmwCar(options) { let newCar = new Bmw(options); return newCar; } } const beemer = Bmw.createNewBmwCar({ model: "fancy", year: "recent", price: "lots", }); console.log(beemer._model); // "fancy"

That has the advantage that if you create a subclass of Bmw , and then call the static method on the subclass, you'll get an instance of the subclass, rather than a Bmw :

 class Car { } class Bmw extends Car { constructor(options) { super(); // <========= this._model = options.model; this._year = options.year; this._price = options.price; } static createNewBmwCar(options) { let newCar = new this(options); return newCar; } } class BmwSpecial extends Bmw { } const beemer = BmwSpecial.createNewBmwCar({ model: "fancy", year: "recent", price: "lots", }); console.log(beemer instanceof BmwSpecial); // true, would be false with the earlier version