Creating a True Abstract class in Javascript

Question

Im' trying to create a true abstract class in javascript where if you try to instantiate an abstract class, it throws an error. Problem is, when I do this, I can't create any default values in the abstract class. Here is my code:

class Abstract {
  constructor() {
    if (new.target === Abstract) {
      throw new TypeError("Cannot create an instance of an abstract class");
    }
  }

  get Num () { return {a: 1, b: 2} }
  get NumTimesTen () { return this.Num.a * 10 }
}

class Derived extends Abstract {
  constructor() {
    super();
  }
}

//const a = new Abstract(); // new.target is Abstract, so it throws
const b = new Derived(); // new.target is Derived, so no error

alert(b.Num.a) // this return 1
b.Num.a = b.Num.a + 1
alert(b.Num.a)  // this also returns 1, but should return 2
alert(b.NumTimesTen) // this returns 10, but should return 20

This is happening because my get function is re-creating that object every time it is called. In a functino class, I would have used this.Num, but that doesn't compile in the class syntax. What should I do?

user3685285 · Accepted Answer · 2017-04-20 22:33:44Z

2

Figured it out. I can still put variable instantiating code in the abstract constructor.

class Abstract {
  constructor() {
    this.thing = {a: 1, b: 2}
    if (new.target === Abstract) {
      throw new TypeError("Cannot create an instance of an abstract class")
    }
  }

  get Thing () { return this.thing }
  get ThingATimesTen () { return this.thing.a * 10 }
}

class Derived extends Abstract {
  constructor() {
    super()
  }
}

//const a = new Abstract(); // new.target is Abstract, so it throws
const b = new Derived(); // new.target is Derived, so no error

alert(b.Thing.a) // this return 1
b.Thing.a = b.Thing.a + 1
alert(b.Thing.a)  // now returns 2
alert(b.ThingATimesTen) // now returns 20

answered Apr 20, 2017 at 22:33

user3685285

6,72614 gold badges60 silver badges101 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Creating a True Abstract class in Javascript

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related