Extending parent class methods in child class in J

Question

I want to extend a child's functionality of a specific method in a parent class. I'm still getting used to OOP in ES6 so I'm not sure if this is possible or breaks rules.

I'm looking for something like this:

class Parent {
  constructor(elem) {
    this.elem = elem;

    elem.addEventListener((e) => {
      this.doSomething(e);
    });
  }

  doSomething(e) {
    console.log('doing something', e);
  }
}

class Child extends Parent {
  constructor(elem) {
    // sets up this.elem, event listener, and initial definition of doSomething
    super(elem)
  }

  doSomething(e) {
    // first does console.log('doing something', e);
    console.log('doing another something', e);
  }
}

In essence I want to append a child-specific callback on a parent method. Is there a creative way of doing this without creating a completely new class with the same functionality?

Answer 1

You can use super in methods:

doSomething(e) {
  super.doSomething(e)
  console.log('doing another something', e)
}

this in child class and in parent class is the same thing, and refers to the actual object, so if you call a method in parent's code, it will call child's implementation, if it is indeed a child. Same works in other languages, for example Python and Ruby.

Working code:

class Parent {
  constructor(elem) {
    this.elem = elem
    this.elem.addEventListener('click', (e) => { this.doSomething(e) })
  }

  doSomething(e) {
    alert('doing something')
  }
}

class Child extends Parent {
  constructor(elem) {
    super(elem)
  }

  doSomething(e) {
    super.doSomething(e)
    alert('doing another something')
  }
}

let child = new Child(document.getElementById('button'))

<button id="button">Alert</button>