CODEWITHSUNDEEP
javascriptecmascript-6es6-class
Oleksiï Nikonov
Oleksiï Nikonov

Reputation: 5598

how redefine 'this' and reuse another class's method. Polymorphism

I would love to make polymorphisme in js with classes reassigning this.

I have one class simplified to this:

class First{
  method1 = function(aVar) { this.list.map (e => e === aVar)}
}

and another class which is already has inharitance from another Parent class:

class Second extends Parent{
  constructor(){
    this.list = ['some elements'];
    this.method1 = First.method1.apply(this, arguments)
}

The Parent cannot be extended from the First; When i run Second it throws an error: apply cannot be applied to method1 because it's undefiend, But when i create an instance it losts Second's this scope : 

class Second extends Parent{
  constructor(){
    this.list = ['some elements'];
    this.method1 = (new First).method1.apply(this, arguments)
}

Also i need to provide arguments to First.method1

i tried this answer but that wasnt working

Upvotes: 0

Views: 378

Answers (1)

Sebastian Kaczmarek
Sebastian Kaczmarek

Reputation: 8515

The thing is that the .apply() fires the function, and you don't want to fire the function, you want to create a function with changed this context. To do so, you want to use a .bind() method which creates a function but does not fire it. Look at this code:

class First {
 
  method1() {
    this.list.map(e => e)
    console.log('I got called');
  }
  
  method2() {
    return this.list;
  }
}

class Parent {}

class Second extends Parent {
  constructor(){
    super();
    
    this.list = ['second class list'];
    this.method1 = (new First()).method1.bind(this)
    
    this.theList = (new First()).method2.apply(this);
  }
}

const sec = new Second();
sec.method1();
console.log(sec.theList);

So the method1 in the Second class is a copy of the method with the same name from class First. It's created using bind() and changed this to the Second class context.

However, the theList field in the Second class is a result of invoking the method2() from the First class with changed this context. It is not a function, it's a result of the function.

See the difference?

Upvotes: 1

Related Questions