Passing an object of a class using 'this' to another class (Javascript)

Question

Passing an object of a class using 'this' to another class in Javascript

So, I come from a background of java, c#, python and other languages of that range, and I'm trying to basically recreate something in Javascript.

I want to pass an object of Class A to Class B using 'this' so an instance of Class A can be accessed in Class B.

import B from .....
class A
{
   constructor()
   {
    Obj = new B // Make object of class B

    b(this) // Send instance of class A to class B
   }

 methodA() // Method to test
 {

    alert("Hello!")

 }

}

class B
{

 constructor(a) //receive class A here in class B
 {

    a.methodA // call method of the instance class A

 }

}

I cannot access methodA in b when passing A into B

You may want to use extends for this: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Joss Classey
– Joss Classey, Commented Aug 7, 2019 at 20:58
No. You have to read up on this and inheritance in javascript. It is not as in Java and IMHO you should not try to shoe horn java-this and java-inheritance into javascript. Instead embrace how Javscript is intended to work. I have no good links for you but the book "Javascript, the good parts" is a good investment in time to read. It tells you where javascript is wrong, quirky and good. Especially the latter is valueable to know, what to embrace. FWIW — LosManos
– LosManos, Commented Aug 7, 2019 at 21:05
You can only use this inside a class method, since it's the object that the method was called on. What do you expect b(this) to do outside of a method? — Barmar
– Barmar, Commented Aug 7, 2019 at 21:08

Barmar · Accepted Answer · 2019-08-07 21:14:47Z

2

Since you're using the object in the B constructor, you need to pass this as an argument to new B. There's no b() function anywhere.

You also forgot the parentheses when calling a.methodA().

class A {
  constructor() {
    let Obj = new B(this) // Make object of class B
  }

  methodA() // Method to test
  {
    alert("Hello!")
  }
}

class B {

  constructor(a) //receive class A here in class B
  {
    a.methodA() // call method of the instance class A
  }
}

let a = new A;

answered Aug 7, 2019 at 21:14

Barmar

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3 Comments

tyvm Over a year ago

Hey, thanks for the help. I got it working with your solution, now I want to access method() in another method which isn't in the constructor, say a method below the constructor, when I call a.method() outside the constructor it says 'undefined is not an object' (this.a.methodA()) I understand this is a problem with the scope of the variable, how may I access it outside the constructor? Thank you

Barmar Over a year ago

You never saved a anywhere. You can do this.a = a; in the constructor, and then use this.a.methodA() elsewhere.

tyvm Over a year ago

Thank you my friend, best wishes

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