Pass a function that uses other functions as parameter in Javascript

Question

I am developing a project in Typescript that has two classes like this:

class A{

  //some attributes

  function_to_pass(){
    //Do something
    this.someFunction();
    //Make changes in object A
  }

  protected someFunction(){
    //Do some stuff
  }

}

class B{
  
  private readonly _functionProvided: () => void;

  constructor(functionProvided: () => void){
    this._functionPassed = functionProvided;
  }

  private myFunction(){
    //Do some stuff
    this._functionProvided();
  }

}

I want the object of class B to call the "function_to_pass" method in the object of class A, so i pass the function as a parameter in the constructor and then call "myFunction" in this object b. Like this

objectA = new A();

objectB = new B(objectA.function_to_pass)

objectB.myFunction()

But, i get the following error:

TypeError: this.someFunction is not a function

I suppose that objectB does not know about objectA, so objectB can't manipulate objectA.

Is there anyway i can do this?

ObjectB.myFunction didnt you declare it private ? Have you tried changing scope to public — Sameh
– Sameh, Commented Mar 30, 2022 at 1:57

danh · Accepted Answer · 2022-03-30 00:52:31Z

1

I see two problems with the code, the first I think is a typo: the class B constructor doesn't initialize the instance data, _functionProvided. (see FIX #1)

The second problem is the real problem, and has to do with the execution context of function_to_pass. In order that this be defined when it runs, the function must be bound to its instance. See (FIX #2).

class A{

  //some attributes

  function_to_pass(){
    //Do something
    this.someFunction();
    //Make changes in object A
  }

  protected someFunction(){
    //Do some stuff
  }

}

class B{
  
  private readonly _functionProvided: () => void;

  constructor(functionProvided: () => void){
    // FIX #1, no such thing as _functionPassed
    // WAS: this._functionPassed = functionProvided;
    this._functionProvided = functionProvided;
  }

  private myFunction(){
    //Do some stuff
    this._functionProvided();
  }

}


let objectA = new A();

// FIX #2: bind the receiver of function_to_pass
// WAS: objectB = new B(objectA.function_to_pass)
let objectB = new B(objectA.function_to_pass.bind(objectA));

objectB.myFunction()

answered Mar 30, 2022 at 0:52

danh

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1 Comment

Jorge Cróquer Over a year ago

Thank you for your quick response! The typo was my mistake while writing the question. The .bind was all I needed. Thank you very much.

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