How function passing works in typescript?-CodePudding


class Test {
    x = 0;

    t2: T2;

    constructor() { 
        this.t2 = new T2(this.display);
    }

    display() {
        console.log(this.x);
    }

}

class T2 {
    constructor(private display: any) {

    }

    print() {
        this.display();
    }

}

let t = new Test();

t.t2.print();

When I run the above code it prints undefined instead 0, can anyone please help me out with an explanation ? Thank you.

CodePudding user response：

This is not related to TypeScript per se, but is rather a consequence of how this works in JavaScript. There are a variety of rules around how this context changes under various circumstances, you can read more about that here.

Here, the value of this is determined by the receiver of the print method, which is in this case the object t.t2. This means inside the print method, this is referring to an instance of the T2 class.

The print method then calls this.display() where again, the value of this is determined by the receiver. Since the receiver this time is just this again, it transitively carries over from the prior print call and is still the t.t2 object.

And finally, since now we know that the this when you try to execute console.log(this.x) is actually the T2 instance, we note that the T2 instance does not have a property x and therefore we get undefined as expected.

Here is a code snippet demonstrating the this chain. Note how you can change the receiver of the print method and how that also changes the value of this within the method.

class Test {
  constructor() {
    this.x = 0
    this.t2 = new T2(this.display);
  }

  display() {
    console.log("Does this in 'display' equal 't.t2'?:", this === t.t2)
    console.log(this.x);
  }

}

class T2 {
  constructor(display) {
    this.display = display
  }

  print() {
    console.log("Does this in 'print' equal 't.t2'?:", this === t.t2);
    this.display();
  }

}

let t = new Test();
t.t2.print();

// And note you can change the `this` by changing the recieving object:
console.log("Swapping reciever!")
t.print = t.t2.print;
t.print();