How to use a class as a type for properties of another class in typescript?

Question

I am new to typescript. I have defined some classes. I have used them as a type for the property of another class. e.g:

fileone.ts

export class A {
   propertyOne: string;
   propertyTwo: string;
}

Now i have another class B in another file:

filetwo.ts

import { A } from './fileone';
export class B {
    myProperty: A;
    mySecondProperty: string;
}

I have instantiated this class B in another file. I have the following code:

myapp.ts

import { B } from './filetwo';
export class C {
    let myObj: B = new B();
    myObj.myProperty.propertyOne = 'hello';
    myObj.myProperty.propertyTwo = 'world'';
    console.log(myObj);
 }

Now when i try to set the property of A through B, it say the following error:

Cannot set the property "propertyOne" of undefined

Can we not do this like in java? And please explain why i cannot do what i am doing right now. And what is the correct approach for this. Please do not just give me solution to my problem but also an explanation.

Answer 1

You have set the correct type of your myProperty member but this variable is not initialized by just declaring the type. So you are trying to set a property propertyOne on an undefined variable on your instance of B.

If you want to have it initialized correctly, you need to do this manually in your class B:

export class B {
  myProperty: A;
  constructor() {
    this.myProperty = new A();
  }
}

Answer 2

myObj.myProperty is type of A which is not defined yet, so you should initialize it

myObj.myProperty = new A();

then use it