Passing a readonly variable to parent constructor | Typescript

Question

I'm new to TypeScript and I'm trying to pass a readonly variable to the constructor of a parent class:

abstract class ChildClass extends ParentClass {
    private readonly READ_ONLY: number= 1;

    public constructor(foo: string)
        // 'super' must be called before accessing 'this' in the constructor of a derived class. ts(17009)
        super(foo, this.READ_ONLY); 
    }
}

ChildClass will be extended by more classes, therefore the abstract.

Is there any way I can pass the read-only variable in the super constructor?

Answer 1

This has nothing to do with readonly .

You cannot reference any property of this before the superclass's constructor is called. So you have to do this in a different way.

Perhaps READ_ONLY is a protected property of ParentClass , and then child classes can set a new value for it.

class ParentClass {
  protected readonly READ_ONLY: number = 1;
  constructor(foo: string) {
    console.log('used READ_ONLY: ', this.READ_ONLY)
  }
}

class ChildClass extends ParentClass {
  protected readonly READ_ONLY: number = 2;
}

new ParentClass('foo') // used READ_ONLY: 1
new ChildClass('foo') // used READ_ONLY: 2

This works because protected just means private to the outside, but public to subclasses.