typescript method with type as parameter

Question

Given this class hierarhcy

export class A {
  static m() { return 'a'};
}


export class B extends A {
  static m() { return 'b'};
}


export class C extends A {
  static m() { return 'c'};
}

I need a method taking an array of classes (not instance) extending A and calls m() on each element of the array:

function looper(classes: A[]) {
  classes.forEach(c => c.m());
}

This expects an array of instances of A or its subclasses.

How can I have a method that takes as argument classes that extend A?

Generics as pointed out by @Oscar Paz

EDIT 1

Moreover the input to looper needs to be stored in a property of an object:

export class Container {
  public klazzes: A[];
}

Do you mean something like "classes: typeof(A)[]" should be the parameter for looper? — user2347763
– user2347763, Commented Apr 5, 2018 at 13:57
My comment didn't mean any offense, I actually thought it was a code suggestion :) — Cec
– Cec, Commented Apr 5, 2018 at 14:06
You are in a hurry! :) Is that what you seek to send a type of class A[] as parameter? — user2347763
– user2347763, Commented Apr 5, 2018 at 14:07

Oscar Paz · Accepted Answer · 2018-04-05 14:34:52Z

6

Well, using generics:

function callM<T extends typeof A>(arr: T[]): void {
    arr.forEach(t => t.m());
}

Now you can do:

callM([A, B, C]); // OK
callM([A, B, string]); // Error

If you want to store the values:

class Container {
    public klazzes: (typeof A)[];
}
const cont: Container = new Container();
callM(cont.klazzes);

edited Apr 5, 2018 at 14:34

answered Apr 5, 2018 at 14:01

Oscar Paz

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3 Comments

Cec Over a year ago

thank you! That works indeed! I updated my question with a related problem of where the input to the method needs to be stored. I'm having a hard time trying to apply the generics concept in that case

Niladri Over a year ago

I missed the edited part while typing my answer .. too late again

Cec Over a year ago

Thank you very much @Oscar Paz, I owe you big time

Niladri · Accepted Answer · 2018-04-05 14:42:04Z

0

Not sure if this is something you were looking for - on top of what @Oscar suggested you can create a generic class to store the array in a property of type T[] like below .

export class A {
  static m() { return 'a'};
}


export class B extends A {
  static m() { return 'b'};
}


 export class C extends A {
  static m() { return 'c'};
}

 export class Container<T extends typeof A> {
    public klazzes: T[];
    constructor() {
        this.klazzes = [];
    }

   public callM<T extends typeof A>(arr: T[]): void {
       arr.forEach(t => 
           console.log(t.m())
        );
   }
}

// usage 
let cont = new Container();
cont.klazzes = [A, B, C];
cont.callM(cont.klazzes);

Here is a working link

Working demo

answered Apr 5, 2018 at 14:42

Niladri

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1 Comment

Cec Over a year ago

Not what I needed, but will come handy in the future maybe :)

user2347763 · Accepted Answer · 2018-04-05 16:56:19Z

0

export class Container<T extends typeof A> {
    public klazzes: T[];
}

is not the same as

export class Container<T extends typeof A> {
    public klazzes: A[];
}

Typing the array as a specific type 'A' undermines the purpose of generics. so, I guess you would want to use the template type 'T'

edited Apr 5, 2018 at 16:56

answered Apr 5, 2018 at 16:47

user2347763

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