Restrict Typescript Class to only contain members that return strings

Question

Is it possible to restrict a typescript class to only contain members (attributes and functions) that return strings?

I've tried defining an interface:

export interface IMessages {
    [index: string]: TReturn;
}

type sfunc = (sVal?: string) => string;
type TReturn = string | sfunc;

And then implementing it in a class:

class MyMessages implements IMessages {

  [index: string]: string;  // silly, see - https://github.com/Microsoft/TypeScript/issues/15300

  a: string;
  b: (sVal?: string) => string;

}

but typescript says that "Property 'b' of type '(sVal?: string | undefined) => string' is not assignable to string index type 'string'."

I get the feeling that I'm missing some kind of symbol definition in the interface that allows functions to be defined. At the end of the day, I really just want to enforce that all properties and functions defined in the class must return strings.

EDIT: after a comment revealed a mismatch in the index signatures, I'd like to bring up a relevant question - is there any way to avoid the index signature in the implementing class altogether?

When you declare the index signature in the class, you need to use your TReturn type. You can get rid of the interface itself, it's just noise. — Aluan Haddad
Oh wow, good catch. If you post that as the answer I'm happy to accept it. For bonus points, I'd love a solution that avoids the index signature in the class altogether (that's what I'm trying to achieve with the interface, but it is admittedly noise if I also have to include the index signature). — RocketMan

RocketMan RocketMan · Accepted Answer · 2020-09-17T16:00:05

Just in case anyone runs into a similar need, I ended up using Record instead of my interface:

class MyMessages implements Record<keyof MyMessages, TReturn>  {

  a: string;
  b: (sVal?: string) => string;
  c: number; // error here, good!

}

Record docs: https://www.typescriptlang.org/docs/handbook/utility-types.html#recordkeystype

Restrict Typescript Class to only contain members that return strings

1 Answers