Generic key string name

Question

I would like to define a response for my request however one thing might change and that's the key name. It's always going to be a string but it will be different depending on the request.

These are the possible responses

{ someRequest: { message: 'success', status: 200 } }
{ someOtherRequest: { message: 'unauthorized', status: 401 } }

Knowing the someRequest might change but the other will not I have created this

export interface Response<T extends string> {
  [key: T]: { message: string; status: number; }
}

but it does not work. I get the

An index signature parameter type must be either 'string' or 'number'

How do I define a generic key string?

Answer 1

You have to use [key in T] instead of [key: T]. However, interfaces don't allow this operation, so switch to a type alias instead:

export type Response<T extends string> = {
    [key in T]: { message: string; status: number; };
};

Alternatively, use the built-in Record type:

export type Response<T extends string> = Record<T, { message: string; status: number; };

Answer 2

You could use a type using Record:

type Key = 'someRequest' | 'someOtherRequest';
type Response<T extends Key> = Record<T, { message: string; status: number; }>;

const foo: Response<'someRequest'> = {someRequest: {status: 200, message: 'success'}};
const bar: Response<'someOtherRequest'> = {someOtherRequest: {status: 401, message: 'unauthorized'}};

Though, type Response<T extends string> = ... does also work, without the need of Key. Or use type Key = 'someRequest' | 'someOtherRequest' | string; to be able to extend the keys.