Generics in typescript object indexers

Question

I've been trying to make a type whose indexer has a return type which is based upon a string literal for the key.

export enum Units {
    Enum = 'enum',
    String = 'string',
    Boolean = 'bool',
    Pounds = 'pounds'
}

export type AConditional<T extends Units> = T extends 'string' ? string : T extends 'bool' ? boolean : number;

type AType = {
    <T extends Units>[Key in `${string}/${T}`]: AConditional<T>;
}

I'm aiming for

const a: AType = {};
typeof a['whatever/pounds'] === 'number'
typeof a['something/bool'] === 'boolean'
typeof a['something/string'] === 'string'
a['something/notaunit'] //Error
a['wrong key'] //Error

The only part I have been unable to solve is how to add generics to an object indexer, obviously adding it before the brackets like I have in the snippet does not work, and putting it in the type's generics does not work either because then the object must have a generic which does not work for my case. Does anyone know any solutions to make it work as shown above?

You can't add generics to an object indexer. Why can't you do AType<"whatever/pounds"> === "number"? — Anthony Ma
– Anthony Ma, Commented Apr 11, 2022 at 17:40

k.tten · Accepted Answer · 2022-04-26 15:34:27Z

2

You were pretty close, but a relatively new feature of TypeScript is using as in mapped types to "rename" keys:

type AType = {
    [T in Units as `${string}/${T}`]: AConditional<T>;
};

And because Units is just an enumeration, we can use T in Units directly.

Playground

answered Apr 26, 2022 at 15:34

k.tten

26.8k4 gold badges34 silver badges63 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Generics in typescript object indexers

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related