Constrain keys using value type

Question

I have a TypeScript function a bit like this:

function lookup<Object, Key extends keyof Object>(object: Object, key: Key): any

Now, can I constrain the key type using the type of the produced value? Example:

interface Animal {
    name: string;
    legs: number;
}

Now can I constrain lookup to only allow keys that have a string value? Ie. that calling lookup(animal, "name") would be valid, but lookup(animal, "legs") would be not.

maybe duplicated to: https://stackoverflow.com/questions/49752151/typescript-keyof-returning-specific-type — linxie, Jun 16 '20 at 17:16

Yury Tarabanko · Accepted Answer · 2020-06-16T15:01:37.960

3

Some play with generics and conditional types might do the trick

type KeysOfType<T, P> = { [K in keyof T]: P extends T[K] ? K : never }[keyof T]

function lookup<T, K extends KeysOfType<T, string>>(object: T, key: K): T[K] {
    return object[key];
}

Playground

edited Jun 16 '20 at 15:01

answered Jun 16 '20 at 14:49

Yury Tarabanko

44,270
9
84
98

Very interesting! – OliverRadini Jun 16 '20 at 14:50
This allows Object types – Murat Karagöz Jun 16 '20 at 14:50
@MuratKaragöz Yep, something more restrictive `type KeysOfType = { [K in keyof T]: T[K] extends P ? P extends T[K] ? K : never : never }[keyof T]` would work I guess. – Yury Tarabanko Jun 16 '20 at 14:57
Yours works well. But you could make it shorter by using `lookup – Murat Karagöz Jun 16 '20 at 15:08

Murat Karagöz · Answer 2 · 2020-06-16T15:16:49.240

1

You could use the following

function lookup<Object, Key extends keyof Object>(object: Object, key: Object[Key] extends string ? Key: never)

edited Jun 16 '20 at 15:16

answered Jun 16 '20 at 14:53

Murat Karagöz

35,401
16
78
107

Constrain keys using value type

2 Answers2