How to limit the keys of a TypeScript type?

Issue

I would like to do:

type PossibleKeys = 'a' | 'b' | 'c'

... and now I would like to create a type which the key has to be necessarily one of the above. Like:

type MyType = {
 a: number;
 b: string;
 c: boolean;
 d: {} // <--- I want it not to be allowed because `d` does not extend `PossibleKeys`
}

How would you do that?

Solution

You can create your own type validator:

type ValidateKeys<
    K,
    T extends ([keyof T] extends [K] ? [K] extends [keyof T] ? unknown : never : never)
> = T

type PossibleKeys = 'a' | 'b' | 'c'

type MyType = ValidateKeys<PossibleKeys, {
    a: number;
    b: string;
    c: boolean;
}>

Answered By - tokland