How to limit the keys of a TypeScript type?

Question

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?

not sure what you're asking here. Do you want to restrict the type definition or create a type that restricts instances based on PossibleKeys. — apokryfos
– apokryfos, Commented Dec 10, 2021 at 17:27

tokland · Accepted Answer · 2024-09-02 20:48:13Z

5

You can create "type validators" using the pattern T extends (Condition ? unknown : never)

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;
}>

playground

edited Sep 2, 2024 at 20:48

answered Dec 10, 2021 at 16:32

tokland

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3 Comments

tmikeschu Over a year ago

Amazing! FWIW this also works without the tuple syntax i.e., T extends keyof T extends K ? (K extends keyof T ? unknown : never) : never

vikyd Over a year ago

How to trigger autocompletion when using ValidateKeys in VSCode ?

tokland Over a year ago

@vikyd I think we do need the tuple wrapping to avoid the distribution of conditional types over unions. Demo:

type T1<T> = T extends 'a' ? 1 : 0; type R1 = T1<"a" | "b">;  type T2<T> = [T] extends ['a'] ? 1 : 0;  type R2 = T2<"a" | "b">;

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