I have a type like this:
type Person = {
name: string;
relatives: Array<Person>;
favoriteRelative: string;
}
But I want to enforce favoriteRelative to be a name from any Person in relatives.
I tried this, but got only string instead of the wanted list of strings (probably due to the names not being considered a const-Array):
type Person<R extends Array<Person>> = {
name: string;
relatives: R;
favoriteRelative: R[number]['name'];
}
I tried the infer keyword like this, but got an error :
type Person<T extends Array<Person>> = T extends infer R ? {
name: string;
relatives: R;
favoriteRelative: R[number]['name']; // Type number cannot be used to index type R
} : never
I don't get the answer either out of the Typescript docs or other closely related SO threads.
Dynamic type inference can only be done via functions
type Person<F extends string = string> = {
name: string;
relatives: ReadonlyArray<Person>;
favoriteRelative: F;
}
type OneOf<T> = T extends ReadonlyArray<infer E> ? E : never
type Fallback<T, F> = [T] extends [never] ? F : T
function Person<
const P extends Person<R>,
R extends string = Fallback<OneOf<P['relatives']>['name'], string>
>(person: P): P {
return person
}