How to 'map' a Tuple to another Tuple type in Typescript 3.0

Question

I have tuple of Maybe types:

class Maybe{ }

type MaybeTuple = [Maybe, Maybe, Maybe];


        

Answer 1

You'll need to use a mapped tuple type, which is supported in TypeScript 3.1.

You can make a mapped type that has properties 0, 1, 2 and length of the correct types, like this:

class Maybe<T> {}

type MaybeTuple = [Maybe<string>, Maybe<number>, Maybe<boolean>];

type MaybeType<T> = T extends Maybe<infer MaybeType> ? MaybeType : never;
type MaybeTypes<Tuple extends [...any[]]> = {
  [Index in keyof Tuple]: MaybeType<Tuple[Index]>;
} & {length: Tuple['length']};

let extractedTypes: MaybeTypes<MaybeTuple> = ['hello', 3, true];

If you're using an older version of typescript, you can use an unreleased version of TypeScript, or as a workaround, you can write a conditional type to match against tuples as long as you think you are likely to have, e.g., like this.