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 {}

type MaybeTuple = [Maybe, Maybe, Maybe];

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

let extractedTypes: MaybeTypes = ['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.