Does typescript not check types on the result of an Array.map?

Question

interface Company {
  id: string;
  name: string;
}

type input = Company;

// This fails as the types don't match
const ACME: input = { id: '123', name: 'ACME', ceo: 'Eric' };

function mapIds(ids: string[]): input[] {
  // This compiles, but it shouldn't, or is Array.map returning something different?
  return ids.map(id => ({ id: '1', name: '1', ceo: 'Eric' }));

  // This fails as types don't match
  return [{ id: '1', name: '2', ceo: 'Eric' }];
}


Given the above code, the typescript compiler will not allow the function to return values that don't belong in the type, however if the return is from an Array.map, it does. You can see this with the above snippet on the Typescript Playground: https://www.typescriptlang.org/play/

Could anyone explain what's up with that?

Answer 1

Your map function does not specify a return type so it can return anything. If you want a stricter check you need to be explicit:

interface Company {
  id: string;
  name: string;
}

type input = Company;

// This fails as the types don't match
const ACME: input = { id: '123', name: 'ACME', ceo: 'Eric' };

function mapIds(ids: string[]): input[] {
  return ids.map((id):Company => ({ id: '1', name: '1', ceo: 'Eric' }));

  // This fails as types don't match
  return [{ id: '1', name: '2', ceo: 'Eric' }];
}

The reason is that the .map function is a mapping operation intended to transform each element in the array to a new type. TypeScript does not know what that new type will be if you don't specify.

To expand on the comments below. TSC objects to line return [{ id: '1', name: '2', ceo: 'Eric' }]; because it expects a type of input[] which it is not. However ids.map(id => ({ id: '1', name: '1', ceo: 'Eric' })); by itself is fine (because .map can return any type) and that is then assigned to input[] which is allowed.

Thanks to @TitianCernicova-Dragomir and @p.s.w.g for their comments on this.