Providing a default user that matches the shape of User without actually providing a return type annotation passes the tests and typecheck, but that obviously avoids the intended lesson.

const makeUser = () => {
  return {
    id: 1,
    firstName: '',
    lastName: '',
    role: 'user',
    posts: [
      {
        id: 1,
        title: '',
      },
    ],
  }
}