bad.ts

type State = {
  song: string | undefined; // undefined represents that no song is playing. if you're less lucky, some might use an empty string, e.g. "", instead of undefined
  position: number; // how many seconds into the song we're in
  duration: number; // how many seconds long the song is
}

// take the state and seek to a particular position
function seek(state: State, position: number): State {
  if (position > state.duration) {
    throw Error
  }
  return {
    ...state,
    position
  }
}

// initial state
let state = {
  song: "Hooked on a Feeling",
  duration: 90,
  position: 0,
}

// seek to some part of the song
state = seek(state, 10)

// stop the current song
let state = {
  song: undefined
  // we still have the fields "duration" and "position" on this object even though no song is playing
}

// this will cause an error!
seek(state, 10)