better.ts

type PlayingState = {
  song: string;
  position: number; // how many seconds into the song we're in
  duration: number; // how many seconds long the song is
}

type StoppedState = {}

type State = PlayingState | StoppedState

// take the state and seek to a particular position
function seek(state: PlayingState, position: number): PlayingState {
  // note that we cannot get rid of this runtime in every case, but it will now be a compile time error if we try to seek if a song is not playing
  if (position > state.duration) {
    throw Error
  }
  return {
    ...state,
    position
  }
}

// initial state
let state: 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 = {}

// this will not compile
seek(state, 10)