Cannot read property of undefined when using react hooks

Question

I'm getting the error "Cannot read property 'map' of undefined" but can't work out why - map will not work because 'movies' is undefined. Can you see what I'm doing wrong? Many thanks.

const Dashboard = () => {
  const discoverUrl = 'https://movied.herokuapp.com/discover';
  const [movieList, setMovies] = useState({ movies: [] });

  useEffect(() => {
    const getAllMovies = () => {
      fetch(discoverUrl)
        .then(response => response.json())
         .then(data => setMovies(data))
        }
        getAllMovies()

      }, [])

  return (
    <ul>
      {movieList.movies.map(movie => (
        <li>
          <p>{movie}</p>
        </li>
      ))}
    </ul>
  );
}

export default Dashboard```

TypeError: Cannot read property 'map' of undefined

Answer 1

You response data mybe a list or undefined you need to do like this:

setMovies(pre => ({
  ...pre,
  movies: data || []
}))

Answer 2

When you set state which is setMovies(data), movieList will be equal to data. So you will lose movies property in movieList(there will be no movieList.movies).

So either initiate state like this

const [movieList, setMovies] = useState([]);

and do setMovies(data).

Or else set state like

setMovies({ movies: [...data] })