API Calls with ComponentWillMount

If you want to use a functional component, use useEffect() instead of componentWillMount():

export default function PostIndex() {
  const [posts, setPosts] = useState();

  //Must use useEffect in non-page component

  useEffect(async () => {
    let res = await fetch(`${Config.apiUrl}/wp-json/wp/v2/posts`);
    res = await res.json();
    setPosts(res);
  }, []);

  return (
    <div>
      <h1>Post Index</h1>
      <ul>
        { !!posts && posts.map((post) => (
          <li key={posts.id}>
            <Link href={`/posts/${post.id}`}>
              <a>{post.title.rendered}</a>
            </Link>
          </li>
        ))}
      </ul>
    </div>
  );
}