next-hash.jsx

const { useEffect, useState } = React;

export default function IndexPage(props) {
  const [currentHash, setCurrentHash] = useState(window.location.hash);

  useEffect(() => {
    function handleHashChange(e) {
      setCurrentHash(window.location.hash);
    }
    window.addEventListener('hashchange', handleHashChange);
    return () => {
      window.removeEventListener('hashchange', handleHashChange);
    }
  }, []);

  return (
    <main>
      <h1>Index Page</h1>
      <h2>{`Current hash: ${currentHash}`}</h2>
      <button onClick={() => {
        location.hash = "foo";
      }}>
        Add hash
      </button>
    </main>
  )
}