Stale data invalidation

hook.js

const swr = new Map;

const useSWR = (path, fetcher, cache) => {
  let [data, update] = useState(null);
  if (!swr.has(path) || swr.get(path) !== cache) {
    fetcher(path).then(update, () => update(new Error(path)));
    swr.set(path, cache);
  }
  const isError = data instanceof Error;
  return {
    data: isError ? null : data,
    error: isError ? data : null
  };
};