staleWhileRevalidate.js

// Implements stale-while-revalidate
self.addEventListener('fetch', event => {
  const cached = caches.match(event.request);
  const fetched = fetch(event.request);

  // Call respondWith() with whatever we get first.
  // If the fetch fails (e.g disconnected), wait for the cache.
  // If there’s nothing in cache, wait for the fetch. 
  // If neither yields a response, return a 404.
  event.respondWith(
    Promise.race([fetched.catch(_ => cached), cached])
      .then(resp => resp || fetched)
      .catch(_ => new Response(null, {status: 404}))
  );

  // Update the cache with the version we fetched
  event.waitUntil(
    Promise.all([fetched, caches.open('cache-v1')])
      .then(([response, cache]) => cache.put(event.request, response))
      .catch(_ => {/* eat any errors */})
  );
});