Service Worker gotchas – https://medium.com/kollegorna/service-worker-gotchas-af20a9dab986

service-worker-online-first.js

self.addEventListener('fetch', event => {
  event.respondWith(
    // try fetching a resource from the network
    fetch(event.request).then(response => {
      // cache the resource and serve it
      let responseCopy = response.clone();
      addToCache(event.request, responseCopy); // this is a custom function, I'll elaborate on it later
      return response;
    })
    .catch(() => {
      // the resource could not be fetched from network
      // so let's pull it out from cache, otherwise serve the "offline" page
      return caches.match(event.request).then(response => response || caches.match('/offline/'));
    });
  );
});