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

service-worker-offline-first.js

self.addEventListener('fetch', event => {
  event.respondWith(
    // check if the resource was cached before
    caches.match(event.request).then(response => {
      // serve the resource if cached, otherwise fetch it through the network
      return response || fetch(event.request).then(response => {
        // cache the newly fetched 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(() => {
        // serve "offline" page if it couldn't be fetched from network
        return caches.match('/offline/');
      });
    });
  );
});