example-service-worker-fetch.js

self.addEventListener('fetch', (e) => {
  if (assetManifest.includes(e.request.url) {
    e.respondWith(
      caches
        .open(cacheKey)
        .then(cache => cache.match(e.request))
        .then(response => {
          if (response) return response;
          return fetch(e.request);
        });
    );
  } else {
    e.respondWith(fetch(e.request));
  }
});