service.worker.fetch.js

self.addEventListener('fetch', function(event) {
  event.respondWith(
    caches.open(CACHE_NAME).then(function(cache) {
      return cache.match(event.request).then(function (response) {
        return response || fetch(event.request).then(function(response) {
          return response;
        });
      });
    })
  );
});