Ejemplo serviceworker

sw.js

const PRECACHE = 'trivia-precache-v12';
const RUNTIME = 'runtime';

const PRECACHE_URLS = [
  '/',
  '/game/img/loader4.gif',
  '/game/css/all.css',
  '/game/js/build.min.js',
  '/game/sound/sounds.mp3', 
];

self.addEventListener('install', event => {
  event.waitUntil(
    caches.open(PRECACHE)
      .then(cache => cache.addAll(PRECACHE_URLS))
      .then(self.skipWaiting())
  );
});

self.addEventListener('activate', event => {
  const currentCaches = [PRECACHE];
  event.waitUntil(
    caches.keys().then(cacheNames => {
      return cacheNames.filter(cacheName => !currentCaches.includes(cacheName));
    }).then(cachesToDelete => {
      return Promise.all(cachesToDelete.map(cacheToDelete => {
        return caches.delete(cacheToDelete);
      }));
    }).then(() => self.clients.claim())
  );
});

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