const cacheName = 'minimal-pwa-v1';
 const assets = [
   './',
   './index.html',
   './manifest.json'
 ];
 
 self.addEventListener('install', (event) => {
   event.waitUntil(
     caches.open(cacheName).then((cache) => {
       return cache.addAll(assets);
     })
   );
 });
 
 self.addEventListener('fetch', (event) => {
   event.respondWith(
     caches.match(event.request).then((response) => {
       return response || fetch(event.request);
     })
   );
 });