sw.js

self.addEventListener("install", async e => {
 caches.open("v1").then(function (cache) {
   return cache.addAll(["/app", "/app.css"]);
 });
});

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