serviceworker.js

const version = "3.0.0";
const staticCache = `${version}staticfiles`;

addEventListener("install", e => {
  skipWaiting();
  e.waitUntil(
    caches.open(staticCache).then(cache => {
      // split into nice to have that isn't returned
      // return must have cache
      return cache.addAll([
        "offline.html",
        "img/typewriter-favicon.png",
        "https://fonts.googleapis.com/css?family=Source+Sans+Pro:400,700&display=swap",
        "styles.css"
      ]);
    })
  ); // end waitUntil
});

addEventListener("activate", e => {
  e.waitUntil(
    // clean old caches
    caches.keys().then(cacheNames => {
      return (
        Promise.all(
          cacheNames.map(name => {
            if (name !== staticCache) {
              return caches.delete(name);
            }
          })
        )
          // take control over any opened tabs
          .then(() => clients.claim())
      );
    })
  ); // end waitUntil
});

addEventListener("fetch", e => {
  const request = e.request;
  e.respondWith(
    // return cache
    caches.match(request).then(resp => {
      if (resp) {
        return resp;
      }
      // return network resp
      return fetch(request).catch(() => caches.match('offline.html'));
    })
  ); // end respondWith
});