Service Worker gotchas – https://medium.com/kollegorna/service-worker-gotchas-af20a9dab986

service-worker-install-cache-critical.js

const criticalResources = [
        '/',
        '/offline/',
        '/assets/css/main.css',
        '/assets/js/main.js'
      ],
      cacheCriticals = () => {
        return caches.open(version).then( cache => {
          return cache.addAll(criticalResources);
        });
      };

self.addEventListener('install', event => {
  event.waitUntil(
    cacheCriticals().then( () => self.skipWaiting())
  );
});