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

service-worker-install-cache-all.js

const otherResources = [
        '/about/',
        '/contact/',
        '/services/'
      ],
      cacheCriticals = () => {
        return caches.open(version).then( cache => {
          cache.addAll(otherResources); // important, but not critical resources
          return cache.addAll(criticalResources); // critical resources
        });
      };