sw.js

const version = '1.0.1';
const cacheName = `v${version}::static`;

const fileList = ['/'];

self.addEventListener('install', e => {
  // once the SW is installed, go ahead and fetch the resources
  // to make this work offline
  e.waitUntil(
    caches
      .open(cacheName)
      .then(cache => {
        return cache.addAll(fileList).then(() => {
          self.skipWaiting();
        });
      })
      .then(() => {
        console.log(`offline ${version} ready`);
      })
  );
});

// when the browser fetches a url, either response with
// the cached object or go ahead and fetch the actual url
self.addEventListener('fetch', event => {
  event.respondWith(
    caches.match(event.request).then(res => res || fetch(event.request))
  );
});

const clearOldCaches = () => {
  return caches.keys().then(keys => {
    return Promise.all(
      keys.filter(key => key !== cacheName).map(key => caches.delete(key))
    );
  });
};

self.addEventListener('activate', event => {
  event.waitUntil(clearOldCaches().then(() => self.clients.claim()));
});