text: const CACHE_NAME = 'version-1';
  const urlsToCache = ['index.html', 'offline.html'];
  const self = this;
  //Install service worker
  self.addEventListener('install', event => {
    event.waitUntil(
      caches.open(CACHE_NAME).then(cache => {
        console.log('Opened cache');
        return cache.addAll(urlsToCache);
      }),
    );
  });
  //Listen for request
  self.addEventListener('fetch', event => {
    event.respondWith(
      caches.match(event.request).then(() => {
        return fetch(event.request).catch(() => caches.match('offline.html'));
      }),
    );
  });
  
  //activate service worker
  self.addEventListener('activate', event => {
    const cacheWhitelist = [];
    cacheWhitelist.push(CACHE_NAME);
    event.waitUntil(
      caches.keys().then(cacheNames =>
        Promise.all(
          cacheNames.map(cacheName => {
            if (!cacheWhitelist.includes(cacheName)) {
              return caches.delete(cacheName);
            }
          }),
        ),
      ),
    );
  });
  