Return an offline page using Service Worker

service-worker-offline-page.js

this.addEventListener('fetch', event => {
  // request.mode = navigate isn't supported in all browsers
  // so include a check for Accept: text/html header.
  if (event.request.mode === 'navigate' || (event.request.method === 'GET' && event.request.headers.get('accept').includes('text/html'))) {
        event.respondWith(
          fetch(event.request.url).catch(error => {
              // Return the offline page
              return caches.match(offlineUrl);
          })
    );
  }
  else{
        // Respond with everything else if we can
        event.respondWith(caches.match(event.request)
                        .then(function (response) {
                        return response || fetch(event.request);
                    })
            );
      }
});