service-worker.js

if ('serviceWorker' in navigator) {
  navigator.serviceWorker
  .register('/static/service-worker.js', {
    scope: '/'
  })
  .then(function(registration) {
      console.log('Service Worker Registered!');
      return registration;
  })
  .catch(function(err) {
      console.error('Unable to register service worker.', err);
  });
}

const CACHE_NAME = 'static-cache';

const FILES_TO_CACHE = [
  '/static/offline.html',
];

self.addEventListener('install', (evt) => {
  console.log('[ServiceWorker] Install');
  evt.waitUntil(
    caches.open(CACHE_NAME).then((cache) => {
      console.log('[ServiceWorker] Pre-caching offline page');
      return cache.addAll(FILES_TO_CACHE);
    })
  );
  self.skipWaiting();
});


self.addEventListener('fetch', function(event) {
  event.respondWith(fetch(event.request));
});

self.addEventListener('fetch', (evt) => {
  if (evt.request.mode !== 'navigate') {
    return;
  }
  evt.respondWith(fetch(evt.request).catch(() => {
      return caches.open(CACHE_NAME).then((cache) => {
        return cache.match('offline.html');
      });
    })
  );
});