sworker.js

const FILES_TO_CACHE = [
  '/offline.html',
  '/src/images/disconnect.png',
  '/manifest.json',
  '/favicon.png'
];

const CACHE_NAME = 'static-cache-v1';

self.addEventListener('install', (evt) => {
  evt.waitUntil(
    caches.open(CACHE_NAME).then((cache) => {
      return cache.addAll(FILES_TO_CACHE);
    })
  );

  self.skipWaiting();
});

self.addEventListener('activate', (evt) => {
  evt.waitUntil(
    caches.keys().then((keyList) => {
      return Promise.all(keyList.map((key) => {
        if (key !== CACHE_NAME) {
          return caches.delete(key);
        }
      }));
    })
  ); 
  self.clients.claim();
});

self.addEventListener('fetch', (evt) => {
  if (evt.request.mode !== 'navigate') {
    // Not a page navigation, bail.
    return;
  }
  evt.respondWith(
      fetch(evt.request)
          .catch(() => {
            return caches.open(CACHE_NAME)
                .then((cache) => {
                  return cache.match('offline.html');
                });
          })
  );
});