service-worker.js

const OFFLINE_VERSION = 2;
const CACHE_NAME = 'offline';

self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open(CACHE_NAME).then((cache) => {
      return cache.addAll([
        './',
        './index.html',
        './src/words.txt',
        './src/images/background.jpg',
      ]);
    })
  );
  self.skipWaiting();
});

self.addEventListener('fetch', (event) => {
  event.respondWith(
    (async () => {
      try {
        // Always try the network first.
        const networkResponse = await fetch(event.request);
        return networkResponse;
      } catch (error) {
        // catch is only triggered if an exception is thrown, which is likely
        // due to a network error.
        console.log(
          'Fetch failed; attempting to return cached asset.',
          event.request.url
        );

        const cache = await caches.open(CACHE_NAME);
        const cachedResponse = await cache.match(event.request);
        return cachedResponse;
      }
    })()
  );
});