sw.js

const CACHE_NAME = 'webplatformapis';

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

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

self.addEventListener('activate', event => {
  console.log("sw.js activated");
  event.waitUntil(
    caches.keys().then(keyList => {
      return Promise.all(keyList.map(key => {
        if (key != CACHE_NAME) {
          console.log('[ServiceWorker] Removing old cache', key);
          return caches.delete(key);
        }
      }));
    })
  );
});

self.addEventListener('fetch', event => {
  if (event.request.mode != 'navigate')
    return;

  event.respondWith(
    fetch(event.request)
        .catch(async () => {
          return caches.open(CACHE_NAME)
              .then(cache => {
                return cache.match('offline.html');
              });
        })
  );
});