sw.js

/**
 * Copyright 2022 Google LLC. All Rights Reserved.
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *     http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

const CACHE_NAME = 'offline';
// Customize this with a different URL if needed.
const OFFLINE_URL = './offline.html';

self.addEventListener('install', (event) => {
  event.waitUntil(
    (async () => {
      const cache = await caches.open(CACHE_NAME);
      // Setting {cache: 'reload'} in the new request will ensure that the
      // response isn't fulfilled from the HTTP cache; i.e., it will be from
      // the network.
      await cache.add(new Request(OFFLINE_URL, { cache: 'reload' }));
    })(),
  );
  // Force the waiting service worker to become the active service worker.
  self.skipWaiting();
});

self.addEventListener('activate', (event) => {
  event.waitUntil(
    (async () => {
      // Enable navigation preload if it's supported.
      // See https://developers.google.com/web/updates/2017/02/navigation-preload
      if ('navigationPreload' in self.registration) {
        await self.registration.navigationPreload.enable();
      }
    })(),
  );
  // Tell the active service worker to take control of the page immediately.
  self.clients.claim();
});

self.addEventListener('fetch', (event) => {
  // We only want to call event.respondWith() if this is a navigation request
  // for an HTML page.
  if (event.request.mode === 'navigate') {
    event.respondWith(
      (async () => {
        try {
          // First, try to use the navigation preload response if it's supported.
          const preloadResponse = await event.preloadResponse;
          if (preloadResponse) {
            return preloadResponse;
          }

          // Always try the network first.
          const networkResponse = await fetch(event.request);
          return networkResponse;
        } catch {
          const cache = await caches.open(CACHE_NAME);
          const cachedResponse = await cache.match(OFFLINE_URL);
          return cachedResponse;
        }
      })(),
    );
  }

  // If our if() condition is false, then this fetch handler won't intercept the
  // request. If there are any other fetch handlers registered, they will get a
  // chance to call event.respondWith(). If no fetch handlers call
  // event.respondWith(), the request will be handled by the browser as if there
  // were no service worker involvement.
});