Service Worker Sample: Custom Offline Page Sample

Available in Chrome 40+ | View on GitHub | Browse Samples


This sample demonstrates basic service worker registration. During the installation step, a custom "Sorry, you're offline." page is cached. The service worker handles all fetch requests for HTML pages (ignoring other requests), and if fetching an HTML page fails due to a network error, it will instead return the cached "offline" page.

Try disconnecting from your network, and then reload this page.

This Page's JavaScript

if ('serviceWorker' in navigator) {

Service Worker's JavaScript

Copyright 2015, 2019 Google Inc. 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
 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 See the License for the specific language governing permissions and
 limitations under the License.

// Incrementing OFFLINE_VERSION will kick off the install event and force
// previously cached resources to be updated from the network.
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;
    // 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'}));

self.addEventListener('activate', (event) => {
  event.waitUntil((async () => {
    // Enable navigation preload if it's supported.
    // See
    if ('navigationPreload' in self.registration) {
      await self.registration.navigationPreload.enable();

  // Tell the active service worker to take control of the page immediately.

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;

        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.
        // If fetch() returns a valid HTTP response with a response code in
        // the 4xx or 5xx range, the catch() will NOT be called.
        console.log('Fetch failed; returning offline page instead.', error);

        const cache = await;
        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.