https://www.secondsout.com/service-worker.js

1const OFFLINE_CACHE = 'offline-v1'
2const OFFLINE_URL = 'offline.html'
3
4self.addEventListener('install', e => {
5    e.waitUntil(
6        caches.open(OFFLINE_CACHE).then(cache => {
7            return cache.add(
8                new Request(OFFLINE_URL, { cache: 'reload' })
9            ).then(() => self.skipWaiting())
10        })
11    )
12})
13
14self.addEventListener('activate', e => {
15    e.waitUntil(self.clients.claim())
16})
17
18self.addEventListener('fetch', e => {
19    const request = e.request
20
21    if (request.method === 'GET') {
22        e.respondWith(
23            fetch(request).catch(error => {
24                return caches.open(OFFLINE_CACHE).then(cache => cache.match(OFFLINE_URL))
25            })
26        )
27    }
28})