pages/sw.js

async function initCache() {
  const cache = await caches.open('v1')
  await cache.addAll([
    '/',
    '/index.html',
    '/app.js'
  ])
}

self.addEventListener("install", event => {
  event.waitUntil(initCache())
})

async function cacheFirst(request) {
  const resp = await caches.match(request)
  if (resp) {
    return resp
  } else {
    return fetch(request)
  }
}

self.addEventListener('fetch', event => {
  event.respondWith(cacheFirst(event.request))
})

self.addEventListener('activate', event => {
  event.waitUntil(clients.claim())
})