serviceWorker.js

const PRECACHE = 'precache-v1'
const RUNTIME = 'runtime'

// A list of local resources we always want to be cached.
const PRECACHE_URLS = ['index.html', 'offline.html']

// The install handler takes care of precaching the resources we always need.
self.addEventListener('install', event => {
  event.waitUntil(
    caches
      .open(PRECACHE)
      .then(cache => cache.addAll(PRECACHE_URLS))
      .then(self.skipWaiting())
  )
})

// The activate handler takes care of cleaning up old caches.
self.addEventListener('activate', event => {
  const currentCaches = [PRECACHE, RUNTIME]
  event.waitUntil(
    caches
      .keys()
      .then(cacheNames => {
        return cacheNames.filter(
          cacheName => !currentCaches.includes(cacheName)
        )
      })
      .then(cachesToDelete => {
        return Promise.all(
          cachesToDelete.map(cacheToDelete => {
            return caches.delete(cacheToDelete)
          })
        )
      })
      .then(() => self.clients.claim())
  )
})

// The fetch handler serves responses for same-origin resources from a cache.
// If no response is found, it populates the runtime cache with the response
// from the network before returning it to the page.
self.addEventListener('fetch', event => {
  console.log('self.location.origin: ' + self.location.origin)
  // Skip cross-origin requests, like those for Google Analytics.
  if (event.request.url.startsWith(self.location.origin)) {
    console.log(event.request.url)
    event.respondWith(
      caches.match(event.request).then(cachedResponse => {
        if (cachedResponse) {
          return cachedResponse
        }

        return caches.open(RUNTIME).then(cache => {
          return fetch(event.request).then(response => {
            // Put a copy of the response in the runtime cache.
            return cache.put(event.request, response.clone()).then(() => {
              return response
            })
          })
        })
      })
    )
  }
})