service-worker.js

const CACHE_NAME = "my-first-pwa-cache-v1";
const urlsToCache = [
  "./",
  "./css/built-in.css",
  "./css/built-in-offline-page.css",
  "./js/babel.js",
  "./js/react.development.js",
  "./js/react-dom.development.js",
  "./js/built-in.js"
];

// Install service worker
self.addEventListener("install", event => {
  event.waitUntil(
    caches.open(CACHE_NAME).then(cache => {
      return cache.addAll(urlsToCache);
    })
  );
});

// Update service worker
self.addEventListener("activate", event => {
  const cacheWhitelist = [
    "my-first-pwa-cache-v1-foo",
    "my-first-pwa-cache-v1-bar"
  ];

  event.waitUntil(
    caches.keys().then(cacheNames => {
      return Promise.all(
        cacheNames.map(cacheName => {
          if (cacheWhitelist.indexOf(cacheName) === -1) {
            return caches.delete(cacheName);
          }
        })
      );
    })
  );
});

// Cache and return requests
self.addEventListener("fetch", event => {
  event.respondWith(
    caches.match(event.request).then(response => {
      if (response) {
        return response;
      }
      const fetchRequest = event.request.clone();
      return fetch(fetchRequest).then(response => {
        if (!response || response.status !== 200 || response.type !== "basic") {
          return response;
        }
        const responseToCache = response.clone();

        caches.open(CACHE_NAME).then(cache => {
          cache.put(event.request, responseToCache);
        });

        return response;
      });
    })
  );
});