lkarch.org/tools/breaklock/service-worker.js

var APP_NAME = 'breaklock',
    APP_VERSION = 12,
    CACHE_NAME = APP_NAME + '_' + APP_VERSION;
var filesToCache = [
  './',
  './?utm_source=homescreen',
  './app.css',
  './app.js',
  './assets/intro.svg',
  './assets/fonts/robotomono-light-webfont.woff2',
  './assets/fonts/robotomono-light-webfont.woff',
  './assets/fonts/robotomono-light-webfont.ttf'
];

// Service worker from Google Documentation

self.addEventListener('install', function(event) {
  // Perform install steps
  event.waitUntil(
    caches.open(CACHE_NAME)
      .then(function(cache) {
        return cache.addAll(filesToCache);
      })
  );
});

self.addEventListener('activate', function(event) {
  event.waitUntil(
    caches.keys().then(function(cacheNames) {
      return Promise.all(
        cacheNames.map(function(cacheName) {
          if (cacheName.indexOf(APP_NAME) === 0 && CACHE_NAME !== cacheName) {
            return caches.delete(cacheName);
          }
        })
      );
    })
  );
});

self.addEventListener('fetch', function(event) {
  event.respondWith(
    caches.match(event.request)
      .then(function(response) {
        // Cache hit - return response
        if (response) {
          return response;
        }

        // IMPORTANT: Clone the request. A request is a stream and
        // can only be consumed once. Since we are consuming this
        // once by cache and once by the browser for fetch, we need
        // to clone the response.
        var fetchRequest = event.request.clone();

        return fetch(fetchRequest).then(
          function(response) {
            // Check if we received a valid response
            if(!response || response.status !== 200 || response.type !== 'basic') {
              return response;
            }

            // IMPORTANT: Clone the response. A response is a stream
            // and because we want the browser to consume the response
            // as well as the cache consuming the response, we need
            // to clone it so we have two streams.
            var responseToCache = response.clone();

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

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