self.addEventListener('activate', event => {
  let expectedCacheNames = Object.keys(CURRENT_CACHES).map(function(key) {
    return CURRENT_CACHES[key];
  });

  event.waitUntil(
    caches.keys().then(cacheNames => {
      return Promise.all(
        cacheNames.map(cacheName => {
          if (expectedCacheNames.indexOf(cacheName) === -1) {
            console.log('Deleting out of date cache:', cacheName);
            return caches.delete(cacheName);
          }
        })
      );
    })
  );
});