Service Worker gotchas – https://medium.com/kollegorna/service-worker-gotchas-af20a9dab986

service-worker-offline-file-external.js

event.respondWith(caches.match(request).then(response => {
  return response // cache-first
  || fetch(request).then(response => { // network
    // ...
  })
  .catch(() => { // offline
    if(type.startsWith('image')) {
      return caches.match('/offline.jpg');
    }
    else {
      // ...
    }
  });
}));