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

service-worker-clean-cache.js

const clearOldCaches = () => {
  return caches.keys().then(keys => {
    return Promise.all(keys.filter(key => !key.startsWith(version)).map(key => caches.delete(key)));
  });
};

self.addEventListener('activate', event => {
  event.waitUntil(clearOldCaches().then(() => self.clients.claim()));
});