Short (naive) Service Worker demos
Please note these are not production level examples.
Please try the push example in Firefox, since not GCM id is provided. Update the subscription in server-push.js and run node server-push.js to send a push to the subscribed client.
When implementing for production, consider authorization through VAPID: https://developers.google.com/web/updates/2016/07/web-push-interop-wins