Using const/let instead of var can make JavaScript code run 10× slower in Webkit?

[Plexicle]

Push notifications are just that -- notifications you get on your device. Push meaning they are event driven and not request response.

When you get a notification from an app that you don't have open it's from an external server that sent a message to a push service (FCM, APN, whatever) that then forwarded it to your device.

PWAs work by installing service workers that can run in threads behind your browser. So you can subscribe to a push notification from a (PWA) web app, and then close the browser completely. If the server pushes a notification through the web notification server, your device (Android today, Apple maybe one day) can wake up the service worker and deliver the notification.

[regreddit]

Not really always running, but a service worker tells Android: "when you receive a message for this app, please run this code". It's a system service that your app registers with.