Service Worker

Definition

A JS script that gets registered with the browser
stays registered with the browser even when offline
data can still be used despite internet being disconnected
can load content even with no connection
they cannot directly access the DOM
- instead, it communicate with the pages it controls
  - by responding to messages sent by the post message interface
  - these pages can manipulate the DOM if needed
it is a programmable network proxy
- allows you to control how network requests from your page are handled
it gets terminated when not being used
- and gets restarted when they are needed again
makes use of ES6 promises
requires HTTPS unless on localhost

web browser > service worker > remote server
service worker acts as a middleman and can intercept request
- it will then decide what should happened after intercepting the request
  - such as decide whether to show the remote version or offline version

most commonly used for offline browsing & caching assets & API calls
push notifications (Push & Notification API) are also a big part of service workers
- these are notifications that will pop up on your desktop or mobile device
  - good for marketing and advertising
background data sync / preload
- isn't yet fully supported for all browsers
- a new API that lets you defer actions until the user has a stable connection
  - e.g.: a user likes a post on a social network,
    - in this case, a request needs to be sent to a server, which isn't possible due to the unstable connection
    - by using this browser sync API, it allows such action to be deferred or put on hold in the cache until connectivity is restored
      - then the post will get liked
used in Progressive Web Apps