building my codeigniter app to become pwa, can work offline and sync

I have point of sale based on codeigniter, this is app still work online only. I wanna build this app to work offline, app can cache assets offline and database offline. users can still do transaction when working offline after online users can sync to server database. example everything change on live server can cache to offline and every transaction like reducing stock will sync to server. searching for indexeddb and service worker developer.