Browser Database for offline application use

Most web applications communicate with server / backend to read and write data, and the browser’s job is just to fetch, and send.
What will happen if the user loses internet connection or if we need to create an offline application?
I encountered this situation in a real project, and to build a solid offline application, we should store data on the front-end side.
The topic of the session is how to implement storage management using IndexedDB.