How to ensure online and offline data synchronization of web app?

Maintaining data synchronization for both online and offline users of a web app like a point of sale software is quite confusing and i hope i could get some ideas on understanding how that could be achieved.

For instance for the point of sale software lets say computer A and B all access the same online database and perform basic CRUD operations.

In case computer B goes offline without syncing data that it was working on with online database and computer A is online and therefore is in sync,how do you reconcile the data from computer B when its now online especially when same table rows are worked on by both computers.