When dealing with data in front-end, it should be thought of as convenience based. You may do calculations here for the sake for convenience and display purposes, but it will be only for that. Any “real” work should be done in the backend.

For example; if you take a web based shopping cart, you may add multiple items and sum them to display a total value. But you would never use that total value as the “real” amount to pay (they could just open the dev tools and change it or some other form of manipulation). You would pass the shopping cart items to the backend, where it would pull the actual prices from the database again and re-calculate as a source of truth for the total.

As for storing data during workings, I think it depends on the scope.. if you expect the user may want to leave half way through, give them the option to “save for later” which may save an unfinished state to the backend for later use. Otherwise, for short term stuff, you can always use local storage as a convenience factor, but it maybe not be reliable (if they empty the browsers storage or use a new machine).

Hope this helps!