JavaScript – AJAX Requests and Client-Side JavaScript

I think real-time, client-side, more responsive interface features sometimes end up being an exception to the DRY rule in cases just like the one you've described. The need for responsiveness merits client-side code, but often one finds themselves calculating these values on the backend before passing them to the front-end; the values are then dynamically recalculated when the user adjusts the interface somehow.

One thing I can say for certain: this is not worth a roundtrip to the server, unless it's ok that your values on the front-end only update only second or more. Any less than that and you may have not only issues with responses returning out of order, but a certain jarring refresh behavior even if the former isn't a problem.

I've always thought that - at some point - this would be the main reason for using Javascript on both the front and backend but I dunno if anyone's ever taken a stab at a common code transport or something like that.

If anything, I would question whether or not it's necessary to have these calculations in your backend model/server-side code at all - perhaps you need to ensure the linear contraints between the variables are met for consistency reasons, ones of security, etc, but - if not, and if you don't explicitly call these calculations from somewhere in your backend - then I would suggest you consider taking them out of there.

It's not so much "fake" performance as real responsiveness. There are a number of reasons why it's popular:

• Internet connections are fairly reliable nowadays. The risk of an AJAX request failing is very low.
• The operations being performed are not really safety critical. If your emails don't get deleted on the server, the worst that happens is you have to delete them again the next time you come to the page.
• You can design your page to undo the action if the request fails, but you don't really have to be that subtle because it means your connection to the server is broken. It's easier to say "Connection lost. Unable to save recent changes. Try again later."
• You can design your page to only allow one pending AJAX request, so your state won't get too out of sync from the server.
• The browser warns you if you try to close or navigate away from a page while an AJAX request is pending.