That frontend code already exists and does that every time you vote (line 27 of https://news.ycombinator.com/hn.js). The response content isn't used for anything at all.

Nice! I hadn't actually looked at the code before writing my comment. That does seem relatively trivial, so I stand corrected. I didn't realize the clientside code was discarding the response; I sorta naively assumed it was doing a DOM replacement or something equally straightforward.

What a pleasantly tiny script file. More site code should be this easy to read.

If you have JavaScript disabled, the response content is used to hide the upvote sign.

We're talking here about something simple like:

  (if *ajax-request*
     (upvote)
     (upvote-and-generate-page))