It's a straight line from Jesusonic to Reaper: Jesusonic JSFX script is in Reaper, and there's a whole selection of stock JS plugins that come with it and it's actually quite easy to program.

I always figured JSFX was javascript :) I've used reaper and plenty of jsfx plugins for a long time, just never bothered to look under the hood.

Whoa TIL. Jsfx feels way cooler now. Fwiw jsfx is easy fodder for llm’s if you need a quick utility. Mine was a midi channel filter