What it is trained on is only necessary to teach it basic language and coding patterns. If you want it to understand js, literally feed it all the relevant js code and documentation you need and then work from there. I've done this with brand new apps and software that has practically zero online conversation and that the LLM has absolutely no knowledge of. It works incredibly well. At this point I like how Kodi Low puts it -- the LLMs have reached the point where if it doesn't do what you want, its not that the LLM isn't capable, its that you have to be more specific or clever about *how you ask it.* Keep exploring, there are huge increases in LLM capability that are entirely in better understanding how to utilize them.