[sn9]

executeprogram is excellent for picking up JS/TS. Not necessarily front end, but the language.

Frontend Masters might be worth checking out.

[Rendello]

I only did Execute Program for a little bit and don't remember much, but the developer's Destroy All Software screencasts are what taught me how to write parsers. I must have rewatched that video 30 times while I was writing my first one.