I love these projects so much. It must be exhilarating to work on them and I'm very tempted to work on one myself as a side project/self learning experience.

There's hundreds or thousands of these indie game engines because, yes!, it is a lot of fun to write them. You'd learn a lot. The #1 tip though is that you're writing a game, or an engine, not both. So commit to doing one of those tasks. Usually you end up with a bunch of toy games and no steam left to want to really polish them into something complete.

Do you know of any that use JavaScript/typescript?

If you want to know about the really small indie ones that are less for actual use and more for just "I want to make a game engine" I would check out the various gamedev Discord and Reddit channels. There's always people showing off their work.