I was looking for a NodeJS Activity Pub library, and the only promising one I found looks to be only partially complete. That said, I think most Fediverse platforms use implementations in other languages: Rust (Lemmy) , PHP (Kbin), Ruby (Mastadon), etc).

Several frontends use NodeJS, though. Perhaps you can contribute there or even contribute to Actvitypub-Express to bring that to feature completion?

Check out Firefish

misskey and it's forks (firefish, iceshrimp, sharkey, and the approx. million others) use a node/ts backend with a vue/ts frontend. peertube also uses node to the best of my knowledge.

that said:

• misskey itself is developed almost entirely in japanese so you probably won't be able to help out too much.
• firefish in my experience is doing a LOT of questionable design decisions (hello scylladb migration!) so i'm not sure if it's in a good shape right now
• iceshrimp is forked off from firefish and seems to be aiming to fix things up as opposed to introduce new features (their matrix chat has been really interesting to lurk in especially when it comes to database performance talk)
• on the other hand, sharkey follows the latest versions of misskey and has been adding a fair amount of useful features on top of it (post imports that actually work without sending notifications to everyone, message editing which misskey itself found too hard to do, etc.)