Not nodejs, but I am in the process of tracking down and event loop loop (e.g. emit( event_id ) somewhere in the path of an event lisener of that name). I assume this can happen within node too.
The code is based on boost asio, so the io_service is on the same thread as your code if you only start 1. Then after that I am in gdb/ldb. Asio's io_service handles the async calls portion and dispatches to my callback. From there on it is no longer necessarily async. The fun is that I have a higher level event handler sitting above this to register/unregister listeners, like one would in node. If one of those listeners happens to emit a message to what is is also listening for, boom.
long store short, it's not the async code per say, but after that.