The implementation of libuv under nodejs (and the state of the art python async/await) actually uses a total of three threads. It's a complicated model and it's not even strictly correct to call it an "event loop." I don't completely understand it and very few people do.

From a high level you can think of it as a single thread that can only context switch during an IO call. Otherwise the function has to run and block all the way to termination. <---- This is what rachel is complaining about.