[Scarbutt]

1. In Node, the main event loop is single threaded. So if you want to do some work that is heavily CPU intensive, until you make an IO call, the Node server isn't free to handle another incoming request. You can test this out with a busy wait loop in a Node request handler. If you have that loop run for, say, 10 seconds, then no other incoming requests can be dispatched for 10 seconds. In other words, Node doesn't allow for preemptive interruption.

Nice. I will add that JS runtimes now have worker threads, while they are still terribly inefficient even when compared to OS threads they can alleviate this problem if you don't need to spawn more than number of cores worker threads. If you are using nodejs and that is not enough, welcome to the microservices world.