What node will do is kick off the process (dns, grab a socket, connect tcp, write the header, wait for response etc.)... but it won't wait for any of this to finish, it just starts it on another thread. That thread doesn't need any JS-level synchronization because it has nothing to do with JS. Furthermore, you can kick off multiple requests like this in parallel.

When a request is done and has data to pass back, the external thread will queue up a V8 event. When V8 is free and is ready for the next event, it will see the finished request and trigger your callback with the data. When you're done handling that data (in Javascipt), V8 will wait for the next event and so on.

So you see, it's parallel but doesn't need any JS synchronizatioon.

The way it's implemented is similar to Windows 3.x times - collaborative multitasking. Then there is no code to execute in your thread, an background event loop runs, listens for messages from another threads and calls appropriate callback function in your thread.

This design has the benefit of being simple to do simple stuff. And near impossible for more complex multithreaded algorithms that require shared memory.