Almost every senior Node question is the same question wearing a different hat: do you understand that there is one thread doing your work, and do you know what happens when you block it.
Node interviews look varied and are not. Streams, timers, clustering, memory, async error handling: pull on any of them and you end up at the event loop, which is why interviewers use them as entry points.
The event loop, said properly
The definition everyone gives is that Node is non-blocking and asynchronous. True, and it does not demonstrate anything. The useful version explains the division of labour.
Your JavaScript runs on one thread. Input and output does not: it is handed to the operating system or to a thread pool, and when it completes, the callback is queued back onto that single thread. That is why thousands of idle connections cost almost nothing, and it is also why one expensive synchronous function stops every other request on that process, not just the one it belongs to.
