How does the Node.js event loop work?

The Node.js event loop allows Node.js to perform non-blocking I/O operations despite JavaScript being single-threaded. It offloads operations to the system kernel or libuv thread pool and resumes execution when they complete.

Phases of the event loop (in order):

1. timers — executes setTimeout and setInterval callbacks
2. pending callbacks — I/O callbacks deferred from the previous iteration
3. idle, prepare — internal use only
4. poll — retrieves new I/O events; blocks here if queue is empty
5. check — executes setImmediate callbacks
6. close callbacks — handles close events

Between each phase, Node.js drains process.nextTick and Promise microtask queues completely.