What is the Node.js event loop?
The event loop allows Node.js to perform non-blocking I/O despite being single-threaded. It offloads I/O to the system kernel and processes results via callbacks. It has phases: timers (setTimeout/setInterval), poll (I/O events), check (setImmediate), and close. Microtasks (nextTick, Promises) run between phases.
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process.nextTick fires first (immediately after current operation), then Promise.then (microtask), then setTimeout and setImmediate (order varies in the same context, but setImmediate always fires after I/O in an I/O callback).
Node.js uses a single thread for JavaScript execution (V8) to avoid context switching and thread synchronization complexity. It uses libuv's thread pool (4 threads by default) for I/O operations and non-blocking I/O to handle thousands of concurrent connections efficiently.
process.nextTick fires immediately after the current operation, before I/O events (can starve I/O if overused). setImmediate fires in the next event loop iteration, after I/O events (safe for recursive calls). nextTick always fires before setImmediate.
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