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Explain the Node.js event loop.

Q: Explain the Node.js event loop.

**TL;DR.** The **event loop** is the heart of Node's concurrency: a single thread that repeatedly runs queued callbacks across ordered **phases** (timers → pending → poll → check → close). Between phases (and after each callback) it drains the **microtask** queues ( process.nextTick , then Promises). This lets one thread juggle many async operations without blocking. timers setTimeout/Interval pending callbacks poll

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A core concept in Node.js, this question tests understanding of its asynchronous nature.

event loopasynchronousconcurrencyarchitecture

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Ordering: sync → nextTick → Promise → timers → immediate

console.log('1 sync');
setTimeout(() => console.log('5 timeout'), 0);
setImmediate(() => console.log('6 immediate'));
Promise.resolve().then(() => console.log('4 promise'));
process.nextTick(() => console.log('3 nextTick'));
console.log('2 sync');

// 1, 2 (sync) → 3 nextTick → 4 promise (microtasks)
// → 5 timeout / 6 immediate (later phases)

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