mediumAmazonDSA2024 · 2022

How does the Node.js event loop work?

Q: How does the Node.js event loop work?

**TL;DR.** Practically: Node runs your synchronous code first, then enters the **event loop**, which in each tick runs a phase's callbacks and, after *every* callback, drains **microtasks** ( process.nextTick → Promises) before moving on. The single thread keeps cycling phases until no timers, I/O, or immediates remain, then exits. Macrotask queue timers, I/O, setImmediate one at a time <te

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Walk through the phases of the Node.js event loop and where microtasks fit.

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Microtasks always finish before the next macrotask

setTimeout(() => console.log('macro: timeout'), 0);

Promise.resolve()
  .then(() => console.log('micro 1'))
  .then(() => console.log('micro 2'));   // chained microtasks

// Output: micro 1, micro 2, then "macro: timeout"
// Both microtasks drain before the loop advances to the timer phase.

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