hardSystem Design

How do you achieve concurrency in Node.js since it's single-threaded?

Q: How do you achieve concurrency in Node.js since it's single-threaded?

**TL;DR.** Node's JS runs on one thread, but it achieves **concurrency** three ways: (1) **non-blocking I/O** via the event loop + libuv (the everyday model), (2) **worker_threads** for CPU-bound work in the same process, and (3) **child_process / cluster** to run multiple processes across CPU cores. Pick based on whether the bottleneck is **I/O** or **CPU**. Event loop non-blocking I/O libuv thread pool best for I/O-bound <rect class='d-box' x='162' y='40' width='135' height='120'

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A common misconception about Node.js; this checks a deeper understanding.

concurrencysingle-threadedevent loopworker threads

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Offload CPU work to a worker so the loop stays free

import { Worker } from 'node:worker_threads';

function runHeavy(n) {
  return new Promise((resolve, reject) => {
    const w = new Worker('./fib-worker.js', { workerData: n });
    w.once('message', resolve);
    w.once('error', reject);
  });
}

// The event loop keeps handling requests while the worker computes.
const result = await runHeavy(45);

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