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What is the difference between blocking and non-blocking I/O in Node.js?

Q: What is the difference between blocking and non-blocking I/O in Node.js?

**TL;DR.** **Blocking** I/O makes the thread **wait** until the operation completes (e.g. fs.readFileSync ) — nothing else runs meanwhile. **Non-blocking** I/O **returns immediately** and notifies you later via a callback/Promise (e.g. fs.readFile , fs.promises.readFile ), letting the single thread serve other work while the OS/libuv does the I/O. Blocking read… (thread frozen) next task (waits) Non-blocking <rect

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This question delves into the core I/O model of Node.js.

i/oblockingnon-blockingasynchronous

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Same read, blocking vs non-blocking

import { readFileSync, readFile } from 'node:fs';

// Blocking: thread frozen until the file is read
const cfg = readFileSync('./config.json', 'utf8');   // OK at startup

// Non-blocking: returns immediately, callback later
readFile('./data.json', 'utf8', (err, data) => {
  if (err) throw err;
  console.log('got data while the thread served other work');
});

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