hardSystem Design

Describe the Node.js architecture.

Q: Describe the Node.js architecture.

**TL;DR.** Node layers **your JS** on top of **V8** (executes JS), **libuv** (event loop + thread pool + async I/O), and **C/C++ bindings** to OS APIs. The single JS thread runs your code and callbacks; libuv offloads blocking work and feeds completed results back through the **event loop**. Your JavaScript + core modules (http, fs…) Node bindings (C++) — bridge JS ↔ C libs V8 execute JS

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A high-level view of how Node.js is structured.

architecturev8libuvevent loop

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One async call touches every layer

import { readFile } from 'node:fs';

console.log('1: sync, runs on JS thread (V8)');

readFile('./data.json', 'utf8', (err, data) => {
  // 4: libuv thread pool finished the read, event loop runs this callback
  console.log('3: callback with file contents');
});

console.log('2: sync, runs before the callback');
// Output order: 1, 2, 3 — the read happened off-thread via libuv.

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