Node.js Architecture

TL;DR

Node.js is not a language or a framework; it’s a C++ program that wraps Google’s V8 JavaScript Engine and libuv (a C library) to allow JavaScript to run outside the browser and perform asynchronous I/O operations.

Mental Model

How It Works

1. V8 Engine: Converts JavaScript code into machine code that the processor can understand.
2. libuv: Provides the Event Loop and the Worker Thread Pool. It handles all asynchronous I/O operations (reading files, making network requests).
3. Node Bindings: C++ code that acts as a bridge, allowing JavaScript running in V8 to communicate with libuv and other C/C++ libraries.

Example

const fs = require('fs'); // A core module mapped via Node bindings

// The JS runs in V8, but reading the file is delegated to libuv
fs.readFile('data.txt', (err, data) => {
    if (err) throw err;
    console.log(data.toString());
});
console.log('Reading file...'); 
// 'Reading file...' prints first because libuv handles the I/O in the background

Common Interview Questions

What is the difference between V8 and libuv?

V8 executes your synchronous JavaScript code and performs memory allocation. libuv handles the asynchronous operations (like file system and network calls) via the event loop and thread pool.

Is Node.js completely single-threaded?

No. The Event Loop (which executes your JS code) is single-threaded. However, libuv maintains a hidden Thread Pool (usually 4 threads by default) to handle heavy operations like file I/O or crypto, making Node.js multi-threaded under the hood.