Node.js is an open-source, cross-platform runtime that executes JavaScript outside the browser by embedding the V8 engine inside a C++ layer. It provides a complete stack—runtime, interpreter, and native bindings—so you can build network services, CLI tools, and full-stack applications using a single language.

Why Use Node.js Instead of Just the Browser?

JavaScript is an interpreted language: every .js file must be parsed and executde by an engine. In the browser, that engine is V8 (Chrome), SpiderMonkey (Firefox), or JavaScriptCore (Safari). When you move to server-side or build tooling, you need the same engine available on the host OS. Node.js fills that gap, acting as a portable "mini server" that:

• Bootstraps your build pipeline (Webpack, Vite, Rollup).
• Serves APIs with minimal overhead.
• Runs scripts for CI/CD, data migrations, or cron jobs.

Installation & Version Switching

Download the LTS installer from nodejs.org or use a version manager:

# Install via nvm (macOS/Linux)
curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.39.0/install.sh | bash
nvm install --lts
nvm use --lts

# Verify
node -v   # e.g., v20.12.2
npm -v    # e.g., 10.5.0

Module Resolution Walk-through

When require('something') is encountered, Node.js searches in the following order:

1. Core modules such as fs, http, crypto.
2. Relative files (./foo.js, ../lib/bar.json).
3. Folder modules (./myLib/) that contain an index.js or a package.json#main entry.
4. node_modules walking upward from the current directory to the filesystem root.

Example project structure:

project/
├── src/
│   └── app.js
├── utils/
│   └── helper.js
├── node_modules/
│   └── lodash/
└── package.json

Package Management & Mirrors

npm ships with Node.js, but its default registry can be slow in some regions. Switch to a mirror:

# One-off install via Taobao mirror
npm install lodash --registry=https://registry.npmmirror.com

# Permanent alias
npm config set registry https://registry.npmmirror.com

Install project dependencies:

npm init -y
npm install express dotenv helmet

REPL: Instant Feedback Loop

Launch the enteractive shell to test snippets:

$ node
> const crypto = require('crypto');
> crypto.randomBytes(16).toString('hex');
'9f8e5b2c...'
> .exit

Common CLI Recipes

Minimal HTTP Server with Express

// server.js
import express from 'express';
const app = express();
const PORT = process.env.PORT || 3000;

app.get('/', (_req, res) => res.json({ uptime: process.uptime() }));

app.listen(PORT, () => console.log(`Listening on ${PORT}`));

Run with:

node server.js
# or
npx nodemon server.js