Node.js: Introduction, Environment Setup, and the REPL

What is Node.js?

Node.js is a JavaScript runtime built on Chrome's V8 engine that lets JavaScript run outside the browser — on a server. It is:

• Single-threaded with an event loop for non-blocking, asynchronous I/O
• Ideal for I/O-heavy applications: APIs, real-time apps, streaming
• Not a language or framework — it's a runtime environment

Node.js vs Traditional Server-Side Models

Environment Setup

1. Download the installer from nodejs.org (LTS version recommended)
2. Verify installation:

node -v      # e.g., v20.11.0
npm -v       # e.g., 10.2.4

1. Run a JavaScript file with Node:

node app.js

The REPL (Read-Eval-Print Loop)

Typing node with no arguments starts the REPL — an interactive shell for running JavaScript line by line, useful for quick testing.

$ node
> 2 + 3
5
> const greet = name => `Hello, ${name}!`;
undefined
> greet("World")
'Hello, World!'
> .exit

Useful REPL Commands

First Node.js Program

// hello.js
console.log("Hello, Node.js!");

const os = require('os');
console.log("Platform:", os.platform());
console.log("CPU cores:", os.cpus().length);

node hello.js

NPM (Node Package Manager)

NPM installs and manages third-party packages and is bundled with Node.js.

npm init -y                  # Create package.json with defaults
npm install express          # Install a package (saved as a dependency)
npm install --save-dev nodemon  # Install as a dev-only dependency
npm install                  # Install all dependencies listed in package.json
npm uninstall express        # Remove a package
npm list                     # List installed packages
npm outdated                 # Check for newer versions

package.json

{
  "name": "my-app",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "start": "node index.js",
    "dev": "nodemon index.js"
  },
  "dependencies": {
    "express": "^4.18.2"
  },
  "devDependencies": {
    "nodemon": "^3.0.1"
  }
}

Key Takeaway: Node.js runs JavaScript on the server using V8 and a non-blocking event loop, making it ideal for I/O-heavy applications. The REPL is great for quick experiments, and NPM (with package.json) manages your project's dependencies.