Chapter 2: Intro to Node.js & Express

Welcome to Chapter 2! In the last chapter, we decided to learn **Node.js** because we already know JavaScript. But what is Node.js exactly? And what is this "Express" framework everyone talks about?

What is Node.js?

Node.js is **not** a language. It's a **runtime environment**. This means it's a program that lets you run your JavaScript code *outside* of a web browser. Node.js lets you use JavaScript to do "server-side" tasks, like creating web servers, connecting to databases, and reading files.

What is Express.js?

Node.js *can* create a web server all by itself, but the code is very complicated. **Express.js** is a **framework** that sits on top of Node.js and makes everything *much* easier. It gives you a clean, simple set of tools to handle common web server tasks.

Let's Build Your First Server (Step-by-Step)

To follow along, you need to have Node.js installed. Open your terminal and follow these steps.

Step 1: Set Up Your Project

Create a new folder, move into it, and initialize it with **npm** (the Node Package Manager).

# Create a new folder and move into it
mkdir my-first-server
cd my-first-server

# Initialize the project
npm init -y

This command creates a `package.json` file, which keeps track of all the packages your project needs.

Step 2: Install Express

# Install Express
npm install express

Step 3: Create Your Server File

Create a new file in your folder named `server.js`.

Step 4: Write the "Hello World" Server Code

Open `server.js` and type in the following code.

// 1. Import the Express library
const express = require('express');

// 2. Create an instance of an Express app
const app = express();

// 3. Define a port for our server to run on
const PORT = 3000;

// 4. Define the main route ('/')
// req = request (what the user sends)
// res = response (what we send back)
app.get('/', (req, res) => {
    res.send('Hello, World! This is my first server!');
});

// 5. Start the server
app.listen(PORT, () => {
    console.log(`Server is running live on http://localhost:${PORT}`);
});

Step 5: Run Your Server!

Save the file. Go back to your terminal and run this command:

node server.js

Your terminal should show: `Server is running live on http://localhost:3000`. Now, open your web browser and go to **http://localhost:3000** to see your message!

Core Concept: Routing

**Routing** is how Express handles different URLs. You just add more `app.get()` handlers for different paths.

// ... (setup code is the same) ...

// Route for the homepage
app.get('/', (req, res) => {
    res.send('This is the HOMEPAGE.');
});

// NEW: Route for the about page
app.get('/about', (req, res) => {
    res.send('This is the ABOUT page.');
});

// ... (app.listen code is the same) ...

Core Concept: Sending JSON (Building an API)

Most of the time, your backend will send **JSON** (JavaScript Object Notation) data to your frontend (like a React app). Express has a built-in method, `.json()`, to do this.

// ... (setup code is the same) ...

// A route for sending user data
app.get('/api/user', (req, res) => {
    const user = {
        id: 1,
        username: 'MSMAXPRO'
    };
    
    // Send the object as JSON
    res.json(user);
});

// ... (app.listen code is the same) ...

Now, if you go to `http://localhost:3000/api/user`, you'll see structured JSON data, ready for your React app to fetch!

Conclusion and Next Steps

You've just learned the fundamentals of Node.js and Express. You know how to start a project, create a server, handle routes, and send API data. The next step is to learn how to connect your Express server to a **Database (like MongoDB)** to save data permanently.