Build Like a Pro: Your First Express.js App with a Modular Structure

So you want to build a backend with Node.js. You’ve heard about Express.js, the minimal and flexible framework that powers countless web applications. Starting with Express is famously simple, but moving from a single "Hello, World!" file to a maintainable, professional application requires a good structure. The most common mistake beginners make is stuffing all their logic into one giant server.js file.

This guide will teach you how to avoid that pitfall from day one by building your first Express.js app with a clean, modular routing structure. It's a simple change that makes a world of difference as your project grows.

Prerequisites

Before we start, make sure you have Node.js and npm (which comes with Node.js) installed on your system. You can download it from the official Node.js website.

Step 1: Project Setup

First, let's create a new project directory and initialize it with npm.


mkdir my-express-app
cd my-express-app
npm init -y

Now, let's install Express:


npm install express

Step 2: The Basic Server (The "Old" Way)

Let's create a file named index.js. A simple Express server looks like this:


// index.js
const express = require('express');
const app = express();
const PORT = 3000;

app.get('/', (req, res) => {
  res.send('Hello, World!');
});

app.get('/users', (req, res) => {
  res.json([{ id: 1, name: 'John Doe' }]);
});

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

You can run this with node index.js. This works perfectly fine, but imagine having 50 more routes for products, orders, comments, etc. This file would become a nightmare to manage. Let's fix that.

Step 3: The Modular Approach - Creating a Routes Folder

The key to a scalable structure is to separate your concerns. Routes related to "users" should live in a "users" file. Routes for "products" should live in a "products" file. We'll organize these into a routes folder.

Create a new folder in your project directory called routes.


mkdir routes

Step 4: Create Your First Route File

Inside the routes folder, create a file named users.js. Here, we'll use the Express Router. It's like a mini-app that can handle its own set of routes.


// routes/users.js
const express = require('express');
const router = express.Router();

// This route corresponds to the /users endpoint
router.get('/', (req, res) => {
  res.json([{ id: 1, name: 'John Doe' }]);
});

// You could add more user-specific routes here
// For example, a route to get a specific user by ID
router.get('/:id', (req, res) => {
  const userId = req.params.id;
  // In a real app, you would fetch the user from a database
  res.json({ id: userId, name: `User ${userId}` });
});

module.exports = router; // Export the router

Notice we defined the path as '/'. This is because we will tell our main index.js file to use this router for any path that starts with /users.

Step 5: Connect Your Routes to the Main App

Now, let's update our main index.js file to use our new modular router. It becomes much cleaner!


// index.js
const express = require('express');
const app = express();
const PORT = 3000;

// Import your route files
const userRoutes = require('./routes/users');

app.get('/', (req, res) => {
  res.send('Welcome to the homepage!');
});

// Tell the app to use the user routes for any path that starts with /users
app.use('/users', userRoutes);

// You can easily add more route modules here!
// const productRoutes = require('./routes/products');
// app.use('/products', productRoutes);

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

Now, when you run node index.js:

A request to http://localhost:3000/ is handled by index.js.
A request to http://localhost:3000/users is passed to the users.js router and handled by its '/' route.
A request to http://localhost:3000/users/123 is passed to the users.js router and handled by its '/:id' route.

Conclusion: Why This Matters

By adopting this simple modular structure from the very beginning, you've set yourself up for success. Your main server file (index.js) remains clean and acts as a central hub, while the specific logic for each part of your application is neatly organized in its own file. As your application grows, you can simply add new files to the routes folder without cluttering up your existing code.

This separation of concerns is a fundamental principle in software engineering, and mastering it early will make you a more effective and professional Node.js developer.

Step 2: The Basic Server (The "Old" Way)

Let's create a file named index.js. A simple Express server looks like this:

// index.js const express = require('express'); const app = express(); const PORT = 3000; app.get('/', (req, res) => { res.send('Hello, World!'); }); app.get('/users', (req, res) => { res.json([{ id: 1, name: 'John Doe' }]); }); app.listen(PORT, () => { console.log(`Server is running on http://localhost:${PORT}`); });

You can run this with node index.js. This works perfectly fine, but imagine having 50 more routes for products, orders, comments, etc. This file would become a nightmare to manage. Let's fix that.

Step 4: Create Your First Route File

Inside the routes folder, create a file named users.js. Here, we'll use the Express Router. It's like a mini-app that can handle its own set of routes.

// routes/users.js const express = require('express'); const router = express.Router(); // This route corresponds to the /users endpoint router.get('/', (req, res) => { res.json([{ id: 1, name: 'John Doe' }]); }); // You could add more user-specific routes here // For example, a route to get a specific user by ID router.get('/:id', (req, res) => { const userId = req.params.id; // In a real app, you would fetch the user from a database res.json({ id: userId, name: `User ${userId}` }); }); module.exports = router; // Export the router

Notice we defined the path as '/'. This is because we will tell our main index.js file to use this router for any path that starts with /users.

Step 5: Connect Your Routes to the Main App

Now, let's update our main index.js file to use our new modular router. It becomes much cleaner!

// index.js const express = require('express'); const app = express(); const PORT = 3000; // Import your route files const userRoutes = require('./routes/users'); app.get('/', (req, res) => { res.send('Welcome to the homepage!'); }); // Tell the app to use the user routes for any path that starts with /users app.use('/users', userRoutes); // You can easily add more route modules here! // const productRoutes = require('./routes/products'); // app.use('/products', productRoutes); app.listen(PORT, () => { console.log(`Server is running on http://localhost:${PORT}`); });

Now, when you run node index.js:

A request to http://localhost:3000/ is handled by index.js.

A request to http://localhost:3000/users is passed to the users.js router and handled by its '/' route.

A request to http://localhost:3000/users/123 is passed to the users.js router and handled by its '/:id' route.

Conclusion: Why This Matters

This separation of concerns is a fundamental principle in software engineering, and mastering it early will make you a more effective and professional Node.js developer.