Express.js Complete Guide: Building Scalable Web Applications

Getting Started

Express.js is a minimal and flexible Node.js web application framework that provides robust features for web and mobile applications.

Basic Setup

javascript
import express from 'express';
const app = express();

app.use(express.json());
app.use(express.urlencoded({ extended: true }));

app.get('/', (req, res) => {
  res.json({ message: 'Hello World' });
});

app.listen(3000);

Routing

Express provides a powerful routing system.

RESTful Routes

javascript
// GET all users
app.get('/api/users', async (req, res) => {
  const users = await User.find();
  res.json(users);
});

// GET single user
app.get('/api/users/:id', async (req, res) => {
  const user = await User.findById(req.params.id);
  if (!user) return res.status(404).json({ error: 'User not found' });
  res.json(user);
});

// POST new user
app.post('/api/users', async (req, res) => {
  const user = await User.create(req.body);
  res.status(201).json(user);
});

// PUT update user
app.put('/api/users/:id', async (req, res) => {
  const user = await User.findByIdAndUpdate(
    req.params.id,
    req.body,
    { new: true }
  );
  res.json(user);
});

// DELETE user
app.delete('/api/users/:id', async (req, res) => {
  await User.findByIdAndDelete(req.params.id);
  res.status(204).send();
});

Middleware

Middleware functions have access to req, res, and next.

Application-Level Middleware

javascript
// Logger middleware
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

// Authentication middleware
app.use('/api/protected', (req, res, next) => {
  if (req.headers.authorization) {
    next();
  } else {
    res.status(401).json({ error: 'Unauthorized' });
  }
});

Error Handling Middleware

javascript
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({
    error: 'Something went wrong!',
    message: process.env.NODE_ENV === 'development' ? err.message : undefined
  });
});

Best Practices

1. 1Use router modules: Organize routes by feature
2. 2Validate input: Use validation middleware
3. 3Secure your app: Helmet, CORS, rate limiting
4. 4Handle errors: Never let errors crash your app
5. 5Use async/await: Modern async patterns

Project Structure

src/
├── routes/
│   ├── auth.js
│   ├── users.js
│   └── posts.js
├── middleware/
│   ├── auth.js
│   ├── validation.js
│   └── errorHandler.js
├── controllers/
├── models/
└── app.js

Conclusion

Express.js provides a solid foundation for building web applications. Follow these patterns to build maintainable, scalable applications.