Middleware in Express.js

In Express.js, middleware is a function that sits between the client request and the server response.
It can inspect, modify, allow, or stop a request before it reaches the final route handler.

1. What Middleware Does

A middleware function can:

• Read request data

• Modify request or response objects

• Execute code

• End the request–response cycle

• Pass control to the next middleware

(req, res, next) => {
  // middleware logic
  next();
}

2. How Middleware Works (Flow)

Request → Middleware → Middleware → Route → Response

Each middleware decides whether to:

• Call next() → continue

• Send a response → stop the chain

3. Types of Middleware in Express.js

3.1 Application-Level Middleware

Applied to the entire app.

app.use((req, res, next) => {
  console.log(req.method, req.url);
  next();
});

3.2 Router-Level Middleware

Applied to specific routes.

const router = express.Router();

router.use((req, res, next) => {
  console.log('Router middleware');
  next();
});

3.3 Built-in Middleware

Provided by Express.

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

3.4 Third-Party Middleware

Installed using npm.

Examples:

• morgan – logging

• cors – enable CORS

• helmet – security

app.use(require('morgan')('dev'));

3.5 Error-Handling Middleware

Handles errors globally.

app.use((err, req, res, next) => {
  res.status(500).send('Server Error');
});

⚠ Must have four parameters.

4. Middleware Execution Order

• Executed in the order they are defined

• Order matters

• Error middleware runs only on errors

5. Conditional Middleware

app.use((req, res, next) => {
  if (req.method === 'POST') {
    console.log('POST request');
  }
  next();
});

6. Custom Authentication Middleware (Example)

function auth(req, res, next) {
  if (req.headers.token) {
    next();
  } else {
    res.status(401).send('Unauthorized');
  }
}

app.get('/secure', auth, (req, res) => {
  res.send('Protected Route');
});

7. Best Practices

• Keep middleware small and focused

• Always call next() when required

• Place error middleware at the end

• Avoid heavy logic in middleware

8. Advantages of Middleware

• Reusable logic

• Cleaner code

• Better request control

• Improved maintainability

9. Summary

• Middleware processes requests before responses

• Can modify, block, or pass requests

• Express supports multiple middleware types

• Execution order is critical