Node.js Middleware
Understanding Node.js Middleware
Middleware in Node.js acts as an intermediary function that operates between the client request and the server response. It manipulates, processes, or validates incoming data before passing it along to the next handler. Middleware is extensively used in frameworks like Express.js to manage tasks such as authentication, logging, request parsing, error handling, and response customization.
Real-World Example: Simulating a Server Request
1. Application-Level Middleware
Directly applies middleware to an entire application or specific routes.
Example
const express = require('express');
const app = express();
// Custom Logging Middleware
app.use((req, res, next) => {
console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
next(); // Continue processing
});
app.get('/', (req, res) => {
res.send('Hello, World!');
});
app.listen(3000, () => console.log('Server running on port 3000'));
Explanation:
This middleware logs each request’s timestamp, method, and URL before routing.
2. Router-Level Middleware
Applied to specific routes instead of the entire application.
Example
const express = require('express');
const app = express();
const router = express.Router();
// Authentication Middleware
const authenticate = (req, res, next) => {
const token = req.headers['authorization'];
if (token === 'secret-token') {
next(); // Valid token, proceed
} else {
res.status(403).send('Access Denied');
}
};
// Protecting a route
router.get('/dashboard', authenticate, (req, res) => {
res.send('Welcome to the dashboard');
});
app.use('/api', router);
app.listen(3000, () => console.log('Server running on port 3000'));
Explanation:
The middleware verifies the presence of a valid token before allowing access.
3. Error-Handling Middleware
Handles exceptions and failures within the request-response cycle.
Example
const express = require('express');
const app = express();
// Sample Route
app.get('/', (req, res) => {
throw new Error('Something went wrong!');
});
// Error Middleware
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).send({ message: 'Internal Server Error' });
});
app.listen(3000, () => console.log('Server running on port 3000'));
Explanation:
Any unhandled errors trigger this middleware, logging the stack trace and responding with a 500 status.
4. Built-in Middleware in Express.js
Express provides several predefined middleware functions:
Example
const express = require('express');
const app = express();
app.use(express.json()); // Enables JSON body parsing
app.post('/data', (req, res) => {
res.send(`Received: ${JSON.stringify(req.body)}`);
});
app.listen(3000, () => console.log('Server running on port 3000'));
Explanation:
express.json() parses incoming JSON payloads into req.body
5. Third-Party Middleware
Middleware libraries that extend functionalities beyond built-in features.
Example
const express = require('express');
const cors = require('cors');
const app = express();
app.use(cors()); // Enables Cross-Origin Resource Sharing
app.get('/public', (req, res) => {
res.send('This is accessible from any domain.');
});
app.listen(3000, () => console.log('Server running on port 3000'));
Explanation:
cors allows external clients to access the server’s resources without security restrictions.
Key Takeaways
- Middleware orchestrates the request-response lifecycle efficiently.
- next() ensures smooth flow control within middleware chains.
- Application-level, router-level, and error-handling middleware serve different use cases.
- Express provides built-in middleware for common functionalities.
- Third-party libraries extend middleware capabilities for advanced scenarios.