Middleware in Express.js: 3 Practical Examples

Setting Up Middleware in Express.js

Middleware functions are an essential part of Express.js applications. They allow you to modify the request and response objects, end requests, and call the next middleware function. In this article, we’ll explore three diverse examples of setting up middleware in Express.js.

Example 1: Logging Middleware

Use Case

This middleware logs every request made to your server, helping you monitor incoming traffic and troubleshoot issues.

const express = require('express');
const app = express();

// Logging middleware function
const logger = (req, res, next) => {
    console.log(`\({req.method} request made to: }\(req.url}`);
    next(); // Pass control to the next middleware function
};

// Use the logger middleware
app.use(logger);

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

app.listen(3000, () => {
    console.log('Server is running on port 3000');
});

Notes

• You can customize the log message to include timestamps or request headers.
• This middleware applies to all routes in your application since it’s set up globally with app.use().

Example 2: Authentication Middleware

Use Case

This example demonstrates how to create middleware that checks if a user is authenticated before allowing access to certain routes, enhancing the security of your application.

const express = require('express');
const app = express();

// Mock authentication check
const isAuthenticated = (req, res, next) => {
    const userLoggedIn = false; // Simulate user not logged in
    if (userLoggedIn) {
        next(); // User is authenticated, proceed to the next middleware
    } else {
        res.status(401).send('Unauthorized: You must log in to access this resource.');
    }
};

// Protect this route using the authentication middleware
app.get('/protected', isAuthenticated, (req, res) => {
    res.send('Welcome to the protected route!');
});

app.listen(3000, () => {
    console.log('Server is running on port 3000');
});

Notes

• Replace the userLoggedIn variable with actual authentication logic (like checking a session or a token).
• This middleware can be applied to any route that requires authentication.

Example 3: Error Handling Middleware

Use Case

In this example, we set up a middleware function specifically for handling errors throughout the application, ensuring that users receive a friendly error message.

const express = require('express');
const app = express();

app.get('/', (req, res) => {
    throw new Error('Something went wrong!'); // Simulate an error
});

// Error handling middleware
app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send('Something broke! We are working on it.');
});

app.listen(3000, () => {
    console.log('Server is running on port 3000');
});

Notes

• The error handling middleware must be defined after all routes to catch any errors that occur.
• You can enhance the error response by providing more context or logging different error levels.

By implementing these examples of setting up middleware in Express.js, you’ll enhance your application’s functionality, security, and user experience. Happy coding!