How to Create Custom Middleware in Node.js with Express: A Step-by-Step Guide

Middleware is the backbone of Express.js applications, acting as a bridge between incoming requests and final responses. Whether you’re handling authentication, logging, or data validation, custom middleware helps you write cleaner, reusable code. In this guide, you’ll learn how to create your own middleware in Node.js using Express.js — even if you’re a beginner!

Table of Contents

1. What is Middleware?
2. Prerequisites
3. Types of Middleware in Express
4. Step-by-Step: Building Custom Middleware
5. Practical Examples
6. Best Practices
7. Conclusion

What is Middleware?

Middleware functions are functions that have access to the request object (req), response object (res), and the next() function in the application’s request-response cycle. They can:

• Execute any code.
• Modify req and res objects.
• End the request-response cycle.
• Call the next middleware using next().

Think of middleware as a chain of security guards checking your ID before letting you into a club. Each guard (middleware) performs a specific task before passing you to the next one.

Prerequisites

Before diving in, ensure you have:

• Node.js and npm installed.
• Basic knowledge of JavaScript.
• Express.js installed (run npm install express if not).

Types of Middleware in Express

Express supports five types of middleware:

1. Application-level middleware (e.g., app.use()).
2. Router-level middleware (similar but bound to express.Router()).
3. Error-handling middleware (has four arguments: err, req, res, next).
4. Built-in middleware (e.g., express.json()).
5. Third-party middleware (like cors or morgan).

We’ll focus on creating application-level middleware and error-handling middleware.

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Step-by-Step: Building Custom Middleware

Step 1: Set Up a Basic Express App

const express = require('express');
const app = express();
const port = 3000;

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

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

Step 2: Create a Simple Logging Middleware

Add this before your routes:

app.use((req, res, next) => {
  console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
  next(); // Pass control to the next middleware/route
});

Now, every request will log its timestamp, method, and URL.

Step 3: Build Authentication Middleware

const authMiddleware = (req, res, next) => {
  const apiKey = req.headers['x-api-key'];
  
  if (apiKey === 'MY_SECRET_KEY') {
    next(); // Authorized
  } else {
    res.status(401).send('Unauthorized!');
  }
};

// Apply to a specific route
app.get('/admin', authMiddleware, (req, res) => {
  res.send('Welcome to the admin panel!');
});

Practical Examples

Example 1: Error-Handling Middleware

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

Example 2: Route-Specific Middleware

// Apply middleware only to /api routes
const router = express.Router();

router.use((req, res, next) => {
  console.log('API request received');
  next();
});

router.get('/users', (req, res) => {
  res.send('List of users');
});

app.use('/api', router);

Best Practices

1. Keep Middleware Focused: Each middleware should do one task (e.g., logging, auth).
2. Order Matters: Middleware executes in the order they’re defined.
3. Always Call next(): Unless you’re ending the request (like sending an error).
4. Use Error-Handling Middleware: Place it last after other routes/middleware.
5. Test Thoroughly: Middleware affects all downstream code.

Conclusion

Creating custom middleware in Express.js is straightforward once you understand the request-response cycle. Start with simple tasks like logging, then move to complex logic like authentication. By following the examples and best practices above, you’ll build scalable, maintainable Node.js applications.

Ready to level up your Express skills? Try building middleware for rate limiting or request validation next! Don’t forget to share this guide if you found it helpful 😊.