⚙️ Middleware in Express JS | Application-Level & Router-Level Middleware Explained

💡 What Is Middleware in Express?

Express.js is a powerful and flexible Node.js framework used to build web apps and APIs.

One of its most important features is middleware, which allows you to intercept requests, modify them, run logic, and control the application's request-response cycle.

🧠 Definition

Middleware functions have access to:

• req - Request Object
• res - Response Object
• next - Function to pass control to the next middleware

If the current middleware does not end the cycle, it must call next().

• ✔ Execute any code
• ✔ Modify req or res objects
• ✔ End the request-response cycle
• ✔ Call the next middleware in the stack

If next() is not called or response not sent, the request will hang.

🔧 Types of Middleware

• Application-Level Middleware - Applied to all routes
• Router-Level Middleware - Applied to specific routes
• Built-in Middleware - Express.js built-in middleware
• Third-Party Middleware - External middleware packages

This middleware logs request details for all routes:

This middleware runs only for specific routes:

This middleware is attached using app.use(). It runs for every request unless restricted.

• app.use() registers middleware globally.
• secondMiddleware is used only on specific routes.
• Middleware runs in the order they are defined.