Concept:

Express.js is a minimalist web framework built on top of Node.js’s http module. Its middleware architecture is what gives it power and flexibility — every request goes through a stack of middleware functions that can modify the request/response or pass control to the next handler.

Express is built around this core idea:

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

Real-World Scenario:

You’re tasked with building an enterprise API where you need:

• Authentication and authorization layers

• Request logging and rate limiting

• Input validation

• Custom error handling

  You achieve all of this cleanly via middleware.

Tough Interview Questions:

• How does middleware chaining work in Express?
• How are errors handled in middleware? What happens when next(err) is called?
• What is the difference between application-level and router-level middleware?
• How would you write a middleware that times each request?
• How does Express internally resolve routes and middleware ordering?

How Middleware Works