Express.js is a minimalist and flexible web application framework for Node.js, widely used for building robust APIs and web applications. Its simplicity, combined with powerful features, makes it a go-to choice for developers creating server-side applications in JavaScript. Express streamlines handling HTTP requests, routing, and middleware integration, enabling rapid development of scalable applications. Its importance in Node.js development lies in:

• Simplicity: Provides a straightforward API for handling routes and requests.
• Flexibility: Supports modular development through middleware and routers.
• Ecosystem: Integrates seamlessly with a vast ecosystem of middleware and Node.js packages.
• Performance: Leverages Node.js's asynchronous nature for efficient request handling.

This article dives into one of Express’s core concepts: middleware. In this first part, we’ll explore what middleware is, how it works, and how to implement it at the application and router levels. Part 2 will cover built-in, third-party, and error-handling middlewares, along with best practices and real-world use cases.

What is Middleware in Express.js?

Middleware in Express.js refers to functions that execute during the request-response cycle. They have access to the request (req), response (res), and the next function, which controls the flow to the next middleware or route handler. Middleware can:

• Modify req or res objects.
• Perform tasks like logging, authentication, or parsing request bodies.
• End the response cycle (e.g., send a response).
• Pass control to the next middleware using next().

Middleware acts as a bridge between the incoming request and the final response, allowing developers to modularize functionality like validation, logging, or error handling.

Middleware Function Signature

A middleware function typically has the following signature: