Middleware Pattern (Express-style)

The Middleware Pattern & Request Lifecycle

The philosophical backbone of Node.js web frameworks (Express, Koa) relies heavily on the Middleware (Chain of Responsibility) pattern. Every incoming HTTP request enters a pipeline of functions sequentially.

The Next Paradigm

Each middleware function has access to the request, response, and the pivotal next callback function. A middleware acts as a discrete, composable unit of logic. If it does not terminate the cycle by sending a response, it must invoke next() to pass control to the subsequent function.

Middleware Categorization

• Application-Level: Executes for all routes globally (e.g., CORS definitions, JSON payload parsers).
• Route-Level: Bound specifically to individual endpoints (e.g., Role-based access validation).
• Error-Handling: Distinguished exclusively by accepting four arguments (err, req, res, next). These intercept any errors passed down the pipeline via next(err).