Middleware is software that sits between different components of an application and processes requests as they flow through the system. In web frameworks, middleware functions run before or after request handlers to perform tasks such as authentication, logging, rate limiting, or request parsing. Middleware helps enforce consistency and reuse common logic across endpoints.
Why it matters
Middleware centralizes cross-cutting concerns, reducing duplication and improving maintainability. It also enhances security by ensuring that checks like input validation or authentication are applied universally.
Examples
An Express.js middleware that checks authentication tokens or logs every incoming request. Lessons like What Is Middleware? introduce middleware flows.