Middleware

Code that runs between a request and a response, before your main logic executes. Used for authentication checks, logging, rate limiting, and request transformation.

Middleware is the layer that protects your app: it checks if users are logged in before showing private pages, validates API tokens before running code, and logs every request for debugging.

A founder's Next.js app has middleware that checks for a valid session cookie on every protected route. Unauthenticated users are redirected to /login before seeing any content.