Code that runs between receiving a request and sending a response — like a security checkpoint.
Middleware is code that runs between receiving a request and sending a response on the server/Backend. It intercepts HTTP requests before they reach your routes and endpoints, allowing you to check authentication, validate payloads, add CORS headers, log activity, or transform requests.
Middleware is essential for protected routes — checking if a user is authenticated before allowing access. It processes JSON request bodies, validates tokens and JWTs, and handles errors.
In Next.js and Express (Node.js), middleware functions chain together. Each can modify the request, send a response, or pass control to the next middleware. Edge Functions, Loaders, and Actions all benefit from middleware logic.
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