Next.js Middleware Builder

Next.js middleware runs before a request is processed, allowing you to modify the response, redirect users, rewrite URLs, or add headers — all at the Edge, before the page renders. It's defined in a middleware.ts file at the root of your project (or src/ directory). Middleware runs on every request that matches its config.matcher pattern.

The matcher config in middleware.ts tells Next.js which paths to run middleware on. It supports regex-like patterns: '/((?!_next/static|_next/image|favicon.ico).*)' skips static assets. You can use arrays for multiple patterns. Matcher patterns must be static strings — no dynamic values or variables.

Check authentication in middleware by reading a cookie (NextResponse.cookies.get), a header (request.headers.get), or a JWT in the Authorization header. If the user is not authenticated, call NextResponse.redirect() to send them to a login page. Always pass the request URL as the callbackUrl parameter so they return after login.

redirect changes the URL visible in the browser and sends a new request to the target path. rewrite proxies the request to a different path without changing the visible URL — the user sees the original URL but gets content from the rewrite target. Rewrites are useful for A/B testing, internationalization, and clean URL routing.

Yes, but with a caveat: middleware runs on the Edge Runtime, which is a restricted JavaScript environment. You can access process.env for server-side variables. However, you cannot use Node.js-specific APIs (fs, path, crypto built-ins). For JWT verification, use the Web Crypto API (crypto.subtle) or edge-compatible libraries like jose.