How to use middleware in Next.js API routes?

In Next.js, you can use middleware to run code before your API route handlers. Middleware can be used for tasks like authentication, logging, or modifying requests and responses.

Example of using middleware for authentication:

// app/api/middleware/auth.js
export async function authMiddleware(request) {
  const token = request.headers.get("Authorization");

  if (!token || token !== "your-secret-token") {
    return new Response(JSON.stringify({ error: "Unauthorized" }), {
      status: 401,
      headers: { "Content-Type": "application/json" },
    });
  }

  return null; // No error, proceed to the next handler
}

You can then use this middleware in your API route:

// app/api/protected/route.js
import { authMiddleware } from "../middleware/auth";

export async function GET(request) {
  const authError = await authMiddleware(request);
  if (authError) {
    return authError; // Return the error response if unauthorized
  }

  const data = await getProtectedData();
  return new Response(JSON.stringify(data), {
    headers: { "Content-Type": "application/json" },
  });
}

In this example, the authMiddleware checks for a valid authorization token before allowing access to the protected API route. If the token is invalid or missing, it returns a 401 Unauthorized response.