After Middleware

After middleware runs after the route handler. Implement it by returning a function from your middleware: that function receives the handler’s Response and returns a new Response (e.g. with added or modified headers).

Common use: adding CORS headers or other response transforms.

const corsMiddleware: Middleware = (req) => {
  return (response) => {
    const headers = new Headers(response.headers);
    headers.set("Access-Control-Allow-Origin", "*");
    return new Response(response.body, { status: response.status, headers });
  };
};

See Middleware Return Types and Middleware.