req.ts

// https://twitter.com/alchemist_ubi/status/1786275138532790439
function proxy(proxy: string, req: Request): Request {
  const url = new URL(proxy);
  const originUrl = new URL(req.url);

  const header = new Headers(req.headers);
  header.set('X-Forwarded-For', 'some-ip');

  url.pathname = originUrl.pathname;

  // Do other stuff mapping the `req` to the proxy req ....
  // doesnot matter for the example

  return new Request(url, {
    ...req,
    headers: req.headers,
  });
}

fetch(proxy('https://example.com', new Request('/api/data')));