Trying to convert html page to xml via nodejs

Im want to add sitemap route to my react website so I fetch the sitemap data and want to render a xml when user enter www.website.om/sitemap so I have this code in the backend:

const generateSitemap = async (req: Request, res: Response) => {    try {        const response = await fetch('https://www.mysitemapdata.app/sitemap.xml');        const xmlData = await response.text();        res.header('Content-Type', 'application/xml');        res.send(xmlData);    } catch (error) {        console.error('Error fetching the sitemap:', error);        res.status(500).send('Error fetching the sitemap');    }};

and I get this error in the backend terminal:

Error: Cannot set headers after they are sent to the client    at new NodeError (node:internal/errors:405:5)    at ServerResponse.setHeader (node:_http_outgoing:648:11)    at ServerResponse.header (C:\Users\97254\Desktop\FeelThere\user-server-production\node_modules\express\lib\response.js:795:10)    at ServerResponse.send (C:\Users\97254\Desktop\FeelThere\user-server-production\node_modules\express\lib\response.js:175:12)    at generateSitemap (C:\Users\97254\Desktop\FeelThere\user-server-production\src\controllers\auth.controller.ts:114:25)    at processTicksAndRejections (node:internal/process/task_queues:95:5) {  code: 'ERR_HTTP_HEADERS_SENT}

and this error in the browser console:POST http://192.168.1.116/auth/sitemap net::ERR_CONNECTION_REFUSED

this is the frontend:

const FetchExternalXML = () => {  const [xmlData, setXmlData] = useState<string | null>(null);  useEffect(() => {    const fetchSitemap = async () => {      try {        const res = await fetch("https://www.feelthere.app/sitemap.xml");        const text = await res.text();        setXmlData(text);        await userService.sitemap(xmlData)      } catch (err) {        console.log("Error while fetching sitemap:", err);      }    };    fetchSitemap();  }, []);  return xmlData ? (<div dangerouslySetInnerHTML={{ __html: xmlData }} />  ) : (<p>Loading sitemap...</p>  );};

const sitemap = (data: any) => handlePostRequest(`auth/sitemap`, data)

My goal is to render this page as xml and not just html file

I tried the code above and its looks that its convert the header to xml but its doesn't