Cant Set Any Header in Fetch Get Request

Im trying to send a get Request from the browser to my backend (node + express). But somehow my headers dont seem to get set.

Heres the Frontend:

let accessToken = localStorage.getItem('accessToken');

    fetch('http://localhost:3000/checkLogin', {
        method: 'GET',
        mode: same-origin,

        withCredentials: true,
        credentials: 'include',
        headers: {
            'Authorization': 'Bearer ' + accessToken,
            'Content-Type': 'text/plain',
    .then(data => {
        console.log('Success:', data);
    .catch((error) => {
        console.error('Error:', error);

In the backend i tried to enable custom headers:

server.use(function(req, res, next) {
res.set('Access-Control-Allow-Origin', 'localhost:3000/testLogin');
res.set('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
res.set('Access-Control-Allow-Headers', 'Authorization, Content-Type, X-Test');

I already tried the answers of similar questions but none of them worked.

when i print the headers received by the server i get:

host: 'localhost:3000',
  connection: 'keep-alive',
  'cache-control': 'max-age=0',
  'sec-ch-ua': '"Google Chrome";v="89", "Chromium";v="89", ";Not A Brand";v="99"',
  'sec-ch-ua-mobile': '?0',
  'upgrade-insecure-requests': '1',
  'user-agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.114 Safari/537.36',
  accept: 'text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9',
  'sec-fetch-site': 'same-origin',
  'sec-fetch-mode': 'navigate',
  'sec-fetch-user': '?1',
  'sec-fetch-dest': 'document',
  referer: 'http://localhost:3000/',
  'accept-encoding': 'gzip, deflate, br',
  'accept-language': 'de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7'


The origin you’re specifying is invalid. It needs to have a scheme (e.g., http:// or https://). Also note that origins don’t have paths. So for instance, instead of 'localhost:3000/testLogin', it would be 'http://localhost:3000':

res.set('Access-Control-Allow-Origin', 'http://localhost:3000');