从javascript获取HTTP Basic Auth用户名？

function getBasicAuthUsername() {
  var headers = new Headers();
  headers.append('Authorization', 'Basic ' + btoa('username:password'));

  var request = new Request('https://example.com', {
    method: 'GET',
    headers: headers,
    mode: 'cors',
    cache: 'default'
  });

  return fetch(request)
    .then(function(response) {
      var authHeader = response.headers.get('WWW-Authenticate');
      if (authHeader) {
        var credentials = authHeader.split(' ')[1];
        var decodedCredentials = atob(credentials);
        var username = decodedCredentials.split(':')[0];
        return username;
      } else {
        throw new Error('HTTP Basic Auth not supported');
      }
    })
    .catch(function(error) {
      console.error(error);
    });
}

getBasicAuthUsername().then(function(username) {
  console.log('Username:', username);
});