gistfile1.txt

<script>
//  alert("test");
  var req = new XMLHttpRequest();
  req.open("POST", "/api/login");
  req.send(JSON.stringify({"username":"test", "password":"password123"});
</script>

var requestBody = {
  username: 'test',
  password: 'password123',
};

fetch('/api/login', {
  method: 'POST',
  body: JSON.stringify(requestBody),
}).then(functino (response) {
  if (response.ok) return response.json(); // Can use response.text() for non-JSON
  throw new Error('invalid response');
}).then(function (response) {
  document.location = '/new/url/path';
}).catch(function (ex) {
  console.error(ex);
  document.location = '/new/url/path?error=true';
});

you misspelled function @mpalmr