asyncXMLHttpRequest.js

function request(method, endpoint, {payload, headers: {}} = {}) {
  return new Promise((resolve, reject) => {
    const ajaxRequest = new XMLHTTPRequest();
    
    ajaxRequest.open(method, endpoint);
    
    for (const key of Object.keys(headers)) {
      ajaxRequest.setRequestHeader(key, headers[key]);
    }
    
    ajaxRequest.onreadystatechange = () => {
      if(ajaxRequest.readyState === 4) {
        if (ajaxRequest.status >= 400) {
          reject(new Error(ajaxRequest.responseText));
        } else {
          resolve(ajaxRequest);
        }
      }
    };
    
    if (payload) {
      ajaxRequest.send(payload);
    } else {
      ajaxRequest.send();
    }      
  });
}

request('POST', 'thatEndpoint.php', {
  headers: {
    'Content-Type': 'multipart/form-data',
  }
}).then((response) => {
  if (response.status === 302) {
    const newLocation = response.getResponseHeader('Location');
    // redirect or whatever
  } else {
    // Show errors from response. 
  }
}).catch((err) => {
  
});