let xhr = new XMLHttpRequest();
xhr.open("POST", url, true);
xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xhr.onreadystatechange = () => {
   if (xhr.readyState == 4) {
       if (xhr.status == 200) {

           resolve(JSON.parse(xhr.responseText));
       } else {

        reject(xhr.responseText);
       }
    }
};
xhr.send('username=lala&password=lala');

xhr.send('username=lala&password=lala');