docs: function xhr (url, responseType) {
    return new Promise((resolve, reject) => {
      var x = new XMLHttpRequest();
  
      if (responseType)
        x.responseType = responseType;
  
      x.open('get', url, true);
  
      x.onreadystatechange = function() {
        if (x.readyState === 4) { // `DONE`
          if (x.status === 200) {
            resolve(x.responseText);
          } else {
            reject(x.status);
          }
        }
      };
      x.send();
    });
  }