vanilla post ajax with promise

postJSON.js

postJSON = function(url,data){
  return new Promise(function(resolve,reject)
    {
      var req = new XMLHttpRequest();
      req.open('POST', url, true);
      req.onreadystatechange = function () {
        if (req.readyState == 4) {
          if(req.status == 200)
            resolve(JSON.parse(req.responseText));
          else
            reject(Error(req.statusText));
        }
      };
      req.onerror = function() {
        reject(Error("network error"));
      };
      req.send(data);
    });
};