Simple ajax post data without jQuery

var serialize = function(obj, prefix) {
  var str = [], p;
  for(p in obj) {
    if (obj.hasOwnProperty(p)) {
      var k = prefix ? prefix + "[" + p + "]" : p, v = obj[p];
      str.push((v !== null && typeof v === "object") ?
        serialize(v, k) :
        encodeURIComponent(k) + "=" + encodeURIComponent(v));
    }
  }
  return str.join("&");
};

function postAjax(url, data, success) {
    var params = typeof data == 'string' ? data : serialize(data);

    var xhr = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");
    xhr.open('POST', url);
    xhr.onreadystatechange = function() {
        if (xhr.readyState>3 && xhr.status==200) { 
   try {
    var obj = JSON.parse(xhr.responseText);
    success(obj);
   } catch(ex) {
    console.log('Error: '+ex.message+'\nTrace: '+xhr.responseText);
   }
  }
    };
    xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    xhr.send(params);
    return xhr;
}