simple ajax - cors request, jsonp, xmlhttprequest

ajax.js

function Ajax() {
}

Ajax.prototype.jsonpHandler = function (url, callback) {
  var scripturl = url + ((url.indexOf("?") !== -1) ? "&" : "?") + "callback=" + callback;
  document.write('<script src="' + scripturl + '"></script>');
  return scripturl;
};

Ajax.prototype.request = function (method, url, fallback, options) {
  var self = this, options = options || {}, sf = options.success, ef = options.error, params = options.params;
  if (options.jsonp) {
    return self.jsonpHandler(url, options.jsonpCallback);
  }
  var xhr = new XMLHttpRequest();
  if ("withCredentials" in xhr) {
    xhr.open(method, url, true);
  } else if (typeof XDomainRequest !== "undefined") {
    xhr = new XDomainRequest();
    xhr.open(method, url);
  } else {
    if (fallback === true) {
      xhr.open(method, url, true);
    } else {
      xhr = null;
      return xhr;
    }
  }
  xhr.onreadystatechange = function (event) {
    if (this.readyState === 4) {
      if (this.status === 200 && sf) {
        sf(this.responseText);
      } else {
        ef("Not OK. " + this.statusText);
      }
    }
  };
  xhr.onerror = function (data) {
    if (ef) {
      ef("ajax error " + this.statusText);
    }
  };
  try {
    if (params) {
      xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
      xhr.send(params);
    } else {
      xhr.send();
    }
  } catch (e) {
    if (ef) {
      ef("error " + e.name + ": " + e.message);
    }
  }
  return xhr;
};