Ajax获取XMLHttp对象的方法

function FactoryXMLHttpRequest() {
  if(window.XMLHttpRequest) {
    return new XMLHttpRequest();
  }else if(window.ActiveXObject) {
    var msxmls = new Array(
      'Msxml2.XMLHTTP.5.0',
      'Msxml2.XMLHTTP.4.0',
      'Msxml2.XMLHTTP.3.0',
      'Msxml2.XMLHTTP',
      'Microsoft.XMLHTTP');
    for (var i = 0; i  msxmls.length; i++) {
      try {
        return new ActiveXObject(msxmls[i]);
      } catch (e) {
      }
    }
  }
  throw new Error("Could not instantiate XMLHttpRequest");
}