Ajax兼容性问题

function CreateXHR() {
          if(XMLHttpRequest) {
             return new XMLHttpRequest;
  } else {
          return new ActiveXObject("Msxml2.XMLHTTP");//兼容老IE浏览器
        }
}
var xhr = CreateXHR()
xhr.open("get", "a.txt");
xhr.onreadystatechange = function() {
    if(xhr.readyState == 4 && xhr.status == 200) {
                rs = xhr.responseText
   }
}
xhr.send(null)