Ajax验证用户名是否存在

<script language="javascript" type="text/javascript">
        var xmlHttp = null;
        function CreateXMLHttp()
        {
            if(window.ActiveXObject)
                {
                    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
                }
            else if(window.XMLHttpRequest)
                {
                    xmlHttp = new XMLHttpRequest();
                }
        }
        function checkUserName(value){
         var patrn=/^[a-z]{1}[a-z0-9]{5,15}$/;
         if (patrn.exec(value)){
          return true
         }else{
          return false
         }
        }
        function CheckLoginName()
        {
            CreateXMLHttp();  
            var txtName=document.g