Ajax 提示缺少对象解决思路

Ajax 提示缺少对象
各位大侠 帮忙看看 提示缺少对象 不知道哪个地方出错了

JScript code

<script type="text/javascript" src="../../inc/ajax.js"></script><script language="javascript">function fetchGX(gyff,gxmc){    createXMLHttpRequest();        xmlHttp.onreadystatechange =handleGX;    var strurl = "../../utils/fetch/fetch_gx.jsp?gyff="+gyff+"&gxmc="+gxmc;    xmlHttp.open("POST", strurl, true);    xmlHttp.send(null);}function handleGX(){    if(xmlHttp.readyState == 4){//0(未初始化);1(正在装载);2 (装载完毕);3 (交互中);4 (完成)            if(xmlHttp.status == 200){//200(OK);404(not found)            var str=xmlHttp.responseText.replace(/[\r\n]/g,"").replace(/(^\s*)|(\s*$)/g,"");            if(str.equals("true"))            {            alert("已存在，重新输入");            return false;            }        }    }}</script>