运行后提示req.readyState为空或不是对象

[www4197]

<script language="javascript">
var req;
function validate() {
var idField = document.getElementById("userid");
var url = "validate.jsp?id=" + escape(idField.value);
if(window.XMLHttpRequest) {
req = new XMLHttpRequest();
} else if (window.ActiveXObject) {
req = new ActiveXObject("Microsoft.XMLHTTP");
}
req.open("GET", url, true);
req.onreadystatechange = callback;
req.send(null);
}

function callback() {
if(req.readyState == 4) {
if(req.status == 200) {
alert(req.responseText);

}
}
}
</script>

我用的是IE7.0，在网页上运行后提示req.readyState为空或不是对象

[www4197]

<script type="text/javascript">
var req;
function validate() {
var idField = document.getElementById("uname");
var url = "validate.jsp?id=" + escape(idField.value);
if(window.XMLHttpRequest) {
req = new XMLHttpRequest();
} else if (window.ActiveXObject) {
req = new ActiveXObject("Microsoft.XMLHTTP");
}
req.open("GET", url, true);
req.onreadystatechange = callback;
req.send(null);
}

function callback() {
if(req.readyState == 4){
if(req.status == 200){
var msg = req.responseXML.getElementsByTagName("msg")[0];
//alert(msg.childNodes[0].nodeValue);
setMsg(msg.childNodes[0].nodeValue);
}
}
}

[wangpeng88888888]

推荐参考http://hi.baidu.com/jt88/blog/item/cda101465a365b0d6b63e5fd.html