AJAX xmlHttp.readyState的返回值总是1

AJAX xmlHttp.readyState的返回值总是1
代码如下：
function QP2001_JF2001_Click(sCode,sPrtPrj,sReferFlag) {
var tStrUrl;
var tStrCondition;
tStrCondition = " AND a.cContractCode = '" + sCode + "' "
+ " AND f.cPPCode = '" + sPrtPrj + "'";
tStrUrl = "../qp/QP2001_A.aspx?MODE=SEARCH"
+ "&CONDITION=" + escape(tStrCondition)
+ "&SHOWMODE=" + sReferFlag
+ "&OTHER=REFER";
xmlHttp.open("GET",tStrUrl,true );
xmlHttp.onreadystatechange = QP2001_JF2001_Click_HttpResponse;
xmlHttp.send(null);
return true;
}
function QP2001_JF2001_Click_HttpResponse() {
if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
var xmlDoc = xmlHttp.responseXML;
if (xmlDoc.documentElement) {
var tStrStatus = xmlDoc.getElementsByTagName("STATUS")[0].text;
var tStrDoc;
if (tStrStatus == "OK") {
var tIntDatanum = xmlDoc.getElementsByTagName("DATANUM")[0].text; //记录数
if (tIntDatanum == 0) {
alert("没有可操作数据.");
return false;
}else{
alert("OK");
}
}else{
alert(tStrStatus);
return false;
}
}
}
}
里面xmlHttp.readyState的返回值总是1
MaxMartin 的回答,各返回值的意义我知道,主要是不知道为啥会总返回1
服务端代码
Web.HttpContext.Current.Response.Clear()
Web.HttpContext.Current.Response.ContentType = "text/xml"
Web.HttpContext.Current.Response.AddHeader("Content-Type","text/xml; charset=utf-8")
Web.HttpContext.Current.Response.AddHeader("Cache-Control","no-cache,private")
Web.HttpContext.Current.Response.AddHeader("Pragma","no-cache")
tStrXml = "" & vbNewLine & _
"" & vbNewLine & _
" OK" & vbNewLine & _
" " & tStrMode & "" & vbNewLine & _
" JF2004" & vbNewLine & _
tStrXmlText & vbNewLine & _
""
Web.HttpContext.Current.Response.Write(tStrXml)
Web.HttpContext.Current.Response.End()