js中回调函数问题，程序进不了回调函数

我这段代码的回调函数哪出错，请各位大神指教

function req(){
createRequest();

var zipcode=document.getElementById("zipcode").value;

var url="b.jsp?zipcode=" + escape(zipcode);
request.open("POST",url,true);
request.onreadystatechage=updatePage;
request.send(null);
}

    function updatePage(){      
        if(request.readyState==4){
            alert("readyState is done");
            if(request.status==200){
                //alert("service is done");
                var response=request.responseText.split(",");
                document.getElementById("city").value=response[0];
                document.getElementById("province").value=response[1];
            }
        }
    }