ajax基础实例 返回xml

//AJAX request for the city data
function ajaxforget(cn) {
    var xmlhttp_request = "";
    try {
        if (window.ActiveXObject) {
            for (var i = 5; i; i--) {
                try {
                    if (i == 2) {
                        xmlhttp_request = new ActiveXObject("Microsoft.XMLHTTP");
                    }
                    else {
                        xmlhttp_request = new ActiveXObject("Msxml2.XMLHTTP." + i + ".0");
                    }
                    break;
                }
                catch (e) {
                    xmlhttp_request = false;
                }
            }
        }
        else if (window.XMLHttpRequest) {
            xmlhttp_request = new XMLHttpRequest();
        }
    }
    catch (e) { xmlhttp_request = false; }
    if (!xmlhttp_request) {
        alert("您的浏览器不支持ajax");
        return;
    }
    xmlhttp_request.open('GET', '../Handler/CityData.ashx?code='+cn+'&time='+new Date().getMilliseconds(), true);

    xmlhttp_request.setRequestHeader("Content-Type", "text/xml"); //setRequestHeader("header", "value")
    xmlhttp_request.setRequestHeader("Charset", "gb2312"); //把指定首部设置为所提供的值。在设置任何首部之前必须先调用open()
    xmlhttp_request.send(null);
    var self = xmlhttp_request; //把引用赋给临时变量self
    xmlhttp_request.onreadystatechange = function() {
        thecallback(self);
    };
}

        //ajax的回调函数
        function thecallback(xmlhttp_request) {
            if (xmlhttp_request.readyState == 4 && xmlhttp_request.status == 200) {
                //绑定项
                var data = xmlhttp_request.responseXML;
                var citys = data.getElementsByTagName("name");
                var cns = data.getElementsByTagName("code");
                var listCity = $("listCity");
                listCity.innerHTML = ""; //先移除所有的项
                var listP = $("listProvince");
                var code = listP.options[listP.selectedIndex].value;
                var item = new Array();
                for (var i = 0; i < citys.length; i++) {
                    if (citys.length != 1 && code == cns[i].childNodes[0].data) continue;
                    item[i] = document.createElement("option");
                    item[i].value = cns[i].childNodes[0].data;
                    item[i].innerHTML = citys[i].childNodes[0].data;
                    listCity.appendChild(item[i]);
                }
                //绑定项结束
                $("txtCode").value = listCity.options[0].value;
            }
        }