AJAX小例一枚（仅GET）

//AJAX request
function AJAX(method, url) {
    this.method = method;
    this.url = url;
    AJAX.Counter++;
}
AJAX.Counter = 0;
AJAX.prototype.CreateXHR = function() { //create XMLHttpRequest
    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');
                        xmlhttp_request.setRequestHeader('Content-Type', 'text/xml');
                        xmlhttp_request.setRequestHeader('Charset', 'gb2312');
                    }
                    break;
                }
                catch (e) {
                    xmlhttp_request = false;
                }
            }
        }
        else if (window.XMLHttpRequest) {
            xmlhttp_request = new XMLHttpRequest();
        }
    }
    catch (e) { xmlhttp_request = false; }
    return xmlhttp_request;
}
AJAX.prototype.EncodeURI = function(_url) {
    var pos = _url.indexOf('?'),
        http = '',
        args = '';
    if (pos != -1) {
        http = _url.substring(0, pos + 1);
        args = _url.substring(pos + 1);
        var hash = args.split('&');
        for (var i = 0; i < hash.length; i++) {
            pos = hash[i].indexOf('=');
            if (pos != -1) {
                hash[i] = hash[i].substring(0, pos + 1) + encodeURIComponent(hash[i].substring(pos + 1));
            }
        }
    }
    else {
        return _url;
    }
    return http + hash.join('&');
}
AJAX.prototype.CallAJAX = function(senderObjID) {
    var xmlhttp_request = this.CreateXHR();
    if (!xmlhttp_request) {
        alert('您的浏览器不支持ajax。');
        return;
    }
    if (this.method === 'GET') {
        xmlhttp_request.open(this.method, this.EncodeURI(this.url), true);
        xmlhttp_request.send(null);
        var self = xmlhttp_request; //把引用赋给临时变量self
        var objID = senderObjID; //由哪个控件触发的
        xmlhttp_request.onreadystatechange = function() {
            thecallback(self,objID);
        };
    }
    else {
        throw new Error('AJAX还没有实现非“GET”的传递方法。');
    }
}

页面端：

var ajax = new AJAX('GET', '../Handler/SelectDept.aspx?PARENT=01&temp=' + new Date().getMilliseconds());
ajax.CallAJAX(senderObjID);

......

function thecallback(xmlhttp_request, senderObjID) {
    var result = xmlhttp_request.readyState;
    switch (result) {
        case 0:
        case 1:
        case 2:
        case 3: break;
                //这里应该显示一个弹窗提示用户正在加载数据

　　　　　　......
        case 4:
            //getObj(senderObjID).removeAttribute('disabled');
            //这里应该删除弹窗

            if (xmlhttp_request.status == 200) {
                //debugger;
                //BindHandler(AJAX.Counter, senderObjID);为DOM绑定事件的方法
                ......
            }
            else {
                alert('请求有错误。错误码：' + xmlhttp_request.status);
                return;
            }
            break;
        default: alert('XMLHTTPRequest返回超出预期的值。');
            break;
    }
}