用太久JQuery了，别忘了Ajax原生是怎么写的

var Ajax = {    get : function (url, callback) {                var req = Ajax.getRequest(callback);                req.open("GET", url, true);        req.setRequestHeader("If-Modified-Since","0");        req.send(null);    },    post : function (url, param, callback) {                var req = Ajax.getRequest(callback);                req.open("POST", url, true);        req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");        req.setRequestHeader("If-Modified-Since","0");        req.send(param);    },    getRequest : function (callback) {        var req;        try {            req = new XMLHttpRequest();        } catch (error) {            try {                req = new ActiveXObject("Microsoft.XMLHTTP");            } catch (error) {                return null;            }        }        req.onreadystatechange = function() {            if (req.readyState != 4)                return;            if (req.status == 200) {                callback(req);            } else                alert("请求失败:" + req.statusText);            return true;        };        return req;    },    encode : function (str) {        return encodeURIComponent(str);    }};