封装ajax

//封装ajax
function ajax(obj) {
    var xhr = (function () {
        if (typeof XMLHttpRequest != 'undefined') {
            return new XMLHttpRequest();
        } else if (typeof ActiveXObject != 'undefined') {
            var version = [
                                        'MSXML2.XMLHttp.6.0',
                                        'MSXML2.XMLHttp.3.0',
                                        'MSXML2.XMLHttp'
            ];
            for (var i = 0; version.length; i ++) {
                try {
                    return new ActiveXObject(version[i]);
                } catch (e) {
                    //跳过
                }    
            }
        } else {
            throw new Error('您的系统或浏览器不支持XHR对象！');
        }
    })();
    obj.url = obj.url + '?rand=' + Math.random();
    obj.data = (function (data) {
        var arr = [];
        for (var i in data) {
            arr.push(encodeURIComponent(i) + '=' + encodeURIComponent(data[i]));
        }
        return arr.join('&');
    })(obj.data);
    if (obj.method === 'get') obj.url += obj.url.indexOf('?') == -1 ? '?' + obj.data : '&' + obj.data;
    if (obj.async === true) {
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4) {
                callback();
            }
        };
    }
    xhr.open(obj.method, obj.url, obj.async);
    if (obj.method === 'post') {
        xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        xhr.send(obj.data);    
    } else {
        xhr.send(null);
    }
    if (obj.async === false) {
        callback();
    }
    function callback() {
        if (xhr.status == 200) {
            obj.success(xhr.responseText);            //回调传递参数
        } else {
            alert('获取数据错误！错误代号：' + xhr.status + '，错误信息：' + xhr.statusText);
        }    
    }
}