封装ajax

function AJAX(json) {
    var url = json.url,
        method = json.method,
        flag = json.flag,
        data = json.data,
        callBack = json.callBack,
        xhr = null;
    if(window.XMLHttpRequest) {
        xhr = new window.XMLHttpRequest();
    }else {
        xhr = new ActiveXObject('Mircosoft.XMLHTTP');
    }            
    if(method == 'get') {
                url += '?' + data + new Date().getTime(); 
        xhr.open('get', url, flag);
    }else {
        xhr.open('post', url, flag);
    }
    xhr.onreadystatechange = function () {
        if (xhr.readyState === 4 && xhr.status === 200) {
            // 数据已经可用了
            callBack(xhr.responseText);
        }
    }
    if(method == 'get') {
        xhr.send();
    }else {
        xhr.setRequestHeader('Content-Type', 'application/x-www-form-urle');
        xhr.send(data);
    }    
}