JS 原生js封装ajax

function ajax(method, url, callback, data, flag) {
    var xhr;
    flag = flag || true;
    method = method.toUpperCase();
    if (window.XMLHttpRequest) {
        xhr = new XMLHttpRequest();
    } else {
        xhr = new ActiveXObject('Microsoft.XMLHttp');
    }
    xhr.onreadystatechange = function () {
        if (xhr.readyState == 4 && xhr.status == 200) {
            console.log(2)
            callback(xhr.responseText);
        }
    }

    if (method == 'GET') {
        var date = new Date(),
        timer = date.getTime();
        xhr.open('GET', url + '?' + data + '&timer' + timer, flag);
        xhr.send()
        } else if (method == 'POST') {
        xhr.open('POST', url, flag);
        xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        xhr.send(data);
    }
}