用 promise写一个ajax请求

function http({ url, type, data, dataType }) {
    return new Promise(function (resolve, reject) {
        var xhr = window.XMLHttpRequest ? new XMLHttprequest() : new ActiveXObject('Microsoft.XMLHttp');
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4 && xhr.status == 200) {
                if (dataType !== undefined
                    && dataType.toLowerCase() === "json")
                    var res = JSON.parse(xhr.responseText)
                else
                    var res = xhr.responseText
                resolve(res);
            }
        }
        if (type.toLowerCase() === "get" && data !== undefined) {
            url += "?" + data;
        }
        xhr.open(type, url, true);
        if (type.toLowerCase() === "post")
            xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        if (type.toLowerCase() === "post" && data !== undefined)
            xhr.send(data);
        else
            xhr.send(null);
    })
}