Ajax packaged with native js (compatible with jsonp)

/* encapsulate the ajax function
 * @param {string}opt.type http connection method, including POST and GET
 * @param {string}opt.url the url to send the request to
 * @param {boolean}opt.async is an asynchronous request, true is asynchronous, false is synchronous
 * @param {object}opt.data The parameter sent, the format is the object type
 * @param {function}opt.success ajax send and receive callback function for successful call
 */
    function ajax(opt) {
        opt = opt || {};
        opt.method = opt.method.toUpperCase() || 'POST';
        opt.url = opt.url || '';
        opt.async = opt.async || true;
        opt.data = opt.data || null;
        opt.success = opt.success || function () {};
        opt.dataType = opt.dataType || "json";
        var xmlHttp = null;
        if (XMLHttpRequest) {
            xmlHttp = new XMLHttpRequest();
        }
        else {
            xmlHttp = new ActiveXObject('Microsoft.XMLHTTP');
        var params = [];
        for (var key in opt.data){
            params.push(key + '=' + opt.data[key]);
        }
        var postData = params.join('&');
        if (opt.method.toUpperCase() === 'POST') {
            xmlHttp.open(opt.method, opt.url, opt.async);
            xmlHttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded;charset=utf-8');
            xmlHttp.send(postData);
        }
        else if (opt.method.toUpperCase() === 'GET') {
            xmlHttp.open(opt.method, opt.url + '?' + postData, opt.async);
            xmlHttp.send(null);
        }
        xmlHttp.onreadystatechange = function () {
            if(opt.dataType != 'jsonp'){
                if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                    opt.success(xmlHttp.responseText);
                }
            }else{
                //alert(1);
                if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                    var oScript = document.createElement('script');
                    document.body.appendChild(oScript);

                    var callbackname = 'wangxiao'
                    oScript.src = opt.url + "?" +  postData+'&callback='+callbackname;

                    window['wangxiao'] = function(data) {
                        opt.success(data);
                        document.body.removeChild(oScript);
                    };
                }


            }

        };
    }
    export default ajax;