XMLHttpRequest发送POST请求

XMLHttpRequest发送POST请求，见如下代码：

 var http = new XMLHttpRequest();
 var params = 'data=123'
 var url = '';
 http.open('POST', url, true);
 http.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
 http.onreadystatechange = function() {
        if(http.readyState == 4 && http.status == 200) {
            console.log(http.responseText);
        }
    }
http.send(params);