原生javascript实现ajax,post参数

  var json = {
           userid: userid,
           cid: cid,
           openid: openid,
           type: 1
         };
         //    原生ajax
         json = (function(obj){ // 转成post需要的字符串.
           var str = "";
           for(var prop in obj){
             str += prop + "=" + obj[prop] + "&"
           }
           return str;
         })(json);
 var xhr = new XMLHttpRequest();
         xhr.open("POST", "/sharecount", true);
         xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");
         xhr.onreadystatechange = function(){
           var XMLHttpReq = xhr;
           if (XMLHttpReq.readyState == 4) {
             if (XMLHttpReq.status == 200) {
               //不做处理
             }
           }
         }
         xhr.send(json);