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);