如何在JavaScript中计算JSON对象

如何计算请求的对象？

我正在使用ajax并向该URL请求json数据，pbxApi+"/conference/participants/"+circle+"/"+data.conference+"/"+data.uniqueid+'?jsonp=response';并且我想计算响应的对象。

这是我的代码

 var uri = pbxApi+"/conference/participants/"+circle+"/"+data.conference+"/"+data.uniqueid+'?jsonp=response';
        getJsonData(uri, function(res){
            console.log(res.length);
});

这是我的功能：

  var getJsonData = function(uri,callback){
    $.ajax({
      type: "GET",
      dataType: "jsonp",
      url: uri,
      jsonpCallback: 'response',
      cache: false,
      contentType: "application/json",
      success: function(json){
        callback(json);
      }
    });
  }

这是我的回应

response({"_id":"561713a78693609968e3bbdd","event":"ConfbridgeJoin","channel":"SIP/192.168.236.15-00000024","uniqueid":"1444352918.94","conference":"0090000293","calleridnum":"0090000288","calleridname":"0090000288","__v":0,"status":false,"sipSetting":{"accountcode":"0302130000","accountcode_naisen":"201","extentype":0,"extenrealname":"UID1","name":"0090000288","secret":"Myojyo42_f","username":"0090000288","context":"innercall_xdigit","gid":101,"cid":"0090000018"}})

谢谢 ：）