extjs 为何得到json返回值却不能作数据判断

Question

不能通过后台的到的json值来判断应该弹出什么样的alert（）；
fieldLabel: "集团登录名",
                id: "groupTypeID",
                width: 150,
                allowBlank: false, //如果是true，则允许，否则不允许，默认是true
                blankText: "请输入集团登录名！", //如果allowBlank 设置为true，并且那个textField的值为空，则会显示blankText属性的字符串以给出错误提示
                emptyText: "请输入集团登录名！",
                listeners: {
                    "blur": function() {
                        var group = Ext.getCmp("groupTypeID").getValue();
                        //Ext.Msg.alert(group);                        Ext.Ajax.request({
                            url: "../ajax/mapHandler.ashx?action=checkGrouptypeId",
                            params: { paramtype: group },
                            success: function(responses) {
                            
                                if (obj == "已存在") {
                                    Ext.Msg.alert("提示", "此集团登录名不可用！", function() {
                                        Ext.getCmp("groupTypeID").setValue();
                                        Ext.getCmp("groupTypeID").focus();
                                        //groupTypeID.disable();
                                    });
                                    //return;
                                }
                            },
                            failure: function(response) {
                                //Ext.Msg.alert("false");
                            }
                        });
                    }
                }
                ////////////////////////
后台代码：
%@ WebHandler Language="C#" Class="mapHandler" %>using System;
using System.Web;
using GPSManagerModels;
using System.Collections.Generic;
using GPSManagerBLL;
using System.Web.SessionState;
using Newtonsoft.Json; 
public class AddDeviceHandler : IHttpHandler {    GISGroupInfoManager gim = new GISGroupInfoManager();
    public void ProcessRequest (HttpContext context) {
        context.Response.ContentType = "text/plain";
        if (HttpContext.Current == null)
        {
            throw new NullReferenceException("The context is not set correct in ProcessRequest().");
        }
        string str = context.Request["action"].ToString();
        switch (str) {
            case "getMap":
                this.getMap();
                return;
            case "checkGrouptypeId":
                string groupTypeID = context.Request["paramtype"].ToString();
                this.checkGroupTypeID(groupTypeID);
                return;
            case "":
                return;
        }
        
    }
 
    public bool IsReusable {
        get {
            return false;
        }
    }    private void checkGroupTypeID(string grouptypeid) {
        if (grouptypeid != null && grouptypeid != "")
        {
            
            int count = gim.checkGroupTypeID(grouptypeid);
            if (count > 0)
            {
                //bool result = true;
                string res = "{success:true}";
                HttpContext.Current.Response.Write(res);
            }
            else {
                //bool result = false;
                string res = "{success:false}";
                HttpContext.Current.Response.Write(res);
            }
        }
    }}

Answer 1

obj == "已存在"obj ?

Answer 2

var obj=Ext.util.JSON.decode(responses.responseText)
if(obj.success){
Ext.Msg.alert("提示","已存在")
}