老师，下拉菜单全是undefined是怎么回事呀

来源：3-7 实现二级联动菜单-1

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2019-04-09 20:14:05

package com.imooc.ajax;

public class Channel {
private String code;
private String name;

public Channel() {

}
public Channel(String code, String name) {
   super();
   this.code = code;
   this.name = name;
}
public String getCode() {
   return code;
}
public void setCode(String code) {
   this.code = code;
}
public String getName() {
   return name;
}
public void setName(String name) {
   this.name = name;
}

}

package com.imooc.ajax;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.alibaba.fastjson.JSON;

/**
* Servlet implementation class ChannelServlet
*/
@WebServlet("/channel")
public class ChannelServlet extends HttpServlet {
   private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public ChannelServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

   /**
   * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
   */
   protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
       String level=request.getParameter("level");
       String parent=request.getParameter("parent");
       List chList=new ArrayList();
       if(level.equals("1")) {
           chList.add(new Channel("ai","前沿/区块链/人工智能"));
           chList.add(new Channel("web","前沿/小程序/JS"));
       }else if(level.equals("2")) {
           if(parent.equals("ai")) {
               chList.add(new Channel("micro","微服务"));
               chList.add(new Channel("blockchain","区块链"));
               chList.add(new Channel("other","..."));
           }else if(parent.equals("web")){
               chList.add(new Channel("html","HTML"));
               chList.add(new Channel("css","CSS"));
               chList.add(new Channel("other","..."));
           }
       }
       String json=JSON.toJSONString(chList);
       response.setContentType("text/html;charset=utf-8");
       response.getWriter().println(json);
   }

}

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<script type="text/javascript" src="js/jquery-3.3.1.js"></script>
<script type="text/javascript">
$(function(){
   $.ajax({
       "url":"/ajax/channel",
        "data":{"level":"1"},
        "type":"get",
        "datatype":"json",
        "success":function(json){
           console.log(json);
           for(var i=0;i<json.length;i++){
               var ch=json[i];
               $("#lv1").append("<option value='"+ch.code+"'>"+ch.name+"</option>");
           }
        }

   })
})
</script>
</head>
<body>
<select id="lv1" style="width:200px; height:30px"><option selected="selected">请选择</option></select>
<select id="lv2" style="width:200px; height:30px"></select>
</body>
</html>