老师您好，请问为什么会显示undefined？？
老师您好，我想问一下为什么会显示undefined，如果将var html初始化为空字符串，虽然不会再显示undefined，但是会显示第一行为空白，请问这是为什么？我使用console.log()打印出来的是正常的。谢谢老师！
<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Insert title here</title>
<style type="text/css">
#div1{
position:absolute;
right:1050px;
}
#div2{
position:absolute;
right:525px;
}
#div3{
position:absolute;
right:10px;
}
#div4{
position:absolute;
top:40px;
right:730px;
}
input{
width:450px;
}
</style>
</head>
<body>
<script type="text/javascript" src = "/ajax/js/jquery-3.4.1.js"></script>
<div id="div1">
<input id = "button1" type="button" value="员工列表">
</div>
<div id="div2">
<input id = "button2" type="button" value="职位列表">
</div>
<div id="div3">
<input id = "button3" type="button" value="部门列表">
</div>
<div id="div4" style = "text-align:center"></div>
</body>
<script type="text/javascript">
var xmlhttp;
var text;
var empName;
if(window.XMLHttpRequest){
xmlhttp = new XMLHttpRequest();
}else{
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
$("#button1").click(function(){
xmlhttp.open("GET","http://localhost:8080/ajax/ajax",true);
xmlhttp.send();
xmlhttp.onreadystatechange=function(){
if(xmlhttp.status == 200 && xmlhttp.readyState == 4){
text = xmlhttp.responseText;
//var empList = JSON.parse(text);
var empList = eval("("+text+")");
console.log(empList);
var html;
for(var i=0;i<empList.length;i++){
empName = empList[i];
console.log(empList[i].name);
html = html + "<br>" + empName.name + i;
}
$("#div4").html(html);
}
}
});
</script>
</html>5
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3回答
1、undefined表示不能识别的类型。如果值为null时，在el表达式中会显示undefined
2、同学把html设置为“”后，就可以识别了，但是因为同学加了换行<br>
所以第一行是空行，
建议同学吧<br> 加到最后试试。
html = html + empName.name + i+"<br>";
3、如果还是有问题，请同学贴一下你的代码：
如果贴代码，
注意不要贴在回复里，会失去代码的格式。可以在回答里贴一下。祝学习愉快。
慕粉121948417
2019-11-02 13:38:14
<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Insert title here</title>
<style type="text/css">
#div1{
position:absolute;
right:1050px;
}
#div2{
position:absolute;
right:525px;
}
#div3{
position:absolute;
right:10px;
}
#div4{
position:absolute;
top:40px;
right:730px;
}
input{
width:450px;
}
</style>
</head>
<body>
<script type="text/javascript" src = "/ajax/js/jquery-3.4.1.js"></script>
<div id="div1">
<input id = "button1" type="button" value="员工列表">
</div>
<div id="div2">
<input id = "button2" type="button" value="职位列表">
</div>
<div id="div3">
<input id = "button3" type="button" value="部门列表">
</div>
<div id="div4" style = "text-align:center"></div>
</body>
<script type="text/javascript">
var xmlhttp;
var text;
if(window.XMLHttpRequest){
xmlhttp = new XMLHttpRequest();
}else{
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
$("#button1").click(function(){
xmlhttp.open("GET","http://localhost:8080/ajax/ajax",true);
xmlhttp.send();
xmlhttp.onreadystatechange=function(){
if(xmlhttp.status == 200 && xmlhttp.readyState == 4){
text = xmlhttp.responseText;
var empList = JSON.parse(text);
//var empList = eval("("+text+")");
console.log(empList);
var html;
for(var i=0;i<empList.length;i++){
var empName = empList[i];
console.log(empList[i].name);
html = html + empName.name + i+ "<br>";
}
// $("#div4").html(html);
document.getElementById("div4").innerHTML = html;
}
}
});
$("#button2").click(function(){
xmlhttp.open("GET","http://localhost:8080/ajax/ajax",true);
xmlhttp.send();
xmlhttp.onreadystatechange=function(){
if(xmlhttp.status == 200 && xmlhttp.readyState == 4){
text = xmlhttp.responseText;
//var empList = JSON.parse(text);
var empList = eval("("+text+")");
console.log(empList);
var html;
for(var i=0;i<empList.length;i++){
var empJob = empList[i];
//console.log(empList[i].name);
html = html + "<br>" + empJob.job + i;
}
$("#div4").html(html);
}
}
});
$("#button3").click(function(){
//发送Ajax请求
xmlhttp.open("GET","http://localhost:8080/ajax/ajax",true);
xmlhttp.send();
//处理服务器响应
xmlhttp.onreadystatechange=function(){
if(xmlhttp.status == 200 && xmlhttp.readyState == 4){
text = xmlhttp.responseText;
//var empList = JSON.parse(text);
var empList = eval("("+text+")");
console.log(empList);
var html;
for(var i=0;i<empList.length;i++){
var empDept = empList[i];
//console.log(empList[i].name);
html = html + "<br>" + empDept.dept + i;
}
$("#div4").html(html);
}
}
});
</script>
</html>老师您好，上面是jsp文件当中的代码，可是还是有问题
慕粉121948417
2019-11-02 13:36:10
package com.imooc.ajax;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.alibaba.fastjson.JSON;
import com.imooc.employee.Employee;
public class Ajax extends HttpServlet{
@Override
protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//response.getWriter().println("test");
String empName ;
Employee emp1 = new Employee("小红", "职员", "人事部");
Employee emp2 = new Employee("小明", "职员", "技术部");
Employee emp3 = new Employee("小白", "经理", "事业部");
List<Employee> empList = new ArrayList<Employee>();
//List<String> nameList = new ArrayList<String>();
empList.add(emp1);
empList.add(emp2);
empList.add(emp3);
// for(Employee emp:empList) {
// empName = emp.getName();
// nameList.add(empName);
//
// }
// String jsonName = JSON.toJSONString(nameList);
String jsonEmp = JSON.toJSONString(empList);
response.setContentType("text/html;charset=utf-8");
response.getWriter().println(jsonEmp);
}
}老师您好，我将换行换到最后面了，但是还是现实undefined，这个是Java代码
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