表单事件，当用户与表单元素进行交互时发生。
实例：
1 <!DOCTYPE html>
2 <html>
3 <head>
4 <title>表单事件</title>
5 <meta charset="UTF-8">
6 <meta name="viewport" content="width=device-width, initial-scale=1.0">
7 </head>
8 <body>
9 <form id ="myform" action="http://www.163.com/" method="get" >
10 用户名：<input type="text" id="username" name="username" >
11 <button type="submit">提交</button>
12 </form>
13
14 <script>
15 var myform = document.getElementById('myform');
16 myform.addEventListener('submit', stop, false);
17 function stop(e) {
18 var username = document.getElementById('username');
19 var str = username.value;
20 if (str === '') {
21 //要求输入内容
22 alert("请输入用户名");
23 } else {
24 alert("输入的值是：" + str);
25 }
26 }
27 </script>
28 </body>
29 </html>
