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>