Form events occur when a user interacts with the form elements.
Example:
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 is IF (STR === '' ) { 21 is // required input content 22 is Alert ( " Please enter the user name " ); 23 } the else { 24 Alert ( " value entered is: " + STR); 25 } 26 is } 27 </ Script> 28 </ body> 29 </html>