javaScript表单验证

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>表单验证</title>
    <script type="text/javascript">
        function checkForm(){  //表单验证
            //01.获取用户的输入
          var userName= document.getElementById("userName");
            //02.验证
            if(userName.value==""||userName.value.length<6){
                alert("用户名不能为空！用户名长度不足！");
                return false;
            }
         return  true;
        }
    </script>

</head>
<body>
<form action="#" method="post" onsubmit="return checkForm()">
   <input type="text" name="userName"  id="userName"  placeholder="请输入用户名">
   <input type="submit" value="提交">

</form>

</body>
</html>
01.表单验证小例子