实例演示classList对象及表单非空验证

实例代码如下：
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>classList对象、blur事件进行表单非空验证</title>
    <link rel="stylesheet" href="css/style.css" />
    <style>
        .bg{
            background-color:aqua;
        }
        .bgn{
            background-color: coral;
        }
    </style>
  </head>
  <body>
    <form action="" method="post" id="login">
      <label class="title">用户登录</label>
      <label for="email">邮箱:</label>
      <input type="email" id="email" name="email" value="" autofocus />
      <label for="password">密码:</label>
      <input type="password" id="password" name="password" />
      <!-- 1. type="button" 来表示这是一个普通按钮,没有提交行为 -->
      <button name="submit" onclick="check(this)">登录</button>
    </form>
    <script>
      // 1. 实例演示classList对象
      document.body.classList.add("bg");
      document.body.classList.remove("bg");
      document.body.classList.toggle("bg");
      document.body.classList.replace("bg","bgn");
      document.body.classList.remove("bgn");
      function check(ele){
        event.preventDefault();
        event.stopPropagation();
        let email = ele.form.email;
        let password = ele.form.password;
        if(email.value.length === 0){
            alert("邮箱不能为空");
            email.focus();
            return false;
        }else if(password.value.length === 0){
            alert("密码不能为空");
            password.focus();
            return false;
        }else{
            return true;
        }
      }
      // 2. 使用blur事件进行表单非空验证
      document.forms.login.email.onblur = function(){
        if(this.value.length === 0){
            alert("邮箱不能为空");
            return false;
        }
      };
      document.forms.login.password.onblur = function(){
        if(this.value.length === 0){
            alert("密码不能为空");
            return false;
        }
      };
    </script>
  </body>
</html>
效果图如下：
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