classList对象、blur事件进行表单非空验证

<!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>表单验证</title>
    <style>
        body {
            background-color: mediumturquoise;
            color: #444;
            font-weight: lighter;
        }
        #login {
            background-color: azure;
            width: 20em;
            border-radius: 0.3em;
            box-shadow: 0 0 1em #888;
            box-sizing: border-box;
            padding: 1em 2em 1em;
            margin: 5em auto;
            display: grid;
            grid-template-columns: 3em 1fr;
            gap: 1em 0;
        }
        .btn1 {
            color: red;
        }
        .btn2 {
            background-color:lightgreen;
        }
        .btn3 {
            background-color: blue;
        }
    </style>
</head>
<body>
    <form action="" method="post" id="login" >
        <label for="email">邮箱：</label>
        <input type="email" id="email" name="email" value="" autofocus>
        <label for="password">密码：</label>
        <input type="password" name="password" id="password">
        <button name="submit" class="btn">登录</button>
    </form>
    <script>
        // 1. 实例演示classList对象
        let btn = document.querySelector('.btn');
        btn.classList.add('btn1');                         // 剩下：btn btn1
        console.log(btn.classList.contains('btn'));
        btn.classList.add('btn2');                         // 剩下：btn btn1 btn2
        btn.classList.remove('btn');                    // 剩下：btn1 btn2
        btn.classList.replace('btn2', 'btn3');          // 剩下：btn1 btn3
        btn.classList.toggle('btn3');                   // 剩下：btn1
        // 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>
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