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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：