表单验证

 <form action="success.html" id="form">
        <p>
            email:<input type="text" name="email" id="email">
        </p>
        <input type="submit" value="提交" id="submit">
    </form>
   
        let form = document.getElementById('form');
        let email = document.getElementById('email');
        /*  let sub = document.getElementById('submit'); */
        form.onsubmit = function () {
            if (email.value == '') {
                alert('邮箱不能为空');
                return false;
            }
            if(email.value.indexOf('@')==-1){
                alert('邮箱必须包括@');
                return false;
            }
            if(email.value.indexOf('.')==-1){
                alert('邮箱必须包括.');
                return false;
            }
            return true;
        }