HTML表单验证后模态框（Modal）的实现与常见问题解决

<form method="post" name="f1" action="">
  <h2 style="padding-left:50px">Application Form</h2>
  <table>
    <tr>
      <td><label for="uname">Name</label></td>
      <td><input type="text" id="uname" name="username" required></td>
    </tr>
    <tr>
      <td><label for="date">Date Of Birth</label></td>
      <td><input type="date" id="date" name="date" required></td>
    </tr>
    <tr>
      <td><label>Gender</label></td>
      <td>
        <label><input type="radio" value="female" name="gender" required> Female</label>
        <label><input value="male" type="radio" name="gender"> Male</label>
      </td>
    </tr>
    <tr>
      <td><label for="district">District</label></td>
      <td>
        <select name="district" id="district">
          <option value="">---Select District---</option>
        </select>
      </td>
    </tr>
    <tr>
      <td><label for="city">City</label></td>
      <td>
        <select name="city" id="city">
          <option value="">---Select District First---</option>
        </select>
      </td>
    </tr>
    <tr>
      <td></td>
      <td>
        <!-- 提交按钮，通过onclick调用验证函数 -->
        <button type="submit" id="myBtn" class="cancelbtn" onclick="return valid();">SUBMIT</button>
      </td>
    </tr>
  </table>
</form>

<!--model div open-->
<div id="myModal" class="modal">
  <div class="modal-content">
    <div class="modal-header">
      <h2>Application Accepted</h2>
      <span class="close">&times;</span>
    </div>
    <div class="modal-body">
      <p>Go Back To Home Page <a href="home">Home</a></p>
    </div>
  </div>
</div>
<!--model div close-->

body {
  font-family: Arial, Helvetica, sans-serif;
}

/* 模态框背景 */
.modal {
  display: none; /* 默认隐藏 */
  position: fixed; /* 固定定位，覆盖整个视口 */
  z-index: 1; /* 确保在其他内容之上 */
  padding-top: 100px; /* 模态框内容顶部填充 */
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto; /* 内容超出时允许滚动 */
  background-color: rgb(0, 0, 0); /* 黑色背景 */
  background-color: rgba(0, 0, 0, 0.4); /* 半透明黑色背景 */
}

/* 模态框内容 */
.modal-content {
  position: relative;
  background-color: #fefefe;
  margin: auto; /* 居中 */
  padding: 0;
  border: 1px solid #888;
  width: 50%; /* 宽度 */
  box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 6px 20px 0 rgba(0, 0, 0, 0.19);
  /* 动画效果 */
  -webkit-animation-name: animatetop;
  -webkit-animation-duration: 0.4s;
  animation-name: animatetop;
  animation-duration: 0.4s;
}

/* 模态框进入动画 */
@-webkit-keyframes animatetop {
  from { top: -300px; opacity: 0; }
  to { top: 0; opacity: 1; }
}

@keyframes animatetop {
  from { top: -300px; opacity: 0; }
  to { top: 0; opacity: 1; }
}

/* 关闭按钮 */
.close {
  color: white;
  float: right;
  font-size: 28px;
  font-weight: bold;
}

.close:hover,
.close:focus {
  color: #000;
  text-decoration: none;
  cursor: pointer;
}

/* 模态框头部和主体 */
.modal-header {
  padding: 10px 15px;
  background-color: #5cb85c; /* 绿色背景 */
  color: white;
}

.modal-body {
  padding: 10px 15px;
  text-align: center;
}

/* 其他表单样式 */
form {
  border: 3px solid #f1f1f1;
  padding-left: 290px; /* 调整表单位置 */
}

input[type=text],
input[type=date],
select {
  width: 75%;
  padding: 5px 10px;
  margin: 8px 0;
  display: inline-block;
  border: 1px solid #ccc;
  box-sizing: border-box;
}

button {
  background-color: #FF5733;
  color: white;
  padding: 10px 15px;
  margin: 8px 0;
  border: none;
  cursor: pointer;
  width: 100px;
  border-radius: 15px;
}

button:hover {
  opacity: 0.8;
}

// 示例数据，用于填充区市下拉菜单
var subObject = {
  "place1": ["place1_city1", "place1_city2", "place1_city3"],
  "place2": ["place2_city1", "place2_city2"],
  "place3": ["place3_city1", "place3_city2", "place3_city3", "place3_city4"]
};

// 获取模态框相关元素，这些变量应在全局或onload中声明，确保只获取一次
var modal;
var btn; // 提交按钮，虽然在这里不直接监听其点击事件，但获取它是一个好习惯
var span; // 关闭按钮

window.onload = function() {
  // 初始化区市下拉菜单
  var distSel = document.getElementById("district");
  var citySel = document.getElementById("city");

  for (var x in subObject) {
    distSel.options[distSel.options.length] = new Option(x, x);
  }

  distSel.onchange = function() {
    citySel.length = 1; // 清空城市下拉菜单，只保留默认选项
    var z = subObject[distSel.value];
    if (z) { // 确保选中的区存在对应的城市数据
      for (var i = 0; i < z.length; i++) {
        citySel.options[citySel.options.length] = new Option(z[i], z[i]);
      }
    }
  };

  // 获取模态框元素
  modal = document.getElementById("myModal");
  btn = document.getElementById("myBtn");
  span = document.getElementsByClassName("close")[0];

  // 为模态框的关闭按钮和背景区域添加事件监听器
  // 这些监听器应在页面加载时设置一次，而不是在每次验证成功时重复设置
  span.onclick = function() {
    modal.style.display = "none"; // 点击关闭按钮隐藏模态框
  };

  window.onclick = function(event) {
    if (event.target == modal) {
      modal.style.display = "none"; // 点击模态框背景区域隐藏模态框
    }
  };
};

// 表单验证函数
function valid() {
  // 校验各项输入是否为空
  if (f1.username.value === "") {
    alert('请输入姓名');
    f1.username.focus();
    return false;
  } else if (f1.date.value === "") {
    alert('请输入出生日期');
    f1.date.focus();
    return false;
  } else if (f1.gender.value === "") { // 注意：对于radio按钮，判断其value是否为空可能不准确，更好的方式是检查是否有选中
    var genderSelected = false;
    var genders = f1.gender; // 获取所有name为gender的radio按钮
    for(var i=0; i<genders.length; i++) {
        if(genders[i].checked) {
            genderSelected = true;
            break;
        }
    }
    if (!genderSelected) {
        alert('请选择性别');
        // f1.gender[0].focus(); // 聚焦第一个radio按钮
        return false;
    }
  } else if (f1.district.value === "") {
    alert('请选择地区');
    f1.district.focus();
    return false;
  } else if (f1.city.value === "") {
    alert('请选择城市');
    f1.city.focus();
    return false;
  } else {
    // 所有验证通过，显示模态框
    modal.style.display = "block";
    // 阻止表单的默认提交行为，否则页面会刷新，模态框会瞬间消失
    return false;
  }
}