html select下拉框 多级联动

多级联动的下拉框可以通过JavaScript实现。以下是一个简单的例子：

HTML代码：

<select id="province" onchange="updateCity()">
  <option value="">请选择省份</option>
  <option value="江苏">江苏</option>
  <option value="浙江">浙江</option>
</select>

<select id="city" onchange="updateDistrict()">
  <option value="">请选择城市</option>
</select>

<select id="district">
  <option value="">请选择区县</option>
</select>

JavaScript代码：

var cityData = {
  "江苏": ["南京", "苏州", "无锡"],
  "浙江": ["杭州", "宁波", "温州"]
};

var districtData = {
  "南京": ["玄武区", "鼓楼区", "建邺区"],
  "苏州": ["姑苏区", "虎丘区", "吴中区"],
  "无锡": ["锡山区", "惠山区", "滨湖区"],
  "杭州": ["西湖区", "上城区", "下城区"],
  "宁波": ["海曙区", "江东区", "江北区"],
  "温州": ["鹿城区", "龙湾区", "瓯海区"]
};

function updateCity() {
  var province = document.getElementById("province").value;
  var citySelect = document.getElementById("city");
  var districtSelect = document.getElementById("district");

  citySelect.innerHTML = "<option value=''>请选择城市</option>";
  districtSelect.innerHTML = "<option value=''>请选择区县</option>";

  if (province != "") {
    var cities = cityData[province];
    for (var i = 0; i < cities.length; i++) {
      var option = document.createElement("option");
      option.value = cities[i];
      option.innerHTML = cities[i];
      citySelect.appendChild(option);
    }
  }
}

function updateDistrict() {
  var province = document.getElementById("province").value;
  var city = document.getElementById("city").value;
  var districtSelect = document.getElementById("district");

  districtSelect.innerHTML = "<option value=''>请选择区县</option>";

  if (province != "" && city != "") {
    var districts = districtData[city];
    for (var i = 0; i < districts.length; i++) {
      var option = document.createElement("option");
      option.value = districts[i];
      option.innerHTML = districts[i];
      districtSelect.appendChild(option);
    }
  }
}

在这个例子中，我们定义了两个对象：cityData和districtData，分别保存了城市和区县的数据。当省份下拉框的值发生变化时，我们会根据选中的省份动态生成城市下拉框的选项。同样地，当城市下拉框的值发生变化时，我们会根据选中的城市动态生成区县下拉框的选项。

你可以根据自己的需求修改cityData和districtData对象，来实现更复杂的多级联动下拉框效果。