如何使用dependent dropdown追加行

<form id="myForm">
  <div>
    <label for="category">Category:</label>
    <select id="category" onchange="loadSubcategories()">
      <option value="">Select category</option>
      <option value="1">Category 1</option>
      <option value="2">Category 2</option>
      <option value="3">Category 3</option>
    </select>
  </div>
  <div>
    <label for="subcategory">Subcategory:</label>
    <select id="subcategory">
      <option value="">Select subcategory</option>
    </select>
  </div>
  <button type="button" onclick="addRow()">Add Row</button>
</form>
<table id="myTable">
  <thead>
    <tr>
      <th>Category</th>
      <th>Subcategory</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>
        <select>
          <option value="">Select category</option>
          <option value="1">Category 1</option>
          <option value="2">Category 2</option>
          <option value="3">Category 3</option>
        </select>
      </td>
      <td>
        <select>
          <option value="">Select subcategory</option>
        </select>
      </td>
    </tr>
  </tbody>
</table>

function loadSubcategories() {
  var category = document.getElementById("category").value;
  var subcategorySelect = document.getElementById("subcategory");
  subcategorySelect.innerHTML = "<option value=''>Select subcategory</option>";

  if (category !== "") {
    // 发送AJAX请求，获取与选择相关的数据
    // 可以使用fetch或XMLHttpRequest等方式发送请求
    // 示例中使用setTimeout模拟异步请求
    setTimeout(function() {
      // 假设返回的数据为JSON格式，包含subcategories字段
      var data = {
        subcategories: [
          { id: 1, name: "Subcategory 1" },
          { id: 2, name: "Subcategory 2" },
          { id: 3, name: "Subcategory 3" }
        ]
      };

      // 动态加载下拉列表选项
      data.subcategories.forEach(function(subcategory) {
        var option = document.createElement("option");
        option.value = subcategory.id;
        option.textContent = subcategory.name;
        subcategorySelect.appendChild(option);
      });
    }, 500); // 模拟延迟
  }
}

function addRow() {
  var table = document.getElementById("myTable");
  var newRow = table.insertRow();

  var categoryCell = newRow.insertCell();
  var categorySelect = document.createElement("select");
  categorySelect.innerHTML = document.getElementById("category").innerHTML;
  categoryCell.appendChild(categorySelect);

  var subcategoryCell = newRow.insertCell();
  var subcategorySelect = document.createElement("select");
  subcategorySelect.innerHTML = "<option value=''>Select subcategory</option>";
  subcategoryCell.appendChild(subcategorySelect);
}