根据第一个下拉列表的值填充第二个下拉列表

<select id="firstDropdown" onchange="populateSecondDropdown()">
  <option value="value1">Value 1</option>
  <option value="value2">Value 2</option>
  <option value="value3">Value 3</option>
</select>

<select id="secondDropdown">
  <!-- 这里的选项将由JavaScript动态填充 -->
</select>

function populateSecondDropdown() {
  var firstDropdown = document.getElementById("firstDropdown");
  var secondDropdown = document.getElementById("secondDropdown");
  var selectedValue = firstDropdown.value;

  // 清空第二个下拉列表的选项
  secondDropdown.innerHTML = "";

  // 根据选择的值填充第二个下拉列表
  if (selectedValue === "value1") {
    var option1 = document.createElement("option");
    option1.value = "option1";
    option1.text = "Option 1";
    secondDropdown.appendChild(option1);

    var option2 = document.createElement("option");
    option2.value = "option2";
    option2.text = "Option 2";
    secondDropdown.appendChild(option2);
  } else if (selectedValue === "value2") {
    var option3 = document.createElement("option");
    option3.value = "option3";
    option3.text = "Option 3";
    secondDropdown.appendChild(option3);

    var option4 = document.createElement("option");
    option4.value = "option4";
    option4.text = "Option 4";
    secondDropdown.appendChild(option4);
  } else if (selectedValue === "value3") {
    var option5 = document.createElement("option");
    option5.value = "option5";
    option5.text = "Option 5";
    secondDropdown.appendChild(option5);

    var option6 = document.createElement("option");
    option6.value = "option6";
    option6.text = "Option 6";
    secondDropdown.appendChild(option6);
  }
}