通过在一个下拉列表中选择基于该选择的值，只有与该值相关的值才会出现在第二个下拉列表中

<select id="firstDropdown" onchange="updateSecondDropdown()">
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

<select id="secondDropdown">
</select>

function updateSecondDropdown() {
  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 = "";

  // 根据选择的值生成第二个下拉列表的选项
  if (selectedValue === "option1") {
    var option1 = document.createElement("option");
    option1.text = "选项1-1";
    secondDropdown.add(option1);

    var option2 = document.createElement("option");
    option2.text = "选项1-2";
    secondDropdown.add(option2);
  } else if (selectedValue === "option2") {
    var option3 = document.createElement("option");
    option3.text = "选项2-1";
    secondDropdown.add(option3);

    var option4 = document.createElement("option");
    option4.text = "选项2-2";
    secondDropdown.add(option4);
  } else if (selectedValue === "option3") {
    var option5 = document.createElement("option");
    option5.text = "选项3-1";
    secondDropdown.add(option5);

    var option6 = document.createElement("option");
    option6.text = "选项3-2";
    secondDropdown.add(option6);
  }
}