如何使用纯javascript激活级联下拉自动选择

<select id="firstSelect">
    <option value="">请选择</option>
    <option value="1">选项1</option>
    <option value="2">选项2</option>
    <option value="3">选项3</option>
</select>

<select id="secondSelect">
    <option value="">请选择</option>
    <option value="1">选项1</option>
    <option value="2">选项2</option>
    <option value="3">选项3</option>
</select>

<select id="thirdSelect">
    <option value="">请选择</option>
    <option value="1">选项1</option>
    <option value="2">选项2</option>
    <option value="3">选项3</option>
</select>

document.getElementById("firstSelect").addEventListener("change", function() {
    var firstSelectValue = this.value;
    var secondSelect = document.getElementById("secondSelect");

    // 清空第二个下拉列表的选项
    secondSelect.innerHTML = '<option value="">请选择</option>';

    // 根据选择的值动态生成第二个下拉列表的选项
    if (firstSelectValue === "1") {
        secondSelect.innerHTML += '<option value="1">选项1</option>';
        secondSelect.innerHTML += '<option value="2">选项2</option>';
    } else if (firstSelectValue === "2") {
        secondSelect.innerHTML += '<option value="3">选项3</option>';
        secondSelect.innerHTML += '<option value="4">选项4</option>';
    } else if (firstSelectValue === "3") {
        secondSelect.innerHTML += '<option value="5">选项5</option>';
        secondSelect.innerHTML += '<option value="6">选项6</option>';
    }
});

document.getElementById("secondSelect").addEventListener("change", function() {
    var secondSelectValue = this.value;
    var thirdSelect = document.getElementById("thirdSelect");

    // 清空第三个下拉列表的选项
    thirdSelect.innerHTML = '<option value="">请选择</option>';

    // 根据选择的值动态生成第三个下拉列表的选项
    if (secondSelectValue === "1") {
        thirdSelect.innerHTML += '<option value="1">选项1</option>';
        thirdSelect.innerHTML += '<option value="2">选项2</option>';
    } else if (secondSelectValue === "2") {
        thirdSelect.innerHTML += '<option value="3">选项3</option>';
        thirdSelect.innerHTML += '<option value="4">选项4</option>';
    } else if (secondSelectValue === "3") {
        thirdSelect.innerHTML += '<option value="5">选项5</option>';
        thirdSelect.innerHTML += '<option value="6">选项6</option>';
    }
});