保存下拉列表的选定值，并默认将其分配到另一个下拉列表/复制下拉列表中

<select id="sourceList">
  <option value="value1">Option 1</option>
  <option value="value2">Option 2</option>
  <option value="value3">Option 3</option>
</select>

<select id="targetList">
  <!-- Target list will be populated based on the selected value from source list -->
</select>

document.getElementById("sourceList").addEventListener("change", function() {
  // Call a function to update the target list based on the selected value
  updateTargetList(this.value);
});

function updateTargetList(selectedValue) {
  var targetList = document.getElementById("targetList");
  
  // Clear existing options
  targetList.innerHTML = "";
  
  // Add new options based on the selected value
  switch(selectedValue) {
    case "value1":
      targetList.add(new Option("Option A", "optionAValue"));
      targetList.add(new Option("Option B", "optionBValue"));
      break;
    case "value2":
      targetList.add(new Option("Option X", "optionXValue"));
      targetList.add(new Option("Option Y", "optionYValue"));
      targetList.add(new Option("Option Z", "optionZValue"));
      break;
    case "value3":
      targetList.add(new Option("Option P", "optionPValue"));
      break;
    default:
      // Add a default option if no match found
      targetList.add(new Option("No options available", ""));
  }
}