如何添加javascript下拉更改？

<select id="mySelect" onchange="changeText()">
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>
<p id="demo">选择一个选项以改变文本。</p>

function changeText() {
  var selectValue = document.getElementById("mySelect").value;
  var text = "";
  
  if (selectValue === "option1") {
    text = "你选择了选项1。";
  } else if (selectValue === "option2") {
    text = "你选择了选项2。";
  } else if (selectValue === "option3") {
    text = "你选择了选项3。";
  }
  
  document.getElementById("demo").innerHTML = text;
}