// HTML代码
<select id="myDropdown">
  <option value="default">默认值</option>
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

// JavaScript代码
document.getElementById("myDropdown").addEventListener("click", function() {
  this.value = "default";
});

// HTML代码
<select id="myDropdown">
  <option value="default">默认值</option>
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

// JavaScript代码
$("#myDropdown").on("click", function() {
  $(this).val("default");
});