在设置了控件的选定值后运行Javascript

 <option value="option1">Option 1</option>
 <option value="option2">Option 2</option>
 <option value="option3">Option 3</option>
</select><script>
function myFunction() {
  var select = document.getElementById("mySelect");
  var selectedValue = select.options[select.selectedIndex].value;
  // 在这里编写您的JavaScript代码
}
</script>

 <option value="option1">Option 1</option>
 <option value="option2">Option 2</option>
 <option value="option3">Option 3</option>
</select><script>
document.getElementById("mySelect").addEventListener("change", function() {
  var select = document.getElementById("mySelect");
  var selectedValue = select.options[select.selectedIndex].value;
  // 在这里编写您的JavaScript代码
});
</script>