Bootstrap-下拉框 Combobox

Bootstrap下拉框 Combobox显示效果如下：

源代码：

<select class="combobox">
  <option></option>
  <option value="PA">Pennsylvania</option>
  <option value="CT">Connecticut</option>
  <option value="NY">New York</option>
  <option value="MD">Maryland</option>
  <option value="VA">Virginia</option>
</select>

<script type="text/javascript">
  $(document).ready(function(){
    $('.combobox').combobox();
  });
</script>