1.新建一个select.html

<select>
  <option value ="东风本田">东风本田</option>
  <option value ="广汽本田">广汽本田</option>
  <option value="一汽大众">一汽大众</option>
  <option value="上海大众">上海大众</option>
</select>

 2.js操作打开下拉框

1 function openselect(){
2 var next=document.getElementById(1); 
3 var cc = next.length; 
4 next.size=cc; 
5 return ''}
6 openselect();

 3.选择其中一个选项

1 function selectone(){
2 var next=document.evaluate('//*[@id="1"]/option[3]', document).iterateNext();
3 next.parentNode.size=1;
4 next.selected=true;
5 return '';}
6 selectone();

 4.执行事件

let evt = document.createEvent('HTMLEvents'); 
evt.initEvent('change', false, true);
next.parentNode.dispatchEvent(evt);