I'm programming for the first time. I want to write a code that moves multiple selections from a list with the up and down buttons in JavaScript. However, I was able to write something that moves up and down, but it does not work well if I move multiple times. Please tell me how to move multiple selected items up and down.

If you select 3 and 5 and press the up button, you want to do as follows.

<option value = "1"> 1 </ option>
<option value = "2"> 2 </ option>
<option value = "3"> 3 </ option>
<option value = "4"> 4 </ option>
<option value = "5"> 5 </ option>
<option value = "6"> 6 </ option>
↓
<option value = "1"> 1 </ option>
<option value = "3"> 3 </ option>
<option value = "2"> 2 </ option>
<option value = "5"> 5 </ option>
<option value = "4"> 4 </ option>
<option value = "6"> 6 </ option>

<table>
   <tr>
       <td>
           <select id="item-list" size="8" multiple="multiple">
               <option value="1">1</option>
               <option value="2">2</option>
               <option value="3">3</option>
               <option value="4">4</option>
               <option value="5">5</option>
           </select>
       </td>
       <td>
           <input type="button" value="↑" onclick="move('up');">
           <br/>
           <input type="button" value="↓" onclick="move('down');">
       </td>
   </tr>
</table>

function move(act)
{
    var s = document.getElementById("item-list");
    if (s.selectedIndex == -1) return;

    var opt = s.options[s.selectedIndex];


            if (act == 'up')
            {
                if (s.options[s.selectedIndex - 1])
                {
                    s.insertBefore(opt, s.options[s.selectedIndex - 1]);
                }
            }
            if (act == 'down')
            {
                if (s.options[s.selectedIndex + 1])
                {
                    s.insertBefore(opt, s.options[s.selectedIndex + 1].nextSibling);
                }
            }
}