JSFiddle - React, Tailwind, and code Playground

HTML

<select multiple="multiple">
    <option value="1">#1 option</option>
    <option value="2">#2 option</option>
    <option value="3">#3 option</option>
    <option value="4">#4 option</option>
    <option value="5">#5 option</option>
</select>
<br />
<input type="button" value="Remove non-selected" />

CSS

select {
  width: 120px;
  height: 90px;   
}

JavaScript

$(':button').click(function()
{
    $('select option:not(:selected)').remove();                   
});