JSFiddle - React, Tailwind, and code Playground

HTML

<select id="sellist" multiple="multiple" size="15">
    <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="7">7</option>
    <option value="8">8</option>
    <option value="9">9</option>
    <option value="10">10</option>
</select>

JavaScript

var str ='4/5/8';
$.each(str.split('/'), function(i, val) {
    $('#sellist option').filter('[value="' + val + '"]').attr('selected', 1);
});