JSFiddle - React, Tailwind, and code Playground

HTML

<select name="liste[]" id="liste" multiple="multiple">
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
</select>
<input type="button" id="calc" value="calcul" />
<div id="result"></div>

JavaScript

$(function () {

    compte_result_select = function () {
        var nb = 0;
        $("#liste :selected").each(function () {
            nb += +$(this).val();
             });
        $("#result").html(nb);
    }

    $('#calc').on('click', compte_result_select)

})