JSFiddle - React, Tailwind, and code Playground

HTML

<select class="mySelectBox" size="3">
    <option value="One">One</option>
    <option value="Two">Two</option>
    <option value="Three">Three</option>
</select>
<br /><br />
<button id="myButton" type="button">Click me!</button>

JavaScript

$(function() {
    $('#myButton').click(function() {
        if ($('.mySelectBox :selected').size() == 0) {
            alert('not selected');
        } else {
            alert($('.mySelectBox :selected').val());
        }
    });
});