JSFiddle - React, Tailwind, and code Playground

HTML

<select name = 'test' id='test' type='text' placeholder='izberi opcijo' data-native-menu='false'><option value="0">prva</option><option value="1">druga</option><option value="2">tretja</option><option value="3">cetrta</option><option value="4">peta</option><option value="5">sesta</option><option value="6">sedma</option><option value="7">osma</option></select><br><br><div id="hidden_div" style="display: none;">Vsebina skritega diva</div>

JavaScript

document.getElementById('test').addEventListener('change', function () {
    var style = this.value == 1 ? 'block' : 'none';
    document.getElementById('hidden_div').style.display = style;
});