JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <select id="monselect">
        <option>truc</option>
        <option>muche</option>
        <option>bidule</option>
    </select>
    <input type="text" id="moninput" />
</form>

JavaScript

$(function(){
    function afficheInput() {
        if ($('#monselect').val() === 'muche') {
            $('#moninput').show();
        }
        else {
            $('#moninput').hide();            
        }
    }
    afficheInput();    
    
    $('#monselect').change(function (){
        afficheInput();
    });
});