JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://code.jquery.com/ui/1.10.3/themes/smoothness/jquery-ui.css">
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<form name="form" method="post" onSubmit="return Controle_choix()">
<select size="1" name="prix" id="prix">
<option value="">- Sélectionnez un prix</option>
<option value="0">&nbsp;</option>
<option value="1000">1000</option>
<option value="2000">2000</option>
<option value="3000">3000</option>
<option value="4000">4000</option>
</select>
<input type="submit" value="Sélectionnez" name="bouton">
</form>

JavaScript

function custom_alert( message, title ) {
    if ( !title )
        title = 'Alert';
    if ( !message )
        message = 'Aucun message à afficher.';
    $('<div></div>').html( message ).dialog({
        title: title,
        resizable: false,
        modal: true,
        buttons: {
            'Ok': function()  {
                $( this ).dialog( 'close' );
            }
        }
    });
}

function Controle_choix()
{
if(document.form.prix.value==0)
{
custom_alert( 'Merci de s\351lectionner un prix', 'Quel est le prix ?' );
document.form.prix.focus();
return false;
}
}