JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post"> 
<input type="text" name="customer">
<input type="submit" id="btn" value="submit:Toggle div green">
</form>

<div id="mydiv">KASTEN: (no data submitted)</div>

CSS

#mydiv {
    width: 30%; 
    height: 100px;
    border: 6px solid green;
}

body {
    color:#FF004C;
}

JavaScript

var $form = $('form');
var $resultDiv = $('#mydiv');
$resultDiv.hide();

var url = '/echo/html/';

var successHandler = function(data, textStatus, jqXhr) {
    $resultDiv.html(data);
    $resultDiv.fadeToggle(200);    
};

$form.submit(function(ev) {
    ev.preventDefault();
    if (! $resultDiv.is(':visible')) {
        var d = $form.serialize();
        $.post(
            url,
            {html: d},
            successHandler
        );
    } else {
        $resultDiv.fadeToggle(1);
    }
});