JSFiddle - React, Tailwind, and code Playground

HTML

<div id="sidebarf">
     <form id="sidebarform" name="myForm" method="post">
         <input type="text" id="sidebarusername" name="fname" placeholder="Name" />
           <input type="submit" id="sidebarformsubmit" value="Submit" />
     </form> 
</div>

JavaScript

$(document).ready(function() {

    $('#sidebarform').on('submit', function() {
        if ($('#sidebarusername').val() == '') {
            alert('First name must be filled out');
            return false;
        }
        
        $("#sidebarform").fadeOut("slow", function(){
              $("#sidebarf").html("hello " + $("#sidebarusername").val() );
        });
        
        return false;
    });
    
});