JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="field"></textarea>
<div id="charNum">0</div>

JavaScript

$("#field").keyup(function(){
    el = $(this);
    if(el.val().length >= 500){
        el.val( el.val().substr(0, 500) );
    } else {
        $("#charNum").text(500-el.val().length);
    }
});