JSFiddle - React, Tailwind, and code Playground

HTML

<div>chars left: <span id="txt-length-left"></span></div>
<textarea id="send-txt"></textarea>

JavaScript

var $txtLenLeft = $('#txt-length-left'); // lets cache this
var maxLen = 20;
$('#send-txt').keydown(function(e){
   var Length = $(this).val().length;
   var AmountLeft = maxLen - Length;
   $txtLenLeft.html(AmountLeft);
   if(Length >= maxLen && e.keyCode != 8){
      e.preventDefault(); // will cancel the default action of the event
   }
});