JSFiddle - React, Tailwind, and code Playground
HTML
<textarea name="post" id="post-input" autocomplete="off"></textarea>
JavaScript
var input = jQuery("#post-input");
input.on('input', function(event){
setTimeout(function(){
eventKeyCode = window.event ? event.keyCode : event.which; // what I already tried is in StackOverflow answer
console.log(eventKeyCode);
}, 200);
});