JSFiddle - React, Tailwind, and code Playground

HTML

<textarea class="test" maxlength="100"></textarea>

JavaScript

var timeoutVariable;
var inputArea = $('textarea');

inputArea.keyup(function(){
   window.clearTimeout(timeoutVariable);
   timeoutVariable = setTimeout(runSum(),100);
});

function runSum() {
   var sum = 100 - inputArea.val().length;
   console.log(sum);
}