JSFiddle - React, Tailwind, and code Playground

HTML

<div><textarea class="test" maxlength="100"></textarea>
<span>100</span> characters left.</div>

JavaScript

var inputArea = $('textarea.test');

var sum = 100;

inputArea.keydown(function (evt) {
    var evt = window.event ? window.event : e;
    var k = evt.keyCode ? evt.keyCode : e.which;
    
    if(k>48) { 
        if(sum>0) sum--;
    }
    else {
         if(sum<100) sum++;   
    }

    $(this).siblings().text(sum);
});