JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="body" id="maxlength" data-maxlength="100" placeholder="O que está acontecendo?"></textarea><br>
<input type="submit" name="send" value="Post">
<span id="content-countdown">100</span>

JavaScript

$(function () {

    $("#maxlength").on("input", function (event) {
	
        var $this = $(this),
        	$target = $("#content-countdown"),
        	max = $this.data('maxlength');
        	len = parseInt($(this).val().length, 10);
        	remain = max - len;
			
        if (len > max) {
            var val = $this.val();
            $this.val(val.substr(0, max));
            remain = 0;
        }

        $target.text(remain);
    });

});

$("[type='submit']").click(function(){
	console.log($('textarea').val().length);
});