<div class="mb-3">
                        <label class="form-label" for="validationTextarea">Textarea</label>
                        <textarea class="form-control is-invalid" id="validationTextarea" placeholder="Required example textarea" required=""></textarea>
                        <div class="invalid-feedback" id="say"></div>
                      </div>

 $(document).ready(function() {
$("#validationTextarea").on("keyup",function(){
var yazi =$("#validationTextarea ").val();
$("#say").html(yazi.length);
});
});