Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
    <textarea class="comment">
    </textarea>
    <script>
        var warning;
        var textarea = document.querySelector('.comment');
        textarea.onkeypress = function() {
            var maxSymbols = 100;

            function createWarningMessage () {
                warning = document.createElement('span');
                textarea.parentNode.insertBefore(warning, this.nextElementSibling);
            }

            function deleteWarningMessage() {
                warning.parentNode.removeChild(warning);
                warning = undefined;
            }

            function updateWarningMessage() {
                warning.innerText = 'Вы превысили лимит на ' + String(textarea.value.length - maxSymbols);
            }

            if (this.value.length > maxSymbols) {
                if (typeof warning === 'undefined') {
                    createWarningMessage();
                }
                updateWarningMessage()
            } else {
                if (typeof warning !== 'undefined') {
                    deleteWarningMessage();
                }
            }
        }
    </script>
</body>
</html>