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(function maxLength(){
	$('.js-maxlength').on('input', function(){
		var $textarea = $(this),
			maxLength = $textarea.attr('maxlength'),
			counterSelector = $textarea.data('maxlength-counter');

		$(counterSelector).text(maxLength - $textarea.val().length);
	});
})();

<textarea class="js-maxlength" data-maxlength-counter="#js-message-counter" maxlength="100"></textarea>
You have <strong id="js-message-counter" class="text-primary">100</strong> characters left.