Making a field mandatory before adding to cart using the "disabled" class on the add to cart button by default

template.html

	<script type="text/javascript">
		if ($('#productextra0').length) {
			$('#productextra0').keyup(function(){
				if ($('#productextra0').val() != '') {
					$('.addtocart').removeClass('disabled');
				}
				else {
					$('.addtocart').addClass('disabled');
				}
			});
		}
		else {
			$('.addtocart').removeClass('disabled');
		}
	</script>