Update active nav item while scrolling

main.js

$(window).scroll(function() {
	var current = $('main section').map(function(){
		if($(this).offset().top < $(window).scrollTop() + $(window).height() / 2) {
			return $(this);
		}
	});
	current = current.last().get(0);
	$('nav.main a').removeClass('active');
	$('a[href=#' + current.attr('id') + ']').addClass('active');
});