jQuery: Scroll sidebar with window. (It's necessary that the sidebar it's absolute positioned and have a wrapper with absolute position too)

app.js

jQuery(document).ready(function() {
	var page = jQuery('.single.post');
	var sidebar = jQuery('.single.post .scrolling-sidebar');
	var content = jQuery('.single.post .content');
 
	// Scroll with screen
	var top = sidebar.offset().top;
	var limit = top + page.height() - sidebar.height();

	// Define sidebar-wrapper height
	jQuery('.single.post .scrolling-sidebar-wrapper').css({
		height : page.css('height')
	});
 
	jQuery(window).scroll(function() {
		// what the y position of the scroll is
		var y = jQuery(this).scrollTop();
 
		// whether that's below
		if ( y >= top && y <= limit ) {
			// if so, ad the fixed class
			sidebar.addClass('fixed');
			sidebar.removeClass('bottom');
		} else if ( y >= limit ) {
			sidebar.addClass('bottom');
			sidebar.removeClass('fixed');
		} else {
			// otherwise remove it
			sidebar.removeClass('fixed bottom');
		}
	});
});