Adds class to the current section that is visible on screen for one page website

Adds class to the current section that is visible on screen for one page website

var sections = $('.section')
                    , nav = $('.menu-main-navigation-container')
                    , nav_height = nav.outerHeight();

            $(window).on('scroll', function() {
                var cur_pos = $(this).scrollTop();

                sections.each(function() {
                    var top = $(this).offset().top - nav_height,
                            bottom = top + $(this).outerHeight();

                    if (cur_pos >= top && cur_pos <= bottom) {
                        nav.find('a').removeClass('active');
                        sections.removeClass('active');

                        $(this).addClass('active');
                        nav.find('a[href="#' + $(this).attr('id') + '"]').addClass('active');
                    }
                });
            });

            nav.find('a').on('click', function(e) {
                e.preventDefault();
                var $el = $(this);
                var id = $el.attr('href');
                var pos = $(id).offset().top;

                $('html,body').stop().animate({
                    scrollTop: pos
                }, 1500, 'easeOutSine');

                //return false;
            });