https://www.bcs.org/js/animatein.js

1/* scroll animations plugin */
2(function($) {
3    $.fn.animateIn = function(options) {
4        return this.each(function() {
5            var settings = $.extend(
6                {
7                    //default settings
8                    offset: 0,
9                    modifier: '__in'
10                },
11                options
12            );
13
14            var item = $(this);
15            var itemOffsetTop = $(this).offset().top;
16            var triggerVal = itemOffsetTop + settings.offset;
17
18            function animateBlock(scrollTop) {
19                // if element is already in view or check if it comes into view
20                if (
21                    scrollTop + $(window).outerHeight() >= triggerVal ||
22                    $(window).outerHeight() >= triggerVal
23                ) {
24                    item.addClass(settings.modifier);
25                }
26            }
27
28            $(window).on('scroll', { passive: true }, function() {
29                var scrollTop = $(window).scrollTop();
30                animateBlock(scrollTop);
31            });
32            $(window).trigger('scroll');
33        });
34    };
35})(jQuery);