https://www.themedemo.co/napoli-light/js/foxlazy.js

1jQuery(function($){
2
3
4    $.fn.foxlazy = function(threshold, callback) {
5
6        var $w = $(window),
7            th = threshold || 0,
8            retina = window.devicePixelRatio > 1,
9            attrib = retina ? "data-lazy-src-retina" : "data-lazy-src",
10            images = this,
11            loaded;
12
13        this.one("foxlazy", function() {
14            var source = this.getAttribute(attrib);
15            source = source || this.getAttribute("data-lazy-src");
16            if (source) {
17                this.setAttribute("src", source);
18                if ($(this).hasClass('hidden-img')) {
19                    $(this).parent().css( 'background-image' , 'url(' + source + ')' );
20                    $(this).hide();
21                }
22            }
23            if (typeof callback === "function") callback.call(this);
24        });
25
26        function foxlazy() {
27            var inview = images.filter(function() {
28                var $e = $(this);
29
30                var scrollTop = $w.scrollTop(),// scrollTop
31                    scrollTopHeight = scrollTop + $w.height(), // scrollTopHeight
32                    offsetTop = $e.offset().top, // offsetTop
33                    offsetTopHeight = offsetTop + $e.height(); // offsetTopHeight
34
35                return offsetTopHeight >= scrollTop - th && offsetTop <= scrollTopHeight + th;
36            });
37
38            loaded = inview.trigger("foxlazy");
39            images = images.not(loaded);
40        }
41
42        $w.on("load.foxlazy scroll.foxlazy resize.foxlazy lookup.foxlazy", foxlazy);
43
44        foxlazy();
45
46        return this;
47
48    };
49
50    $("img[data-lazy-src]").foxlazy();
51
52});