willpower232/TOTPBTF3

(function(d) {
    var withIntersectionObserver = function(callback) {
        // apparently we can't test for false very well so base it off true instead
        if ('IntersectionObserver' in window) {
            callback();
        } else {
            var teh = d.createElement('script');
            teh.src = '/js/intersectionobserver.min.js';
            teh.onload = function() {
                callback();
            };

            d.body.appendChild(teh);
        }
    };
    
    withIntersectionObserver(function() {
        var observer = new IntersectionObserver(function(entries) {
            entries.forEach(function(entry) {
                if (entry.intersectionRatio > 0) {
                    observer.unobserve(entry.target);
                    preloadImage(entry.target);
                }
            });
        }, {
            rootMargin: '50px 0px', // if the image gets within 50px in the X axis, start the download
            threshold: 0.01
        });

        Array.from(d.querySelectorAll('[data-src]')).forEach(function(image) {
            observer.observe(image);
        });
    });
    
    var preloadImage = function(el) {
        var img = new Image();

        img.onload = function() {
            el.src = el.dataset.src;
            delete el.dataset.src;
        };

        // not relevant but might be
        // img.onerror = function(err) {
        //     el.parentNode.classList.add('error');
        // };
    
        img.src = el.dataset.src;
    };
})(document);