jensmemes/website/script/preload.js

document.addEventListener("DOMContentLoaded", function() {
    var lazyloadImages = document.querySelectorAll("img.lazy");
    var lazyloadThrottleTimeout;

    function lazyload () {
        if(lazyloadThrottleTimeout) {
            clearTimeout(lazyloadThrottleTimeout);
        }

        lazyloadThrottleTimeout = setTimeout(function() {
            var scrollTop = window.pageYOffset;
            lazyloadImages.forEach(function(img) {
                if(img.offsetTop < (window.innerHeight + scrollTop)) {
                    img.src = img.dataset.src;
                    img.classList.remove('lazy');
                }
            });
            if(lazyloadImages.length == 0) {
                document.removeEventListener("scroll", lazyload);
                window.removeEventListener("resize", lazyload);
                window.removeEventListener("orientationChange", lazyload);
            }
        }, 20);
    }

    document.addEventListener("scroll", lazyload);
    window.addEventListener("resize", lazyload);
    window.addEventListener("orientationChange", lazyload);
});