function BackgroundNode( {node, loadedClassName}) {
        let src = node.getAttribute('data-background-image-url');
        let show = (onComplete) => {
            requestAnimationFrame(() => {
                node.style.backgroundImage = `url(${src})`
                node.classList.add(loadedClassName);
                onComplete();
            })
        }

        return {
            node,

            // onComplete is called after the image is done loading.
            load: (onComplete) => {
                let img = new Image();
                img.onload = show(onComplete);
                img.src = src;
            }
    }
    }

    let defaultOptions = {
        selector: '[data-background-image-url]',
        loadedClassName: 'loaded'
    }

    function BackgroundLazyLoader( {selector, loadedClassName} = defaultOptions) {
        let nodes = [].slice.apply(document.querySelectorAll(selector))
                .map(node => new BackgroundNode({node, loadedClassName}));

        let callback = (entries, observer) => {
            entries.forEach(({target, isIntersecting}) => {
                if (!isIntersecting) {
                    return;
                }

                let obj = nodes.find(it => it.node.isSameNode(target));

                if (obj) {
                    obj.load(() => {
                        // Unobserve the node:
                        observer.unobserve(target);
                        // Remove this node from our list:
                        nodes = nodes.filter(n => !n.node.isSameNode(target));

                        // If there are no remaining unloaded nodes,
                        // disconnect the observer since we don't need it anymore.
                        if (!nodes.length) {
                            observer.disconnect();
                        }
                    });
            }
            })
        };

        let observer = new IntersectionObserver(callback);
        nodes.forEach(node => observer.observe(node.node));
    };

    BackgroundLazyLoader();