parallax.js

(function ($) {
    function parallaxImg(img, imgParent) {
        const speed = img.data('parallax-speed')
            ? img.data('parallax-speed')
            : 0.25;
        const imgY = imgParent.offset().top;
        const winY = $(this).scrollTop();
        const winH = $(this).height();
        const parentH = imgParent.innerHeight();
        const winBottom = winY + winH;

        if (winBottom > imgY && winY < imgY + parentH) {
            const imgBottom = (winBottom - imgY) * speed;
            const imgTop = winH + parentH;
            const imgPercent = (imgBottom / imgTop) * 100 + (50 - speed * 50);

            img.css({
                transform: 'translate(-50%, -' + imgPercent + '%)',
            });
        }
    }

    $('[data-behaviour="parallax-image"').each(function () {
        const img = $(this).find('img');
        const imgParent = $(this);

        $(document).on({
            scroll: function () {
                parallaxImg(img, imgParent);
            },
            ready: function () {
                parallaxImg(img, imgParent);
            },
        });
    });
})(jQuery);