优化部分JavaScript代码

Question

有人可以帮助我，如何最好地优化javacript代码的下方部分？

我想让浏览器最轻松，但我不知道 - 我对javascript知之甚少。

if ($(window).width() > 1200) {
    (function() {
        var parallaxTop = document.querySelectorAll(".parallax-top");
        if ($(".parallax-center").length) {
            var parallaxCenter = document.querySelectorAll(".parallax-center");
            var centerPosition = $(".parallax-center").position().top;
        }

        if ($(".parallax-bottom").length) {
            var parallaxBottom = document.querySelectorAll(".parallax-bottom");
            var bottomPosition = $(".parallax-bottom").position().top;
        }
        var speed = 0.4;
        window.onscroll = function() {

            [].slice.call(parallaxTop).forEach(function(el, i) {
                var windowYOffset = window.pageYOffset,
                    elBackgrounPos = "center " + "-" + (windowYOffset * speed) + "px";
                el.style.backgroundPosition = elBackgrounPos;
            });
            if ($(".parallax-center").length)
                [].slice.call(parallaxCenter).forEach(function(el, i) {
                    var windowYOffset = window.pageYOffset,
                        elBackgrounPos = "center " + "" + ((windowYOffset - centerPosition) * speed + 1700) + "px";
                    el.style.backgroundPosition = elBackgrounPos;
                });
            if ($(".parallax-bottom").length)
                [].slice.call(parallaxBottom).forEach(function(el, i) {
                    var windowYOffset = window.pageYOffset,
                        elBackgrounPos = "center " + "" + ((windowYOffset - bottomPosition) * speed + 1400) + "px";
                    el.style.backgroundPosition = elBackgrounPos;
                });
        };
    })();
}