parallax scrolling

HTML

<div>
    <div class="boxs"></div>
    <img src="http://placehold.it/1000x1000" alt="bg_img" />
</div>

CSS

img {
    display:block;
    position: relative;
}
div.boxs {
    background-color:red;
    display:block;
    width:200px height:200px;
}

JavaScript

parallaxScrolling: function () {
        var position = 0;

        window.addEventListener('scroll', function () {
            position = document.body.scrollTop;
            document.body.style.backgroundPosition = "0px " + (-1 * position) + "px";
        });
    }