Center a fullscreen image of any aspect ratio

gistfile1.js

function centerImage() {   
    $img = $('img');
    var newimage = new Image();
    var originalImageWidth;
    var originalImageHeight;
    
    newimage.src = $img.attr('src');
    newimage.onload = function()
    {
        // resize/reposition image based on aspect ratio relative to viewport aspect to make sure it is always fullscreen and centered
        originalImageWidth = this.naturalWidth;
        originalImageHeight = this.naturalHeight;

        var imageAspect = originalImageWidth / originalImageHeight;
        var windowAspect = $(window).width()/$(window).height();
    
        if (windowAspect > imageAspect) {
            $img.width($(window).width()).height('auto');
            // shift the element up
            var height = $img.height();
            var shift = (height - $(window).height()) / 2;
            if (shift < 0) shift = 0;
            $img.css({
                "top" : -shift,
                "left" : 0
            });
        }
        else {
            $img.height($(window).height()).width('auto');
            // shift the element left
            var width = $img.width();
            var shift = (width -  $(window).width()) / 2;
            if (shift < 0) shift = 0;
            $img.css({
                "left" : -shift,
                "top" : 0
            });
        }
    }
}