Question

I’ve tried to find a way to animate my body’s background-position using following jQuery plugin (by Alexander Farkas):

(function($) {
    if(!document.defaultView || !document.defaultView.getComputedStyle){ // IE6-IE8
        var oldCurCSS = $.curCSS;
        $.curCSS = function(elem, name, force){
            if(name === 'background-position'){
                name = 'backgroundPosition';
            }
            if(name !== 'backgroundPosition' || !elem.currentStyle || elem.currentStyle[ name ]){
                return oldCurCSS.apply(this, arguments);
            }
            var style = elem.style;
            if ( !force && style && style[ name ] ){
                return style[ name ];
            }
            return oldCurCSS(elem, 'backgroundPositionX', force) +' '+ oldCurCSS(elem, 'backgroundPositionY', force);
        };
    }

    var oldAnim = $.fn.animate;
    $.fn.animate = function(prop){
        if('background-position' in prop){
            prop.backgroundPosition = prop['background-position'];
            delete prop['background-position'];
        }
        if('backgroundPosition' in prop){
            prop.backgroundPosition = '('+ prop.backgroundPosition;
        }
        return oldAnim.apply(this, arguments);
    };

    function toArray(strg){
        strg = strg.replace(/left|top/g,'0px');
        strg = strg.replace(/right|bottom/g,'100%');
        strg = strg.replace(/([0-9\.]+)(\s|\)|$)/g,"$1px$2");
        var res = strg.match(/(-?[0-9\.]+)(px|\%|em|pt)\s(-?[0-9\.]+)(px|\%|em|pt)/);
        return [parseFloat(res[1],10),res[2],parseFloat(res[3],10),res[4]];
    }

    $.fx.step. backgroundPosition = function(fx) {
        if (!fx.bgPosReady) {
            var start = $.curCSS(fx.elem,'backgroundPosition');
            if(!start){//FF2 no inline-style fallback
                start = '0px 0px';
            }

            start = toArray(start);
            fx.start = [start[0],start[2]];
            var end = toArray(fx.end);
            fx.end = [end[0],end[2]];

            fx.unit = [end[1],end[3]];
            fx.bgPosReady = true;
        }
        //return;
        var nowPosX = [];
        nowPosX[0] = ((fx.end[0] - fx.start[0]) * fx.pos) + fx.start[0] + fx.unit[0];
        nowPosX[1] = ((fx.end[1] - fx.start[1]) * fx.pos) + fx.start[1] + fx.unit[1];           
        fx.elem.style.backgroundPosition = nowPosX[0]+' '+nowPosX[1];

    };
})(jQuery);

Actually, it says it is compatible from IE6+, but when executing this:

$('body').stop().animate({backgroundPosition: '50% -800px'}, 2000);

..it throws following error message in IE8 or a lower version:

No retrieved value for property “1”: The object either is null or undefined

what referrs to this line in the plugin above:

return [parseFloat(res[1],10),res[2],parseFloat(res[3],10),res[4]]; 

Answer 1

It is simply Internet Explorer 8, handling the script differently and throwing errorss like:

No retrieved value for property "1": The object either is null or undefined