getDefinitionByName

trace(getDefinitionByName("GalleryDoc"));//[class GalleryDoc]
trace(_prevBtn.parent);//[object GalleryDoc]

_prevBtn.parent.setChildIndex(_prevBtn,_prevBtn.parent.numChildren - 1 );//this works

I am trying to do this:

public static function onTop( ...args ):void {
            
            for each(var item in args){
                getDefinitionByName("GalleryDoc").setChildIndex(item, getDefinitionByName("GalleryDoc").numChildren - 1);
            }
            
}

but ofcourse i am getting na error…

how do i refer to an [object GalleryDoc] instead of a [class GalleryDoc]?