jQuery .is(":visible) for an element which is absolute positioned out of viewport

Question

I am trying to find out if this is the correct way to get state of visibility of an div container which has position: absolute.

I have a sidebar on the left of my viewport. When clicking the "sidebar-button" the "sideBar" gets animated out of the visible area.

jQuery:

$('#sideBarButton').click(function(){
    sidebar = $("#sideBar").outerWidth();
    if(!$("#sideBar").hasClass('outof'))
    {
       $("#sideBar").animate({left: screenW},100).addClass('outof');
       $("#boardContent").animate({width: screenW},200);
       $("#sideBarButton").animate({left: "+=" + (sidebar -3)},100);
    }
    else 
    {
       $("#boardContent").animate({width: bcW}, 50);
       setTimeout(function(){
           $("#sideBar").animate({left: screenW - sidebar}, 200).removeClass('outof');
           $("#sideBarButton").animate({left: "-=" + (sidebar -3)},200);
       }, 120);
    }
    $(this).find('img').toggle();
});

Since the sideBar now is not visible at all (for my eyes) and the left position of the sidebar is bigger than document.width my question is can I get the state of visibility (of sideBar) like this:

if($("#sideBar").is(":visible"))...

or is there a better way? I mean how does jQuery decide if an element is visible or not?

Answer 1

To quote jQuery itself

Elements are considered visible if they consume space in the document. Visible elements have a width or height that is greater than zero.

Elements with visibility: hidden or opacity: 0 are considered visible, since they still consume space in the layout.

So with visible, jQuery does not take into account elements that are pulled out of the viewport. Eg. if you scroll down a page, the header is still deemed visible, although you cannot see it with the eye.

To check if the sidebar is visible, some (not so) complicated calculations have to be done by yourself, or just add a class or data attribute when the sidebar is out, like you have already done using the outof class.