Test in JQuery if an element is at the top of screen

Question

I have a div that is positioned about 100px from the top of the browser window. When the user scrolls down, I want the div to stay where it is

Answer 1

when you have header. and then aside bar. for fixing aside bar, when it is at top of the screen:

Javascript:

var scroll_happened = false;
var aside_from_top = $('aside').offset().top;
$window = $(window);

$window.scroll(function()
{
    scroll_happened = true;
});

setInterval(function()
{
    if(scroll_happened == true)
    {
        scroll_happened = false;
        if($window.scrollTop() >= aside_from_top)
        {
            $('#aside_container').addClass('fixed_aside');
        }
        else
        {
            $('#aside_container').removeClass('fixed_aside');
        }
    }
} , 250);

Css:

.fixed_aside
{
    position: fixed;
    top: 0;
    bottom: 0;
}

Html: