jQuery: Hide/Show Divs on page scroll

Question

jsfiddle: http://jsfiddle.net/MFUw3/5/

jQuery:

function showDiv() {
    if ($(window).scrollTop() > 610) {
        $(".a").css({"position": "fixed", "top": "10px"});
    } else {
        $(".a").css({"position": "relative", "top": "0px"});
    }
}
$(window).scroll(showDiv);
showDiv();

HTML:

<div>
    <div class="a">
        A
    </div>
    <div class="b">
        B
    </div>
</div>

I want to make it so when the user has scrolled past div "B" (A and B are out of sight), then div "A" will fade in and fix itself to the top of the browser.

When you scroll up and div "B" is back in sight, I want div "A" to fade out and reposition itself back to where it was originally.

My code currently does just this, EXCEPT it doesn't do fading.

I've tried messing around with .is(":visible"), .is(":hidden"), .hide(); so that I can use fadeIn(); and fadeOut();, but no matter what I try, I can't figure it out, and I know this isn't efficient in the first place. There's probably some way to detect if it's passed a div instead of passed a certain coordinate?

Answer 1

Here's something that should suit your needs:

function showDiv() {
    if ($(window).scrollTop() > 610 && $('.a').data('positioned') == 'false') {
        $(".a").hide().css({"position": "fixed", "top": "10px"}).fadeIn().data('positioned', 'true');
    } else if ($(window).scrollTop() <= 610 && $('.a').data('positioned') == 'true') {
        $(".a").fadeOut(function() {
            $(this).css({"position": "relative", "top": "0px"}).show();
        }).data('positioned', 'false');
    }
}
$(window).scroll(showDiv);
$('.a').data('positioned', 'false');

And the link to the working example: http://jsfiddle.net/MFUw3/10/

Edit: I have added the code improvements suggested by Sparky672 and the (initially omitted) fadeout.