jQuery .scroll无法启动

Question

I'm trying create an alert saying "True" when the user scrolls past the "#topp" element, yet it isn't doing anything, the element is just supposed to be a tiny div at the top of the page.

HTML

<div id="topp"></div>

jQuery

    $(window).scroll(function() {
    var vpH = $(window).height(),
        st = $(window).scrollTop(),
        y = $('#topp').offset().top;
    if(y > (st + vpH)) alert('true');
});

Answer 1

Why do you need the window height? If you have the top and the scroll to top variables then theirs no need for the height of the window.

$(document).scroll(function() 
{
    var scrollTop = $(window).scrollTop();
    var toppOffset = $('#topp').offset().top;

    if(toppOffset > scrollTop)
        alert('true');
});​

A clearer representation http://jsfiddle.net/zDpw3/1/