how to determin if element is in viewport with jquery

1// this function runs every time you are scrolling
2
3$(window).scroll(function() {
4    var top_of_element = $("#element").offset().top;
5    var bottom_of_element = $("#element").offset().top + $("#element").outerHeight();
6    var bottom_of_screen = $(window).scrollTop() + $(window).innerHeight();
7    var top_of_screen = $(window).scrollTop();
8
9    if ((bottom_of_screen > top_of_element) && (top_of_screen < bottom_of_element)){
10        // the element is visible, do something
11    } else {
12        // the element is not visible, do something else
13    }
14});
15

1$.fn.isInViewport = function() {
2    var elementTop = $(this).offset().top;
3    var elementBottom = elementTop + $(this).outerHeight();
4
5    var viewportTop = $(window).scrollTop();
6    var viewportBottom = viewportTop + $(window).height();
7
8    return elementBottom > viewportTop && elementTop < viewportBottom;
9};
10