JS do something when element is in viewport

js-do-something-when-element-is-in-viewport.js

$(window).scroll(function() {
    var top_of_element = $("#my_element").offset().top;
    var bottom_of_element = $("#my_element").offset().top + $("#my_element").outerHeight();
    var bottom_of_screen = $(window).scrollTop() + $(window).innerHeight();
    var top_of_screen = $(window).scrollTop();

    if ((bottom_of_screen > top_of_element) && (top_of_screen < bottom_of_element)){
        // element is in view, do something
    } else {
        // element is out of view, do something else
    }
});