elementIsVisible.js

function elementIsVisible(el) {
    var docViewTop = $(window).scrollTop();
    var docViewBottom = docViewTop + $(window).height();

    var elTop = $(el).offset().top;
    var elBottom = elTop + $(el).height();
    return ((elBottom - $(el).height() <= docViewBottom)
        && (elTop + $(el).height() >= docViewTop));
}