Test if an element is in the viewport.

inViewport.js

var inViewport = function(el, side) {
    var win = window;

    var viewport = {
        top: win.scrollY,
        left: win.scrollX
    };
    viewport.right = viewport.left + win.outerWidth;
    viewport.bottom = viewport.top + win.outerHeight;

    var element = {
        top: el.offsetTop,
        left: el.offsetLeft
    };
    element.right = element.left + el.offsetWidth;
    element.bottom = element.top + el.offsetHeight;

    switch (side) {
        case 'top':
            return (viewport.top <= element.top);
            break;
        case 'right':
            return (viewport.right >= element.right);
            break;
        case 'bottom':
            return (viewport.bottom >= element.bottom);
            break;
        case 'left':
            return (viewport.left <= element.left);
            break;
        default:
            return (!(viewport.right <= element.left || viewport.left >= element.right || viewport.bottom <= element.top || viewport.top >= element.bottom));
    }
}