This gist will help detect if an element is visible on the viewport. For example, if the footer is visible, do something ...

jquery-element-visible

$.fn.is_on_screen = function(){
     
    var win = $(window);
     
    var viewport = {
        top : win.scrollTop(),
        left : win.scrollLeft()
    };
    viewport.right = viewport.left + win.width();
    viewport.bottom = viewport.top + win.height();
     
    var bounds = this.offset();
    bounds.right = bounds.left + this.outerWidth();
    bounds.bottom = bounds.top + this.outerHeight();
     
    return (!(viewport.right < bounds.left || viewport.left > bounds.right || viewport.bottom < bounds.top || viewport.top > bounds.bottom));
     
};

if( $('.target').length > 0 ) { // if target element exists in DOM
	if( $('.target').is_on_screen() ) { // if target element is visible on screen after DOM loaded
        $('.log').html('<div class="alert alert-success">target element is visible on screen</div>'); // log info		
	} else {
        $('.log').html('<div class="alert">target element is not visible on screen</div>'); // log info
	}
}
$(window).scroll(function(){ // bind window scroll event
	if( $('.target').length > 0 ) { // if target element exists in DOM
		if( $('.target').is_on_screen() ) { // if target element is visible on screen after DOM loaded
			$('.log').html('<div class="alert alert-success">target element is visible on screen</div>'); // log info
		} else {
			$('.log').html('<div class="alert">target element is not visible on screen</div>'); // log info
		}
	}
});