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var $document = $(document),
    $element = $('#top-bar'),
    className = 'shadow';

$document.scroll(function() {
  if ($document.scrollTop() >= 5) { // this is the amount of pixels needed to scroll
    // user scrolled 5 pixels or more;
    // do stuff
    $element.addClass(className);
  } else {
    $element.removeClass(className);
}
});