If supplied, the callback is fired once the animation is complete. This can be useful for stringing different animations together in sequence. The callback is not sent any arguments, but this is set to the DOM element being animated. If multiple elements are animated, it is important to note that the callback is executed once per matched element, not once for the animation as a whole.
Note: To avoid unnecessary DOM manipulation, .fadeOut() will not hide an element that is already considered hidden.
<div class="third" style="display: none;"></div>
<div class="third"></div>$( '.third' ).fadeOut( 250, callback );