Find nth last occurrence of element

by Akram kamal

HTML

<div class="X">
    <ul>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
        <li><a><img /></a></li>
    </ul>
</div>
<div>
    <input type="text" id="value" /><br />
    <input type="button" value="Position Darth Vader" id="button" />
</div>

JavaScript

// from : Find nth last occurrence of element

(function($){
    $(function(){
        var icon1 = 'http://icons.iconarchive.com/icons/jonathan-rey/star-wars-vehicles/48/Hyena-Bomber-icon.png';
        var icon2 = 'http://icons.iconarchive.com/icons/jonathan-rey/star-wars-vehicles/48/Tie-Advanced-icon.png';
        
        var $img = $('.X img');        
        $img.attr('src', icon1);
        
        $('#button').click(function(e){
           e.preventDefault();
           var n = parseInt($('#value').val());
           if(n == window.NaN) throw "Not a number";                  
           $img.attr('src', icon1).filter(':eq('+($img .length - (n+1))+')').attr('src', icon2);  
        });
    });
})(jQuery);