CODE EXAMPLE FOR JAVASCRIPT

js how to know if element touch border

$(function(){
    var $window = $(window),
        $header = $('.header'),
        $this   = $(this); // <-----here you can cache your selectors

    $window.on('scroll', function(){
       if($this.scrollTop() > 0){
           $header.addClass('shadow');
       }else{
           $header.removeClass('shadow');
       }
    }).scroll();
});