How can I return a value from getJSON?

  $("#search-box").keyup(function(){
            var filter = $(this).val();

            var req = {
                    action: 'getItem',
                    filter: filter              
                };

            this.getJSON('videos.php', req, function(serverData)
            {

            });
   });

I have above function which returns values. How can I return the serverData?