code: function(direction) {
      var self = this;
  
      // Get the next track based on the direction of the track.
      var index = 0;
      if (direction === 'prev') {
        index = self.index - 1;
        if (index < 0) {
          index = self.playlist.length - 1;
        }
      } else {
        index = self.index + 1;
        if (index >= self.playlist.length) {
          index = 0;
        }
      }
  
      self.skipTo(index);
    }
docs: Skip to the next or previous track.
  @param  {String} direction 'next' or 'prev'.