docs: function(index) {
      var self = this;
  
      // Stop the current track.
      if (self.playlist[self.index].howl) {
        self.playlist[self.index].howl.stop();
      }
  
      // Reset progress.
      progress.style.width = '0%';
  
      // Play the new track.
      self.play(index);
    }