docs: function() {
      var self = this;
      var display = (volume.style.display === 'block') ? 'none' : 'block';
  
      setTimeout(function() {
        volume.style.display = display;
      }, (display === 'block') ? 0 : 500);
      volume.className = (display === 'block') ? 'fadein' : 'fadeout';
    }