code: function cacheTracksArray(self) {
    self.tracksInOrder = Object.keys(self.playlist).map(trackById);
    self.tracksInOrder.sort(asc);
    self.tracksInOrder.forEach(function(track, index) {
      track.index = index;
    });
  
    function asc(a, b) {
      return operatorCompare(a.sortKey, b.sortKey);
    }
    function trackById(id) {
      return self.playlist[id];
    }
  }
docs: generate self.tracksInOrder from self.playlist