function trackToJson_(track) {
  var ret = ['kind', 'label', 'language', 'id', 'inBandMetadataTrackDispatchType', 'mode', 'src'].reduce(function (acc, prop, i) {

    if (track[prop]) {
      acc[prop] = track[prop];
    }

    return acc;
  }, {
    cues: track.cues && Array.prototype.map.call(track.cues, function (cue) {
      return {
        startTime: cue.startTime,
        endTime: cue.endTime,
        text: cue.text,
        id: cue.id
      };
    })
  });

  return ret;
}