The language read-only property of the TrackDefault interface returns a default language for an associated SourceBuffer to use when an initialization segment does not contain language information for a new track.
The type read-only property of the TrackDefault interface returns the type of track that this SourceBuffer's media segment data relates to (i.e. audio, video, or text track.)
The TrackDefault() getter method of the TrackDefaultList interface allows the TrackDefault objects in the list to be accessed with an array operator (i.e. [].)