The ImageTrackList interface of the WebCodecs API represents a list of image tracks.
ImageTrackList
MDN Reference
The length property of the ImageTrackList interface returns the length of the ImageTrackList.
length
The ready property of the ImageTrackList interface returns a Promise that resolves when the ImageTrackList is populated with ImageTrack.
ready
The selectedIndex property of the ImageTrackList interface returns the index of the selected track.
selectedIndex
index
The selectedTrack property of the ImageTrackList interface returns an ImageTrack object representing the currently selected track.
selectedTrack