The id property of the TextTrackCue interface returns and sets the identifier for this cue.
A string containing the ID of this cue.
In the following example a new VTTCue (which inherits from TextTrackCue) is created. The id property is then set to "first", before the cue is added.
let video = document.querySelector("video");
let track = video.addTextTrack("captions", "Captions", "en");
track.mode = "showing";
let cue = new VTTCue(0, 0.9, "Hildy!");
cue.id = "first";
track.addCue(cue);