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 cue1 = new VTTCue(0, 0.9, "Hildy!");
cue1.id = "first";
track.addCue(cue1);