The line property of the VTTCue interface represents the cue line of this WebVTT cue.
A number, or "auto" representing the cue line of this WebVTT cue.
In the following example a new VTTCue is created, then the value of line is set to 1. The value is then printed to the console.
let video = document.querySelector("video");
let track = video.addTextTrack("captions", "Captions", "en");
track.mode = "showing";
let cue = new VTTCue(0, 0.9, "Hildy!");
cue.line = "1";
console.log(cue.line);
track.addCue(cue);