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 cue1 = new VTTCue(0, 0.9, "Hildy!");
cue1.line = "1";
console.log(cue1.line);

track.addCue(cue1);