The id read-only property of the TextTrack interface returns the ID of the track if it has one.

A string containing the ID, or an empty string.

In the following example the value of id is printed to the console.

const video = document.querySelector("video");
const track = video.addTextTrack("captions", "Captions", "en");
track.mode = "showing";
console.log(track.id);