The read-only TextTrackList property length returns the number of entries in the TextTrackList, each of which is a TextTrack representing one track in the media element.

A value of 0 indicates that there are no text tracks in the media.

A number indicating how many text tracks are included in the TextTrackList. Each track can be accessed by treating the TextTrackList as an array of objects of type TextTrack.

This snippet gets the number of text tracks in the first media element found in the DOM by querySelector().

const mediaElem = document.querySelector("video, audio");
let numTextTracks = 0;

if (mediaElem.textTracks) {
  numTextTracks = mediaElem.textTracks.length;
}

Note that this sample checks to be sure HTMLMediaElement.textTracks is defined, to avoid failing on browsers without support for TextTrack.