The length read-only property of the TextTrackCueList interface returns the number of cues in the list.

An unsigned long which is the number of cues in the list.

The TextTrack.cues property returns a TextTrackCueList containing the current cues for that particular track. Calling cues.length returns the number of cues in the list. Using the WebVTT track below, the value of length is 5.

WEBVTT

first
00:00:00.000 --> 00:00:00.999  line:80%
Hildy!

second
00:00:01.000 --> 00:00:01.499 line:80%
How are you?

third
00:00:01.500 --> 00:00:02.999 line:80%
Tell me, is the ruler of the universe in?

fourth
00:00:03.000 --> 00:00:04.299 line:80%
Yes, they're in - in a bad humor

fifth
00:00:04.300 --> 00:00:06.000 line:80%
Somebody must've stolen the crown jewels

const video = document.getElementById("video");
video.onplay = () => {
  console.log(video.textTracks[0].cues.length); // 5
};