TextTrackCueList: length 属性

length 是 TextTrackCueList 接口的只读属性，它返回列表中提示的数量。

一个 unsigned long，表示列表中提示的数量。

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
};