扩展
构造函数
new TrackEvent()
new TrackEvent(type, target, track): TrackEvent
参数
type
string
target
EventTargettrack
TextTrack|AudioTrack|VideoTrack
返回值
TrackEvent属性
track
track: TextTrack|AudioTrack|VideoTrack
https://html.spec.whatwg.org/multipage/media.html#dom-trackevent-track(仅提供英文版)

