属性
length
length:number
https://html.spec.whatwg.org/multipage/media.html#dom-texttrackcuelist-length
方法
getCueById()
getCueById(id:string): TextTrackCue
https://html.spec.whatwg.org/multipage/media.html#dom-texttrackcuelist-getcuebyid
参数
id
string
