readableStreamBYOBReader.closed

类型：<Promise> 当关联的 <ReadableStream> 被关闭或拒绝时，如果流发生错误或在流完成关闭之前释放读取器的锁，则由 undefined 完成。

¥Type: <Promise> Fulfilled with undefined when the associated <ReadableStream> is closed or rejected if the stream errors or the reader's lock is released before the stream finishes closing.