writableStreamDefaultWriter.closed

类型：<Promise> 当关联的 <WritableStream> 被关闭或拒绝时，如果流发生错误或写入器的锁在流完成关闭之前被释放，则由 undefined 完成。

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