stream.isWritable(stream)

• stream <Writable> | <Duplex> | <WritableStream>

• 返回：<boolean> | <null> - 仅当 stream 不是有效的 Writable、Duplex 或 WritableStream 时才返回 null。

  ¥Returns: <boolean> | <null> - Only returns null if stream is not a valid Writable, Duplex or WritableStream.

返回流是否可写。

¥Returns whether the stream is writable.