新增于: v10.0.0
zlib.bytesWritten 属性指定写入引擎的字节数，在这些字节被处理（压缩或解压缩，根据派生类的不同而适用）之前。
【The zlib.bytesWritten property specifies the number of bytes written to
the engine, before the bytes are processed (compressed or decompressed,
as appropriate for the derived class).】