新增于: v10.0.0
zlib.bytesWritten 属性指定在处理字节（压缩或解压缩，视派生类而定）之前写入引擎的字节数。
Added in: v10.0.0
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).