A duplex stream is just that, one that works both ways. It combines a Readable and Writable stream into a single interface. With this type of stream, we can now just pipe from the socket into our custom stream instead of wrapping the stream like we have been (even though we will still implement it as a wrapped stream).
There is not much more to talk about with Duplex streams other than one fact that trips up newcomers to the stream type. There are two separate buffers: one for Readable and one for Writable. We need to make sure to treat them as separate instances. This means whatever we use for the _read method in terms of variables, should not be used for the _write and _writev method implementations, otherwise, we could run into bad bugs.
As stated before, the following code implements a Duplex stream along with a counting mechanism so, that way...