Out of the four streams, this may be the most useful and possibly the most used stream out of the group. A Transform stream hooks up the readable and writable portions of a stream and allows us to manipulate the data that comes across it. This may sound similar to a Duplex. Well, a Transform stream is a special type of Duplex stream!

Built-in implementations of Transform streams include any of the streams implemented in the zlib module. The basic idea is that we are not just trying to pass information from one end to the other; we are trying to manipulate that data and turn it into something else. That is what the zlib streams give us. They compress and decompress the data. Transform streams change the data into another form. This also means that we can make a transform stream be a one-way transformation; whatever is output from the transform stream cannot be undone. We will create one of these Transform streams here, specifically creating a hash of a string.

First, let's go through the interface for a Transform stream.