The utility function duplexPair returns an Array with two items, each being a Duplex stream connected to the other side:
const [ sideA, sideB ] = duplexPair();
Whatever is written to one stream is made readable on the other. It provides behavior analogous to a network connection, where the data written by the client becomes readable by the server, and vice-versa.
The Duplex streams are symmetrical; one or the other may be used without any difference in behavior.