isDuplexStream

• Check if the value is different from null.
• Use typeof to check if a value is of type object and the pipe property is of type function.
• Additionally check if the typeof the _read, _write and _readableState, _writableState properties are function and object respectively.

const isDuplexStream = (val) =>
  val !== null &&
  typeof val === "object" &&
  typeof val.pipe === "function" &&
  typeof val._read === "function" &&
  typeof val._readableState === "object" &&
  typeof val._write === "function" &&
  typeof val._writableState === "object";
Examples;
const Stream = require("stream");

isDuplexStream(new Stream.Duplex()); // true