isDuplexStream

Checks if the given argument is a duplex (readable and writable) stream.

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";

const Stream = require("stream");
isDuplexStream(new Stream.Duplex()); // true