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