index.js

'use strict';

var isStream = module.exports = function (stream) {
	return stream !== null && typeof stream === 'object' && typeof stream.pipe === 'function';
};

isStream.writable = function (stream) {
	return isStream(stream) && stream.writable !== false && typeof stream._write === 'function' && typeof stream._writableState === 'object';
};

isStream.readable = function (stream) {
	return isStream(stream) && stream.readable !== false && typeof stream._read === 'function' && typeof stream._readableState === 'object';
};

isStream.duplex = function (stream) {
	return isStream.writable(stream) && isStream.readable(stream);
};

isStream.transform = function (stream) {
	return isStream.duplex(stream) && typeof stream._transform === 'function' && typeof stream._transformState === 'object';
};