Updated the test system

test/isDuplexStream.test.js

@@ -0,0 +1,19 @@
+const expect = require('expect');
+const {isDuplexStream} = require('./_30s.js');
+const Stream = require('stream');
+
+test('isDuplexStream is a Function', () => {
+  expect(isDuplexStream).toBeInstanceOf(Function);
+});
+test('isDuplexStream returns false for read streams', () => {
+  expect(isDuplexStream(new Stream.Readable())).toBeFalsy();
+});
+test('isDuplexStream returns false for write streams', () => {
+  expect(isDuplexStream(new Stream.Writable())).toBeFalsy();
+});
+test('isDuplexStream returns true for duplex streams', () => {
+  expect(isDuplexStream(new Stream.Duplex())).toBeTruthy();
+});
+test('isDuplexStream returns false for non-streams', () => {
+  expect(isDuplexStream({})).toBeFalsy();
+});