Updated the test system

test/isWritableStream.test.js

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