Add readable, writable and duplex

test/isWritableStream/isWritableStream.js

@@ -0,0 +1,7 @@
+const isWritableStream = val =>
+  val !== null &&
+  typeof val === 'object' &&
+  typeof val.pipe === 'function' &&
+  typeof val._write === 'function' &&
+  typeof val._writableState === 'object';
+module.exports = isWritableStream;

test/isWritableStream/isWritableStream.test.js

@@ -0,0 +1,20 @@
+const expect = require('expect');
+const isWritableStream = require('./isWritableStream.js');
+const fs = require('fs');
+const Stream = require('stream');
+
+test('isWritableStream is a Function', () => {
+  expect(isWritableStream).toBeInstanceOf(Function);
+});
+test('isWritableStream returns false for read streams', () => {
+  expect(isWritableStream(fs.createReadStream('isWritableStream.js'))).toBeFalsy();
+});
+test('isWritableStream returns true for write streams', () => {
+  expect(isWritableStream(fs.createWriteStream('isWritableStream.js'))).toBeTruthy();
+});
+test('isWritableStream returns true for duplex streams', () => {
+  expect(isWritableStream(new Stream.Duplex())).toBeTruthy();
+});
+test('isWritableStream returns false for non-streams', () => {
+  expect(isWritableStream({})).toBeFalsy();
+});