MelbJS talk about CommonJS and Mocking With Jest

1. CommonJS &
Mocking with Jest
James Hunter – @cccc00

2. CommonJS
var thing = require(“thing”);

3. “For a unit test to operate on a truly isolated unit
of the application, we need to mock every
module except the on we are testing.”
– facebook.github.io/react/blog/2014/09/24/testing-flux-applications.html

4. Classicist vs Mockist
State vs Behaviour

5. Doubles
Dummy, Fake, Stub & Mock

6. “Dummy objects are passed around but never
actually used. Usually they are just used to fill
parameter lists.”
–Martin Fowler [Mocks Aren’t Stubs]

7. “Fake Objects actually have working
implementations but usually take some shortcut
which makes them not suitable for production
(i.e.: In memory database)”
–Martin Fowler [Mocks Aren’t Stubs]

8. “Stubs provide canned answers to calls made
during the test, usually not responding at all to
anything outside what’s programmed in for the
test.”
–Martin Fowler [Mocks Aren’t Stubs]

9. “Mocks objects pre-programmed with
expectations which form a specification of the
calls they are expected to receive.”
–Martin Fowler [Mocks Aren’t Stubs]

10. Stub External State

11. Mock External
Behaviour

12. To Mock Something Properly
You Need Intimate Knowledge
of How It Works

13. Jest

14. Jest
• Developed and used internally at Facebook
• Runs tests in parallel
• Mock by default
• Jasmine style syntax (describe, it, beforeEach)
• Automatically finds tests to execute
• Can test asynchronous thing synchronously
• Fake DOM available via jsdom
• Support for preprocessors (jsx, es6, coffee)

15. Installing & Using Jest
• $ npm install --save-dev jest-cli
• “scripts”:{ “test”: “./node_modules/.bin/jest” }
• put tests in any `__tests__` directory
• $ npm test

16. Tacos App

17. Learn More
• facebook.github.io/jest
• facebook.github.io/jest/docs/api.html
• martinfowler.com/articles/mocksArentStubs.html
• facebook.github.io/react/blog/2014/09/24/testing-flux-applications.
html
• Taco App Repo: github.com/orodio/melbjs-mock

18. CommonJS &
Mocking with Jest
James Hunter – @cccc00