Assuming the audience already knows how to test in Javascript

1. PAVITHRA KODMAD | DEVELOPER
Introduction to Jest

2. Jest is
Javascript
Testing
Framework/
Platform
INTRODUCTION

3. Why Jest?

4. Why Jest?
Sandboxed
Tests
Feedback loop
One stop shop
Community
Sandboxed Tests
• Tests run in parallel
• No shared state between tests

5. Why Jest?
Sandboxed
Tests
Feedback loop
One stop shop
Community
Better Feedback loop
• Rich CLI
• Pretty logging
• Better error outputs
• Failed tests run first
• Coverage

6. Why Jest?
Sandboxed
Tests
Feedback loop
One stop shop
Community
One stop shop

7. Why Jest?
Sandboxed
tests
Feedback loop
One stop shop
Community
Community
Back in 2015 👇

8. Why Jest?
Sandboxed
tests
Feedback loop
One stop shop
Community
Community
Now 👇

9. Why Jest?
Sandboxed
tests
Feedback loop
One stop shop
Community
Community

10. Why Jest?
Sandboxed
tests
Feedback loop
One stop shop
Community
Community
• Atlaskit actively using and developing
tools on Jest
• Jest codemods

11. Anatomy of a test

12. What’s different?
• Globals - describe/expect/test

13. What’s different?
• Matchers

14. What’s different?

15. What’s different?

16. How to mock?

17. How to mock?

18. Mock Hoisting
Hoists mocks to top of file by default
Prevent with doMock

19. Spy behavior
jest.spyOn always called the spied function.

20. Tip : toBe() vs toEqual()

21. Tip : jest.resetModules
Call to clear any pending state in module cache between tests

22. Tip : mockImplementationOnce()

23. Tip : expect.anything()

24. Tip : expect.extend()

25. Tip : mockFn.mock.instances
Useful to get the source mockImplementation

26. Tip : jest.isMockFunction()
Useful to check!

27. Tip : coverage and coverage thresholds

28. Tip : Snapshot Testing
Can take snapshots of
• Components
• Json
• String
• Anything serializable

29. Tip : —runInBand
Jest always run in parallel
• Makes it run on a single thread
• Easier to debug

30. Tip : Vscode and Jest

31. Tip :Async testing
Very similar to what we had with Mocha and Chai

32. Cheers to “Painless” testing

33. References
• Jest Documentation - https://github.com/kalcifer/awesome-jest
• Jest as a Platform - https://github.com/rogeliog/jest-as-a-platform-talk/blob/maste
Jest%20as%20a%20Platform.pdf
• Jest historically - http://rea.tech/painless-javascript-testing-surely-you-jest/
• Jest codemods - https://github.com/skovhus/jest-codemods
• Chai and Jest Matchers - https://medium.com/@RubenOostinga/combining-chai-
matchers-d12d1ffd0303
• VsCode Jest - https://github.com/orta/vscode-jest
• Jest codemods and Sinon - https://github.com/skovhus/jest-codemods/issues/68
• Manjula Dube for this slide - https://twitter.com/manjula_dube/status/9402921548

34. Thank you