How To Write Great Unit Tests

How do you learn to write great unit tests? Why are some unit tests good and some bad? Here I talk about 7 tips for writing great unit tests and gaining confidence in your code.

1. www.marclittlemore.com MARC LITTLEMORE 7 TIPS FOR WRITING GREAT UNIT TESTS www.marclittlemore.com @marclittlemore github.com/marcl
2. WHAT IS A UNIT?
3. A unit is the smallest component that it makes sense to test
4. Class Single method Function
5. Your team can decide what makes sense to be your unit under test
6. WHY DO WE NEED UNIT TESTS?
7. Unit tests are extremely important to your code quality
8. WHY DO WE NEED UNIT TESTS? DEVELOPER UNDERSTANDING Helps to express the expectations of the code
9. WHY DO WE NEED UNIT TESTS? DOCUMENTS FEATURES Tells future-you why the code is written
10. WHY DO WE NEED UNIT TESTS? AIDS IN REFACTOR If tests pass then developer refactor has not changed functionality
11. WHY DO WE NEED UNIT TESTS? HELPS WITH DESIGN Define your API
12. WHY DO WE NEED UNIT TESTS? QUALITY ASSURANCE Helps the team to ship quality code
13. Unit tests should be the first line of defence for your project
14. 7 TIPS FOR WRITING GREAT UNIT TESTS
15. MAKE TEST NAMES UNDERSTANDABLE 1.
16. Test names should be human readable with a clear description
17. Assume you’ll have forgotten what the test does by tomorrow!
18. Write test names for your fellow developers to understand
19. Unit tests should become living documentation for features in your project
20. BAD TEST NAMES
21. BAD TEST NAMES “multiply money by 1.2” “adds 30 to balance” “is username null”
22. Name your tests with the why and not the how
23. BETTER TEST NAMES
24. GOOD TEST NAMES “add expected tax to purchase price” “should increase balance when deposit is made” “should set username to null when user is not signed in”
25. ISOLATION OF DEPENDENCIES 2.
26. Unit tests should be isolated from external influences
27. Unit tests should test only the code that you’ve written
28. Replace production dependencies
29. Stub or mock external services
30. STUB OR MOCK
31. STUB: CALLS TO SERVICES 3rd party APIs e.g. Twitter Calls to own services
32. STUB: SERIALISATION Avoid serialising data Stub data stores Return fake objects
33. STUB: STATE Initialise pre-conditions by mocking or stubbing data in test setup
34. ONE REASON TO FAIL 3.
35. Try to avoid multiple expectations in tests
36. Give only one reason to fail per test
37. Makes the tests much easier to reason about
38. MAKE TESTS FAST 4.
39. Unit tests should execute as quickly as possible
40. Unit tests should be small in scope by stubbing dependencies
41. Unit tests should run in seconds not minutes
42. INDEPENDENT OF ENVIRONMENT 5.
43. Avoid dependencies on specific machines or environments
44. Avoid “...well it works on my machine...”
45. Stub initial state Mock external data Avoid environment dependencies
46. Each test should be independent
47. Each test should run in isolation
48. Don’t rely on results of previous tests
49. Each test should have no side effects e.g. adding to database
50. The team will lose faith if your unit tests begin to fail
51. TEST UNITS COMPLETELY 6.
52. Some unit tests are better than none
53. Test the happy paths
54. But also test the unhappy paths
55. And also test the edge cases
56. Try to test all the things!
57. FOUND A NEW BUG?
58. ADD A NEW UNIT TEST!
59. MAKE TESTING EASY 7.
60. Make unit testing really easy!
61. RUN TESTS From the terminal From the task runner Automatically
62. ADD GIT HOOKS Run tests before code can be pushed to remote repositories
63. ADD TO CI SERVERS Don’t deploy code until all tests are passing
64. ASK THESE QUESTIONS OF YOUR UNIT TESTS
65. WHY DO WE NEED UNIT TESTS? 1. What is being tested? 2. What should the code do?
66. WHY DO WE NEED UNIT TESTS? 3. What is the actual output? 4. What is the expected output?
67. 7 TIPS FOR WRITING GREAT UNIT TESTS 1. MAKE TEST NAMES UNDERSTANDABLE 2. ISOLATION OF DEPENDENCIES 3. ONE REASON TO FAIL 4. MAKE TESTS FAST 5. INDEPENDENT OF ENVIRONMENT 6. TEST UNITS COMPLETELY 7. MAKE TESTING EASY
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