This document contains advice from Marcin Dryka on test-driven development (TDD) and best practices for writing unit tests. Some of the key points made include: 1. TDD is a design process, not just a testing process. Tests should describe desired functionality before writing code. 2. Tests should have single, well-defined purposes and only test one interaction at a time. Avoid complex logic in tests. 3. Mock external dependencies but not the system under test. Tests should be deterministic and independent from each other.

1. Marcin Dryka
@brainhub 2017

2. in the wild

4. tdd
father of

6. bdd
father of

8. ruby on rails
father of

9. What gives you
tdd?

10. TDD is a
design process,
not a testing process

11. title it1
2 design it
describe it3

12. title it

13. describe('Links API', () => {
it('returns proper code on duplicate create', () => {
/* ... */
});
});

14. do not use
truisms

15. Given When Then
design it

16. it('returns proper code on duplicate create', () =>
request(app)
.post('/links')
.set('Accept', 'application/json')
.send(link)
.then(() => request(app)
.post('/links')
.set('Accept', 'application/json')
.send(link)
)
.then(({text}) => request(text)
.get('')
.expect(`Found. Redirecting to ${link.url}`)
)
);

17. Single test should invoke
only one interaction

19. it('creates links', () => {
const model = Links(arango);
const link1 = {
profileId: 'profile-1',
id: 'a-custom-id',
url: 'https://google.com',
};
const link2 = {
profileId: 'profile-2',
id: 'a-custom-id',
url: 'https://google.com',
};
return model.create(link1)
.then(() => model.create(link2))
.then(() => model.findAll())
.then(result => {
expect(result).to.have.length(1);
});
});

20. Describe
failing reason

23. it('sends hook', () => {
const {promise, postedData, profile} = prepare();
return promise
.then(() => {
expect(postedData).to.deep.equal([{profile}]);
});
});

24. Keep the testing co
compact & readab

25. it('mocks external express library', () => {
const res = {
status: sinon.spy(),
};
NotFoundHandler(res);
expect(res.status.withArgs(404)).to.be.calledOnce();
});

26. mock
own
do not
types you don't

27. Don't comment
tests

28. it('should properly encode strings', () => {
let result;
const testValues = {
'Marcin': 'nicraM',
'Dryka!': 'akyrD!',
'Brainhub?!': 'buhniarB!?'
};
for (result in testValues) {
expect(encodeString(testValues[result])).toEqual(result);
}
});

29. Avoid logic
in your tests

30. it('introduces user', () => {
const user = new User();
user.loginFacebook = () => {
return {
firstname: "Marcin",
lastname: "Dryka",
}
};
user.login('facebook');
return expect(user.getFullname())
.to.be.equal('Marcin Dryka');
});

31. Don’t mock
SUT

32. it('creates user', () => {
const user = new User({id: 1});
return repository.persist(user).to.be.fulfilled();
});
it('removes user', () => {
const user = repository.find(1);
return repository.remove(user).to.be.fulfilled();
});

33. Unit tests should be
deterministic
&
independent

34. it('returns objects list, total count, page and limit', () => {
/**
* ...
*/
return profiles.findAllWithCount({ page, limit })
.then((result) => {
const items = result.items;
expect(items).to.be.an('array');
expect(items).to.have.length(limit);
expect(items.map(profile => profile.id)).to.containSubset([
'data 2',
'data 3',
]);
items.map(profile => expect(profile).to.contain.all.keys(schemaKeys));
const EXPECTED_AMOUNT = 6;
expect(result.total).to.equal(EXPECTED_AMOUNT);
expect(result.page).to.equal(page);
expect(result.limit).to.equal(limit);
expect(result.pagesCount).to.equal(expectedPagesCount);
});
});

35. In the same test don’t test
multiple concerns

36. it('provides user's email', () => {
return facebookUserProvider.get('/me')
.then(user => {
expect(user.email).to.match(EMAIL_PATTERN)
});
});

37. Don’t test
external code

38. function generateRandom() {
if (TEST_MODE) {
return 'constant';
}
return Math.random();
}

39. Don’t mix test code and
production code

40. it('validates valid email', () => {
const email = {
html: '<html/>',
text: 'lorem ipsum',
};
return expect(validate(‚email’,
email)).to.be.fulfilled();
});

41. Don’t unit test
configuration settings

42. code smells

43. #1

44. Write tests

45. #2

46. Write tests

47. #3

48. Refactor!

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Parks, Daniil Silantev, Caleb Martin,
Harm Weustink, Kelly Sikkema, Billy
Pasco, Luis Perdigao, CloudVisual,
Sergey Pesterev, Søren Astrup
Jørgensen, Andrew Rice, James
Fuller, Matthew Smith, Ambitious
Creative Co. - Rick Barrett,
Armando Tobar, Vincent van
Zalinge, Edouard TAMBA, Patti
Black, Jeffrey Hovland, Cris Saur,
Emiel Molenaar, Dominik Scythe,
chuttersnap, Tom Pumford