Test doubles ni vitu vinavyoweza kusimama nafasi ya utegemezi halisi katika majaribio — ikiwa ni pamoja na mocks, stubs, spies, fakes, na dummies. Zinatenganisha msimbo unaojaribiwa kutoka kwa utegemezi wake (hifadhi za data, APIs, huduma), na kuwezesha majaribio ya haraka, yenye kuzingatia, na yenye udhibiti wa kitengo.
To test a unit in ISOLATION, replace its real dependencies (DB, API, services) with
TEST DOUBLES (fake stand-ins):
→ isolate the unit (test ONLY it, not its dependencies)
→ FAST (no real DB/network); CONTROLLED (set up exact scenarios, including errors);
DETERMINISTIC (no external flakiness)
STUB → provides canned RESPONSES to calls (returns preset data):
→ "when getUser is called, return this fake user" — controls inputs to the unit
MOCK → a stub that also VERIFIES interactions (was it called? how? with what?):
→ "verify save() was called once with these arguments" — checks behavior/interactions
SPY → wraps a real (or fake) object and RECORDS calls (verify how it was used)
FAKE → a working but simplified implementation (e.g. an in-memory database)
DUMMY → a placeholder passed but not actually used
// stub: control what a dependency returns
const userService = { getUser: jest.fn().mockReturnValue({ name: 'Ann' }) };
// mock verification: check an interaction happened
expect(emailService.send).toHaveBeenCalledWith('[email protected]');
STUB → controls INPUTS (provides data the unit needs) — "state" verification
MOCK → verifies OUTPUTS/interactions (was the dependency called correctly?) — "behavior"
verification
→ Use stubs to provide data; mocks to verify the unit called a dependency correctly.
⚠️ Over-mocking → tests coupled to implementation (brittle); mock at the right boundaries.
Kuelewa test doubles (mocks, stubs, nk.) ni muhimu kwa sababu ni muhimu kwa kuandika majaribio ya kitengo yenye kutengana, kwa hivyo ni maarifa ya kujaribisha yenye thamani, yanayotumiwa mara kwa mara.
Kujaribisha kitengo kipekee (lengo la kujaribisha kitengo), lazima ubadilishe utegemezi wake halisi (hifadhi za data, APIs, huduma) na test doubles — kuwezesha majaribio ambayo ni mabilis (hakuna I/O halisi), yenye udhibiti (kuweka hali mahususi ikiwa ni pamoja na hali ya hitilafu ambayo ni ngumu kueneza na utegemezi halisi), na deterministic (hakuna uzushi wa nje).
Kuelewa aina na madhumuni yao ni muhimu: stubs (kutoa majibu yaliyoandaliwa ili kudhibiti kile utegemezi kinarudi — kulisha kitengo data maalum), mocks (stubs ambayo pia inathibitisha mwingiliano — kuangalia kitengo kilijita utegemezi kwa usahihi), spies (kurekodi simu kwenye vitu halisi au bandia), fakes (utekelezaji wa kazi ulioboreshwa kama hifadhi za data za kumbukumbu), na dummies (mahali pa jumla).
Kuelewa tofauti muhimu ya mock-vs-stub — stubs kudhibiti ingizo (uthibitisho wa hali: kulisha data) wakati mocks inathibitisha matokeo/mwingiliano (uthibitisho wa tabia: kuangalia kitengo kilijita utegemezi kwa usahihi) — ni maarifa muhimu ya kimahali ambayo mara nyingi huchanganywa, na kutumiia kila mmoja kwa usahihi ni muhimu.
Kuelewa jukumu la mocking kupita kiasi (mocking kupita kiasi inaungana na majaribio kwa maelezo ya utekelezaji, na kuifanya kuwa nyingi, kwa hivyo unapaswa kumock kwa mipaka sahihi) inaonyesha hamu ya kujaribisha.
Test doubles zinatumiwa kila wakati katika kujaribisha kitengo kutenganisha msimbo unaojaribiwa.
Kwa kuwa kujaribisha kitengo kimetengana kumhitaji ubadilishe utegemezi halisi na test doubles (kwa kasi, udhibiti, na determinism), na kwa kuwa kuelewa aina, tofauti ya mock-vs-stub, na jukumu la mocking kupita kiasi ni muhimu kuandika majaribio mazuri yenye kutengana, kuelewa test doubles ni maarifa ya kujaribisha yenye thamani, yanayotumiwa mara kwa mara — muhimu kwa kujaribisha kitengo, inatumiwa kila wakati kutenganisha msimbo unaojaribiwa, na muhimu kwa kuandika majaribio yenye kuzingatia, yenye udhibiti wakati wa kuzuia hasara ya mocking kupita kiasi, ujuzi wa msingi kwa kujaribisha kwa ufanisi.