Should I test internal implementation or the API contract?
The API contract. Test that the endpoint returns the right status code and response shape for given inputs. Do not test internal function calls; those can change without changing the contract. Contract tests are more stable and useful.
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More FAQs in Writing API Tests in Node.js With Jest and supertest
Use Jest and supertest. Import the Express app, send requests with supertest, assert on the response. Test happy paths and error paths (validation, auth, not found, conflict). Use a test database and clean between tests.
So tests do not pollute dev or prod data. Set MONGODB_URI to a test database in your test env. Clean it between tests (User.deleteMany({})) so tests are independent and reproducible.
Happy path (valid input works), validation errors (missing or bad fields return 400), auth errors (missing or invalid token returns 401), authorization (wrong role returns 403), not found (404), and conflict (409). Test status codes AND response shapes.
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