Tdd Rubric

TDD Rubric — Three Checks

Answer each one. If any answer is wrong, fix it before moving on.

1. Stub or Mock?

• Stub = incoming dep (data flowing in). Never assert on it.
• Mock = outgoing dep / exit point. Assert on it. Max one per test.

Wrong choice here means you're either asserting on the wrong thing or missing the real exit point entirely.

~2–5% of tests need mocks. If you're reaching for a mock, ask first: can I test this via return value or state change instead?

Anti-patterns to catch now:

• Asserting on a stub — it's not an exit point
• Verifying internal calls between things you own — only assert on calls to deps you don't control
• Over-specifying — exact object schemas, exact argument order unless order is a stated requirement; these break on refactoring even when behavior is correct
• Hardcoded dep (e.g. new Date() inline, moment() inline) — inject it or the test is flaky by design

Jest:

jest.mock("./module-name")          // hoisted before imports
mockFn.mockReturnValue(value)       // stub indirect input
expect(mock).toHaveBeenCalledWith() // assert exit point
afterEach(jest.resetAllMocks)       // prevent state leaking between tests

2. Is this async? Split it.

Don't write async tests against logic you can test synchronously.

Split into:

• Async shell — one integration test for the fetch/IO
• Sync logic core — many unit tests for the processing; extract to a plain function, test it directly

If you can't avoid async:

• Timers → jest.useFakeTimers() + jest.advanceTimersToNextTimer(), not global.setTimeout monkey-patching
• Callbacks → extract success/error logic to named public fns, call via done()
• Await → move logic into plain fns that return/throw; the test stays synchronous

3. Should this test change?

Before touching an existing test, identify why it's failing:

Logic in tests (if, for, try-catch, string concatenation) creates lying tests — a bug in the algorithm makes the expected value wrong in exactly the same way. Always hardcode expected values.