TDD Best Practices

Use this skill when the main question is how to drive implementation from tests, how to structure a test suite, or how to recover discipline in a codebase where tests were written after the fact.

Working Style

1. Write one failing test, then make it pass with the minimum code needed.
2. Refactor only when all tests are green.
3. Test behaviors, not implementations — the public API, not private methods.
4. Keep tests as simple as the production code they verify.
5. A test suite that is hard to change is as expensive as production code that is hard to change.

The Red-Green-Refactor Cycle

RED    → Write a failing test that describes the next behavior.
GREEN  → Write the minimum code to make it pass.
REFACTOR → Clean up both the code and the test — no new behavior.

The discipline is in the order. Never refactor on Red. On Green, add only the behavior demanded by the current failing test.

The 3 Laws (Uncle Bob)

1. You may not write production code unless you have a failing unit test.
2. You may not write more of a unit test than is sufficient to fail. An intentionally missing public API/type can be a valid first Red; unrelated compilation or setup failures are not.
3. You may not write more production code than is sufficient to make the currently failing test pass.

Design Workflow

1. Discover the test command and describe the behavior — use the repository's tooling and name the next observable outcome.
2. Write a failing test — expose the intended assertion or missing new capability, rather than an unrelated setup error. A regression test must reproduce the existing defect.
3. Make it pass — take the simplest path; you can clean up after.
4. Refactor — remove duplication, improve names, reduce complexity.
5. Repeat — the next test should be the smallest step forward.

Choosing Test Granularity

Start with units for logic-heavy code. Start with acceptance tests when following outside-in TDD. Integration tests fill the seams.

Heuristics

When to mock

Mock or fake I/O boundaries in unit/application tests. Use real disposable infrastructure in adapter integration tests where SQL, collation, constraints, search analyzers, transactions, or protocol behavior are the subject.

When testing an Aggregate

Use real Value Objects and child Entities. Test commands through the root, including the rule that a rejected command leaves both state and pending events unchanged.

When testing Domain Events

Assert exact facts after Act: type/name, aggregate identity, business payload, count, and controlled metadata. Test subscriber handling separately from real-bus registration and prove durable duplicate/retry behavior with integration tests.

When testing a Value Object

Test the applicable public contract: semantic equality with distinct instances, exact invariant boundaries, and any exposed normalization, hashing, defensive-copy, operation, or serialization behavior. Inject clocks or policies instead of reading ambient context.

When a test is too big

Large setup is a signal to review fixture clarity and test scope, not proof that the test is too big. Split when multiple behaviors or failure reasons are coupled.

When tests break on every refactor

Tests are coupled to implementation, not behavior. Move the assertion to the public API surface.

When you can't write a test first

Treat difficulty writing a test first as a design or boundary signal; legacy constraints, framework coupling, or missing seams may require characterization tests before redesign.

Warning Signs

• Tests that only pass in a specific execution order.
• Mocks that reproduce the production logic (over-mocking).
• Tests with no assertion (assertTrue(true)).
• Tests named after methods, not behaviors.
• A test suite that takes more than 10 minutes to run on CI.
• Tests that break when internal details change, not when behavior changes.

References

• Read references/behavioral-feedback.md when selecting a test boundary, checking an independent oracle, or proving a regression.
• Read references/tdd-core-practices.md for Red-Green-Refactor detail, FIRST properties, the test pyramid, triangulation, and baby steps.
• Read references/test-doubles.md for the Meszaros taxonomy (Dummy, Fake, Stub, Spy, Mock), when to use each, and the classicist vs mockist distinction.
• Read references/tdd-schools.md for the London (outside-in/mockist) vs Chicago (inside-out/classicist) schools, BDD, and ATDD.
• Read references/tdd-anti-patterns.md for James Carr's 15 anti-patterns, Ian Cooper's "TDD, Where Did It All Go Wrong" insights, and recovery strategies.
• Read references/tdd-language-examples.md for Red-Green-Refactor walkthroughs in TypeScript, Java, Python, C#, Ruby, and PHP with their respective test frameworks.
• Read references/projection-testing.md for duplicate, ordering, concurrency, checkpoint, schema migration, replay/rebuild, and eventual-consistency tests for read models.
• Read references/migration-testing.md for transformation, dirty-data, idempotent rerun, batch, checkpoint, concurrent-write, reconciliation, cutover, and recovery tests for data migrations.
• Read references/value-object-testing.md for semantic equality, invariant boundaries, deep immutability, normalization, optionality, serialization, and property-based Value Object tests.
• Read references/aggregate-testing.md for invariant-first Aggregate tests, repository contract and adapter tests, including atomic failures, deterministic Mothers, collection equality, transaction propagation, and concurrency integration tests.
• Read references/domain-event-testing.md for Aggregate event assertions, application handoff, subscriber and real-bus tests, Outbox/Inbox delivery tests, Integration Event contracts, and CDC mapping tests.
• Read references/transaction-testing.md for atomic commit/rollback, connection propagation, concurrency, cleanup, commit ambiguity, decorators, and side-effect boundary tests.
• Read references/cache-testing.md for cache hit/miss contracts, invalidation, TTL, key isolation, corruption, failure degradation, stampede, and HTTP privacy tests.
• Read references/domain-error-testing.md for typed failure assertions, Result short-circuit tests, exhaustive boundary mappings, public redaction, unknown 500 behavior, Effect execution, and error-test-double caveats.
• Read references/criteria-testing.md for parser/AST tests, shared query semantics, real-adapter operator/security tests, pagination traversal, joins, nested boolean filters, and false-confidence warnings.

Related Skills

• Use oop-best-practices when a class is hard to test — the design needs improvement first.
• Use refactoring-best-practices when adding tests to untested legacy code (characterization tests).
• Use ddd-best-practices to decide Aggregate boundaries and invariant ownership before testing them.
• Use data-migration-best-practices to design the operational migration workflow and cutover gates before testing it.

Source Influences

This skill is synthesized from:

• Test Driven Development: By Example by Kent Beck
• Growing Object-Oriented Software, Guided by Tests (GOOS) by Steve Freeman & Nat Pryce
• Working Effectively with Legacy Code by Michael Feathers
• xUnit Test Patterns by Gerard Meszaros
• Ian Cooper — "TDD, Where Did It All Go Wrong" (talk, Vimeo)
• James Carr — "TDD Anti-Patterns" (blog)
• Tim Ottinger & Jeff Langr — "Unit Tests Are FIRST" (Pragmatic Bookshelf)
• Dan North — "Introducing BDD"
• CodelyTV Aggregates course (including testing counterexamples)
• CodelyTV Value Objects course (including testing and TypeScript counterexamples)
• CodelyTV Repository Pattern course (including repository-double and integration-test counterexamples)
• CodelyTV Domain Events course (including self-asserting-double and Event Bus coverage counterexamples)
• CodelyTV Domain Modeling Errors course (including Result, Effect, stale-contract, and mock counterexamples)
• CodelyTV Criteria Pattern course (including converter, pagination, injection, and test-coverage counterexamples)
• CodelyTV Infrastructure Design Transactions course (including transaction-scope, shared-connection, deferred-side-effect, and coverage counterexamples)
• CodelyTV Infrastructure Design Cache course (including key-collision, invalidation, ETag, destructive-test, and self-asserting-double counterexamples)
• CodelyTV Four Rules of Simple Design course (including private-method coupling and passing repository-mock counterexamples)