Agent-Skills.md

Agent Skills: Test-Driven Development (TDD)

Enforces Test-Driven Development (Red-Green-Refactor). Use when writing unit tests, implementing TDD, or improving test coverage for any feature. (triggers: **/*.test.ts, **/*.spec.ts, **/*_test.go, **/*Test.java, **/*_test.dart, **/*_spec.rb, tdd, unit test, write test, red green refactor, failing test, test coverage)

UncategorizedID: hoangnguyen0403/agent-skills-standard/common-tdd

Repository

hoangnguyen0403/agent-skills-standard
HoangNguyen0403License: Apache-2.0
362103

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pnpm dlx add-skill https://github.com/HoangNguyen0403/agent-skills-standard/tree/HEAD/.agent/skills/common/common-tdd

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Skill Metadata

Name
common-tdd
Description
"Enforces Test-Driven Development (Red-Green-Refactor). Use when writing unit tests, implementing TDD, or improving test coverage for any feature. (triggers: **/*.test.ts, **/*.spec.ts, **/*_test.go, **/*Test.java, **/*_test.dart, **/*_spec.rb, tdd, unit test, write test, red green refactor, failing test, test coverage)"

Test-Driven Development (TDD)

Priority: P1 (OPERATIONAL)

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST. Code written before the test must be deleted. Start over.

The TDD Cycle

  1. RED: Write a minimal failing test. Verify failure (Expected error, not typo).
  2. GREEN: Write simplest code to pass. Verify pass.
  3. REFACTOR: Clean up code while staying green.

AAA Structure (Mandatory)

Every test must follow Arrange-Act-Assert:

  • Arrange: Set up inputs, stubs, mocks, and expected values.
  • Act: Call the single unit under test.
  • Assert: Verify output and side effects. One logical assertion per test. (See AAA Example for code structure).

Coverage Thresholds

  • Minimum: 80% (Statements, Functions, Lines), 75% (Branches).
  • Target: 90% (Statements, Functions, Lines), 85% (Branches).
  • Configure in test runner config (e.g. jest.config.ts, vitest.config.ts). Coverage below minimum is a build-gate failure.

Test Runner Commands

| Language | Runner | Watch Mode | Coverage | | ------------- | ----------------- | --------------------------- | ---------------------------- | | TypeScript/JS | jest / vitest | vitest --watch | vitest run --coverage | | Go | go test | go test -v ./... -count=1 | go test -cover ./... | | Java | JUnit 5 + Maven | mvn test | mvn verify -P coverage | | Kotlin | JUnit 5 + Kotest | ./gradlew test | ./gradlew jacocoTestReport | | Dart/Flutter | flutter test | flutter test --watch | flutter test --coverage |

Core Principles

  • Watch it Fail: Prove the test works.
  • Minimalism: Don't add features/options beyond current test (YAGNI).
  • Real Over Mock: Prefer real dependencies unless slow/flaky.
  • One Reason to Fail: Test one behavior per test.
  • Descriptive Names: e.g. should_returnError_when_emailIsInvalid.

When to Use Mocks vs Real Dependencies

  • Database: Real DB (test container) or in-memory; mock as last resort.
  • HTTP/External APIs: Always mock.
  • Filesystem: Use temp dir; mock for unit isolation.
  • Time/Dates: Always mock/control.
  • Internal services: Real if fast (<200ms); mock if cross-network.

Verification Checklist

  • [ ] Every new function/method has a failing test first?
  • [ ] Failure message was expected?
  • [ ] Minimal code implemented passed?
  • [ ] AAA structure followed?
  • [ ] Coverage thresholds met?

Expert References

Anti-Patterns

  • No test-after: Writing tests post-implementation defeats TDD. Delete and restart.
  • No assertion-free tests: A test without an assert is not a test.
  • No testing implementation: Test behavior and contracts, not internal calls.