msd-core/get-shit-done/references/tdd.md

TDD is about design quality, not coverage metrics. The red-green-refactor cycle forces you to think about behavior before implementation, producing cleaner interfaces and more testable code.

Principle: If you can describe the behavior as expect(fn(input)).toBe(output) before writing fn, TDD improves the result.

## When TDD Improves Quality

TDD candidates (add tdd="true", include <test-first>):

• Business logic with defined inputs/outputs
• API endpoints with request/response contracts
• Data transformations, parsing, formatting
• Validation rules and constraints
• Algorithms with testable behavior
• State machines and workflows
• Utility functions with clear specifications

Skip TDD (standard type="auto"):

• UI layout, styling, visual components
• Configuration changes
• Glue code connecting existing components
• One-off scripts and migrations
• Simple CRUD with no business logic
• Exploratory prototyping

Heuristic: Can you write expect(fn(input)).toBe(output) before writing fn? → Yes: TDD will help → No: Implement first, add tests after if needed

<execution_flow>

Red-Green-Refactor Cycle

RED - Write failing test:

1. Create test file following project conventions
2. Write test describing expected behavior (from <test-first> element)
3. Run test - it MUST fail
4. If test passes: feature exists or test is wrong. Investigate.
5. Commit: test: add failing test for [feature]

GREEN - Implement to pass:

1. Write minimal code to make test pass
2. No cleverness, no optimization - just make it work
3. Run test - it MUST pass
4. Commit: feat: implement [feature]

REFACTOR (if needed):

1. Clean up implementation if obvious improvements exist
2. Run tests - MUST still pass
3. Only commit if changes made: refactor: clean up [feature]

Atomic commits: Each TDD task produces 2-3 commits following this pattern. </execution_flow>

<test_quality>

Good Tests vs Bad Tests

Test behavior, not implementation:

• Good: "returns formatted date string"
• Bad: "calls formatDate helper with correct params"
• Tests should survive refactors

One concept per test:

• Good: Separate tests for valid input, empty input, malformed input
• Bad: Single test checking all edge cases with multiple assertions

Descriptive names:

• Good: "should reject empty email", "returns null for invalid ID"
• Bad: "test1", "handles error", "works correctly"

No implementation details:

• Good: Test public API, observable behavior
• Bad: Mock internals, test private methods, assert on internal state </test_quality>

<framework_setup>

Test Framework Setup (If None Exists)

When a TDD task exists but no test framework is configured:

1. Detect project type:

# JavaScript/TypeScript
if [ -f package.json ]; then echo "node"; fi

# Python
if [ -f requirements.txt ] || [ -f pyproject.toml ]; then echo "python"; fi

# Go
if [ -f go.mod ]; then echo "go"; fi

# Rust
if [ -f Cargo.toml ]; then echo "rust"; fi

2. Install minimal framework:

Project	Framework	Install
Node.js	Jest	npm install -D jest @types/jest ts-jest
Node.js (Vite)	Vitest	npm install -D vitest
Python	pytest	pip install pytest
Go	testing	Built-in
Rust	cargo test	Built-in

3. Create config if needed:

• Jest: jest.config.js with ts-jest preset
• Vitest: vitest.config.ts with test globals
• pytest: pytest.ini or pyproject.toml section

4. Verify setup:

# Run empty test suite - should pass with 0 tests
npm test  # Node
pytest    # Python
go test ./...  # Go
cargo test    # Rust

5. Create first test file: Follow project conventions for test location:

• *.test.ts / *.spec.ts next to source
• __tests__/ directory
• tests/ directory at root </framework_setup>

<error_handling>

Error Handling

Test doesn't fail in RED phase:

• Feature may already exist - investigate
• Test may be wrong (not testing what you think)
• Fix before proceeding

Test doesn't pass in GREEN phase:

• Debug implementation
• Don't skip to next task
• Keep iterating until green

Tests fail in REFACTOR phase:

• Undo refactor
• Commit was premature
• Refactor in smaller steps

Unrelated tests break:

• Stop and investigate
• May indicate coupling issue
• Fix before proceeding </error_handling>

<commit_pattern>

Commit Pattern for TDD Tasks

Each TDD task produces atomic commits:

test: add failing test for email validation

feat: implement email validation

refactor: extract regex to constant (optional)

This pattern:

• Creates clean git history
• Each commit is independently revertable
• Shows TDD discipline in commit log </commit_pattern>