TDD Skill

Autonomous test-driven development. Write comprehensive tests FIRST, then implement until all tests pass. Prevents "Buggy First-Pass Implementations".

Workflow: Red → Green → Refactor

Phase 1: Understanding (READ)

1. •

   Read existing codebase to understand:

   • •Current architecture and patterns
   • •Existing types and interfaces
   • •Testing patterns already in use
2. •

   Identify edge cases that commonly cause bugs:

   • •Empty data / null values
   • •Boundary conditions (0, negative, max values)
   • •Sorting bias (alphabetical, temporal)
   • •Normalization edge cases (division by zero, all same values)
   • •Missing configuration (.env variables)
   • •Type mismatches (string vs number, undefined fields)

Phase 2: Test Design (RED)

BEFORE writing ANY implementation code:

1. •

   Write comprehensive tests covering:

   • •Happy path (expected usage)
   • •Edge cases (empty, null, boundary)
   • •Error conditions (invalid input, missing config)
   • •Integration points (API calls, data flow)
2. •

   Run tests — they should ALL FAIL:

   bash

   npx vitest run <test-file>
   # or
   pytest <test-file> -v
   

3. •

   Verify tests fail for the right reasons:

   • •✓ Function not implemented yet → correct
   • •✗ Test itself is broken → fix the test

Phase 3: Implementation (GREEN)

1. •

   Implement the feature — minimal code to make tests pass

2. •

   Run tests iteratively after each change until ALL pass

3. •

   Verify no regressions — run full test suite:

   bash

   npx vitest run
   

Phase 4: Verify

1. •Build verification:
   bash

   npm run build
   npm run typecheck
   

Phase 5: Report

1. •Provide summary:
   code

   ✓ RED: Wrote 8 tests (edge cases, normalization, sorting bias)
   ✓ GREEN: All 8 tests passing
   ✓ Build passed, no regressions
   Files: src/utils/score.ts (+45), test/score.test.ts (+85, new)
   

Common Edge Cases to Always Test

Scoring/Ranking: empty dataset, all same values, single item, division by zero Field References: wrong field name, undefined/null, type mismatches Sorting: alphabetical bias, temporal bias, tie-breaking Configuration: missing .env, dotenv not loaded, API keys unset

Important Rules

• •NEVER write implementation before tests — core TDD principle
• •NEVER skip edge case tests — these catch the most common bugs
• •NEVER ask clarifying questions — make assumptions and document them
• •DO iterate until 100% pass — partial success is not done
• •DO run full test suite — catch regressions