dsh-skills/skills/test-driven-development/SKILL.md

name: test-driven-development description: Use when building new features, refactoring existing modules, or fixing bugs using Test-Driven Development (TDD: Red-Green-Refactor cycle).

Test-Driven Development (TDD)

Purpose

Build robust, well-designed, and thoroughly tested software by writing automated tests before writing production code.

The Red-Green-Refactor Cycle

1. 🔴 RED — Write a Failing Test

• Define the desired behavior from the caller's perspective.
• Write a focused test asserting that behavior.
• Run the test and verify that it fails for the expected reason (e.g. missing method or assertion mismatch, not a syntax/import error).

2. 🟢 GREEN — Write the Minimal Implementation

• Write just enough code to make the failing test pass.
• Resist the temptation to implement speculative future features or premature optimizations.
• Run the test suite and verify that all tests pass.

3. 🔵 REFACTOR — Clean Up & Optimize

• Eliminate duplication and code smells.
• Improve naming, types, and module structure.
• Ensure all invariants hold while keeping the test suite green at all times.

Best Testing Practices

• Test Behavior, Not Implementation Details: Avoid asserting private state or internal methods. Test through public interfaces.
• Fast and Deterministic: Unit tests should run in milliseconds without relying on real network calls or unstable timing delays.
• AAA Pattern: Arrange (setup data), Act (call function/method), Assert (check outcome and side-effects).