tdd-typescript

# TDD TypeScript Skill TypeScript/JavaScript TDD using strict red-green-refactor with Vitest — sharpened by **DDD at Red** (model the domain with branded types) and **DRY at Refactor** (deduplicate same-concept repetition only). ## Description Implement features by writing failing tests first, minimal code to pass, then refactoring. Types *are* the first test; branded types extend that to catch domain errors the compiler wouldn't otherwise see. ## Discipline Layering - **Before Red — Model the Domain**: Name the entity, value object, or aggregate in the business's language. Use **branded types** for primitives that carry rules (`type EmailAddress = string & { readonly __brand: 'EmailAddress' }`). Prefer `readonly` classes or frozen records for multi-field value objects. Zod/Valibot schemas inferred into branded types give runtime + compile-time safety. - **Red**: Write a failing test. Name it in domain terms (`it('rejects refund when amount exceeds daily limit')`) — not mechanical (`it('returns false')`). - **Green**: Minimal code to pass. - **Refactor — DRY with discipline**: Deduplicate only when the *same domain concept* appears ≥3 times with the same meaning. Distinguish true DRY from accidental similarity (same shape, different meaning). Promote recurring primitives to branded types. Keep domain logic out of components, routes, and stores. ## What's Included - **Examples**: TDD cycles, React component TDD, branded-type value objects, utility function TDD - **Reference**: Red-green-refactor patterns, Vitest best practices, branded-type idioms - **Templates**: Test templates, implementation workflows ## Use When - Implementing TypeScript features with TDD - Modeling domain concepts (entities, value objects, aggregates) with branded types - React component development (keeping domain logic out of components) - Refactoring duplication after a green test - Ensuring high test coverage ## Related Agents - `tdd-typescript-implementer` **Skill Version**: 1.1