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Zhongwei Li
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skills/practicing-tdd-with-jest/SKILL.md
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skills/practicing-tdd-with-jest/SKILL.md
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name: practicing-tdd-with-jest
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description: Implements features following Test-Driven Development RED/GREEN/REFACTOR cycle with Jest. Use when adding new features, fixing bugs, or refactoring existing code
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allowed-tools: Read, Write, Grep, Glob, Bash
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---
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# Test-Driven Development avec Jest
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Implémente code en suivant cycle TDD RED/GREEN/REFACTOR avec Jest.
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## Table des matières
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- [Quand utiliser](#quand-utiliser)
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- [Cycle TDD : RED → GREEN → REFACTOR](#cycle-tdd--red--green--refactor)
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- [Scripts npm](#scripts-npm)
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- [Structure tests (Arrange-Act-Assert)](#structure-tests-arrange-act-assert)
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- [Assertions Jest essentielles](#assertions-jest-essentielles)
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- [Mocking](#mocking)
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- [Tests spécialisés](#tests-spécialisés)
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- [Couverture](#couverture)
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- [Bonnes pratiques](#bonnes-pratiques)
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- [Workflow](#workflow)
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- [Checklist](#checklist)
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- [En cas de doute](#en-cas-de-doute)
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## Quand utiliser
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- Implémentation nouvelles features
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- Correction bugs avec reproduction
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- Refactoring code existant
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- Ajout comportements modules
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- Validation logique complexe
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## Cycle TDD : RED → GREEN → REFACTOR
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**RED (Test échoue) :**
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1. Identifier comportement
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2. Écrire test AVANT code
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3. Test DOIT échouer ❌
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**GREEN (Test passe) :**
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1. Code minimum possible
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2. Pas d'optimisation/généralisation
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3. Faire passer test uniquement ✅
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**REFACTOR (Améliorer) :**
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1. Nettoyer code (noms, structure)
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2. Éliminer duplications
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3. Vérifier tests passent toujours ✅
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## Scripts npm
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- `npm test` - Tous tests
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- `npm run test:watch` - Mode watch
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- `npm run test:coverage` - Avec couverture
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## Structure tests (Arrange-Act-Assert)
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**AAA Pattern :**
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- **Arrange :** Préparer données
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- **Act :** Exécuter action
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- **Assert :** Vérifier résultat
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**Grouping :**
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- `describe()` - Grouper tests liés
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- `test()` / `it()` - Tests individuels
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- `beforeEach()` / `afterEach()` - Setup/cleanup
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**Nommage :** Descriptif ("should do X when Y", "handles edge case Z")
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## Assertions Jest essentielles
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- Égalité: `toBe()` (strict), `toEqual()` (profonde)
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- Booléens: `toBeTruthy()`, `toBeFalsy()`, `toBeNull()`, `toBeUndefined()`
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- Nombres: `toBeGreaterThan()`, `toBeLessThan()`, `toBeCloseTo()`
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- Arrays: `toContain()`, `toHaveLength()`
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- Exceptions: `toThrow()`, `toThrow(ErrorClass)`
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Voir documentation Jest pour liste complète.
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## Mocking
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**Fonctions :**
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- `jest.fn()` - Mock simple
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- `mockReturnValue()` - Valeur
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- `mockResolvedValue()` / `mockRejectedValue()` - Promesses
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- `toHaveBeenCalled()`, `toHaveBeenCalledWith(args)`, `toHaveBeenCalledTimes(n)`
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**Modules :** Isoler code testé, éviter dépendances externes (API, localStorage)
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## Tests spécialisés
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**Async :** `async/await` recommandé, ou `expect().resolves/rejects`
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**DOM/UI :** jsdom simule DOM - créer, attacher, exécuter, vérifier, cleanup
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**Canvas (jeux arcade) :** Mock context, mock performance.now(), tester logique séparément rendu, tests d'état
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## Couverture
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**Cibles :** Branches > 80%, Functions > 80%, Lines > 80%, Statements > 80%
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Lance `npm run test:coverage`, examine rapport.
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## Bonnes pratiques
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**Do's :**
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- Test AVANT code TOUJOURS
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- Un test à la fois (RED → GREEN → REFACTOR)
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- Tests descriptifs (noms clairs)
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- Isolés, indépendants
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- Edge cases (null, undefined, limites)
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- Rapides (< 100ms idéal)
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**Don'ts :**
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- Pas de code avant test
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- Pas de tests trop larges
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- Pas de tests fragiles (timing, random)
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- Pas de RED ignoré (test inutile)
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- Pas de .skip tests
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## Workflow
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1. **Requirement :** Cas normaux + edge cases
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2. **Test RED :** Doit échouer
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3. **Code GREEN :** Minimum pour passer
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4. **REFACTOR :** Améliorer, tests verts
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5. **Répéter :** Couverture > 80%
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## Checklist
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- [ ] Test écrit AVANT code
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- [ ] Test échoue (RED)
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- [ ] Code minimum
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- [ ] Test passe (GREEN)
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- [ ] Refactorisé si besoin
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- [ ] Noms descriptifs
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- [ ] AAA pattern
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- [ ] Mocks isolent
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- [ ] Edge cases
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- [ ] Couverture > 80%
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- [ ] Tests rapides
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## En cas de doute
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**Règles absolues :**
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1. Cycle RED → GREEN → REFACTOR OBLIGATOIRE
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2. Test doit échouer d'abord (sinon inutile)
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3. Code minimum (pas sur-engineering)
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4. Refactor après GREEN seulement
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5. Tests restent verts après refactor
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6. Couverture > 80%
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**Références :**
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- Tests existants patterns
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- jest.config.cjs - Configuration
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- npm run test:coverage - État actual
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