Frontend Best Practices

Technical Standards

• Language: TypeScript with strict mode enabled
• Code Style: ESLint + Prettier, consistent with project config
• Type Safety: No any types without explicit justification
• Testing: Vitest or Jest with Testing Library, minimum 80% coverage
• Accessibility: axe-core automated checks, manual keyboard testing
• Performance: Lighthouse CI, bundle size budgets
• Documentation: One-line JSDoc for every public function; expand only for complex logic

Best Practices

• Semantic HTML elements over generic divs
• CSS custom properties for theming
• Component composition over prop drilling
• Lazy loading for code splitting
• Optimistic UI updates where appropriate
• Proper error boundaries and fallback UI
• Accessible forms with proper labels, error messages, focus management
• Progressive enhancement

Common Patterns

• State Management: React Context for simple state, Zustand/Jotai for complex; Redux only when justified
• Data Fetching: TanStack Query or SWR for server state, avoid raw useEffect
• Forms: React Hook Form or Formik with Zod/Yup schema validation
• Routing: Framework router (Next.js App Router, React Router, Vue Router)
• Styling: CSS Modules, Tailwind CSS, or styled-components — consistent with project
• Testing: Testing Library (query by role/label, not test IDs), MSW for API mocking
• Error Handling: Error boundaries per route/feature, toast for recoverable errors

Common Pitfalls

• Don't use any — use unknown and narrow, or define proper types
• Don't mutate state directly — return new references
• Don't fetch in useEffect without cleanup — use a data fetching library
• Don't skip key props on lists or use array index as key for dynamic lists
• Don't inline object/function literals in JSX props — causes re-renders
• Don't ignore useEffect dependency arrays or suppress the lint rule
• Don't use dangerouslySetInnerHTML without sanitization (XSS risk)
• Don't store derived state — compute during render
• Don't forget loading, error, and empty states in data-driven components

Package.json

• Use peerDependencies for shared framework deps in libraries
• Keep devDependencies vs dependencies accurate
• Audit with npm audit or pnpm audit before releases

Design Quality

For high-fidelity UI work, invoke the frontend-design:frontend-design skill for design quality guidance.

Code Quality Tools

• Linting: ESLint with TypeScript plugin
• Formatting: Prettier
• Type Checking: tsc --noEmit
• Testing: vitest run --coverage
• Accessibility: eslint-plugin-jsx-a11y, axe-core
• Bundle Analysis: vite-plugin-visualizer or webpack-bundle-analyzer

Code Review Checklist

• TypeScript strict mode compliance, no unjustified any
• Component composition and prop management
• Accessibility: ARIA attributes, keyboard navigation, semantic HTML
• CSS/styling consistency and maintainability
• State management patterns appropriate to scope
• DRY compliance: duplicated components, repeated logic
• Naming clarity: components, hooks, utilities, types
• Performance: unnecessary re-renders, missing memoization, bundle size
• Test quality: component tests, user interaction tests, proper mocking
• Code brevity: flag code that can be expressed in fewer lines without losing clarity

Use FE-NNN prefix for all findings.