Frontend Patterns

When to Apply

• Classification: feature, architecture-change (if touching UI)
• Phase: /implement (component design & build), /review (pattern compliance), /test (UI state coverage)
• Trigger: Task involves creating or modifying UI components, pages, routes, or client-side state

Conventions

Customize after /app-init: Replace these generic conventions with your project's ADR and framework-specific patterns.

Component Structure

• One component per file
• Component file name matches export name (PascalCase)
• Co-locate component-specific styles, tests, and types
• Separate page-level components from reusable UI components

Suggested Directory Pattern

src/
├── components/        # Reusable UI components (Button, Modal, Card)
│   └── <Name>/
│       ├── <Name>.tsx
│       ├── <Name>.test.tsx
│       └── index.ts
├── pages/             # Page-level components (route targets)
│   └── <PageName>/
│       ├── <PageName>.tsx
│       ├── components/ # Page-specific components (not reusable)
│       └── hooks/      # Page-specific hooks
├── hooks/             # Shared custom hooks
├── services/          # API client functions
├── stores/            # State management (if applicable)
├── types/             # Shared TypeScript types
└── utils/             # Pure utility functions

State Management Principles

• Local state first: useState/useReducer for component-specific state
• Lift state up only when sibling components need it
• Global state (store) only for truly app-wide state: auth, theme, user preferences
• Server state (API data) managed by data-fetching library (React Query, SWR, etc.) — NOT in global store
• Never duplicate server state in client store

Data Fetching

• Centralize API calls in services/ directory
• Use data-fetching hooks (React Query, SWR, or equivalent) instead of raw fetch/axios in components
• Handle loading, error, and empty states for every data-dependent component
• Implement optimistic updates for better UX where appropriate

Form Handling

• Use a form library for complex forms (React Hook Form, Formik, or equivalent)
• Validate on client AND server (client validation is UX, server validation is security)
• Show inline errors next to fields, not just at form top
• Disable submit button during submission (prevent double-submit)

Error Handling

• Global error boundary for unexpected crashes
• Per-component error handling for expected failures (API errors)
• User-friendly error messages (never show raw error objects)
• Retry mechanism for transient failures (network errors)

Checklist

During /implement:

• Every data-dependent component handles: loading, error, empty, and success states
• No business logic in components (extract to hooks or services)
• No hardcoded strings (use constants or i18n keys)
• Accessible: semantic HTML, aria labels on interactive elements, keyboard navigation
• Responsive: works on mobile viewport (unless spec explicitly excludes mobile)
• No direct API calls in components (use service layer)

During /review:

• No prop drilling deeper than 2 levels (use context or composition)
• No unnecessary re-renders (check memo/callback usage)
• No sensitive data stored in localStorage/client state
• Route guards for authenticated pages
• Loading indicators for async operations

Heading-Scoped Read Note

For phase-entry loading, read only:

• When to Apply
• Checklist

Load Conventions, Anti-Patterns, and References on full read or cache miss only.

Anti-Patterns

• God component: Component with 300+ lines doing everything. Split into smaller components.
• Prop drilling: Passing props through 3+ levels of components. Use context or composition.
• useEffect for everything: Using useEffect for derived state (use useMemo) or event handling (use event handlers).
• Index as key: Using array index as React key in dynamic lists.
• Fetch in render: Calling fetch/axios directly in component body or useEffect without a data-fetching library.
• CSS-in-JS soup: Inline styles everywhere instead of consistent styling approach.
• Ignoring empty state: Showing a blank page when data array is empty instead of a helpful message.
• Alert/Console for errors: Using alert() or only console.error() instead of proper UI error handling.

References

• Project ADR: docs/adr/ADR-002-project-architecture.md § Directory Structure, Naming
• Spec template: .agentcortex/templates/spec-app-feature.md § Frontend
• Security guardrails: .agent/rules/security_guardrails.md (A03: XSS prevention, A07: Auth)