React Frontend Patterns

React Frontend Patterns

Overview

State flows down, events flow up. Colocate state with consumers. Fetch at route level.

Patterns for React 19 + TypeScript + Zustand + TanStack Query + Tailwind v4. Use in Phase 1 (architecture decisions) and Phase 3 (implementation).

Quick Reference

State Colocation

Where does this state live?
│
├── From server? ──────────► TanStack Query cache
│                            (products, user profile, orders)
│
├── Global UI? ────────────► Zustand store
│                            (theme, sidebar, notifications)
│
├── In URL? ───────────────► Router params/​search
│                            (filters, pagination, selected tab)
│
├── Form input? ───────────► react-hook-form
│                            (field values, validation)
│
└── Local to component? ───► useState
                             (modal open, hover, dropdown)

Rule: Start with the most local option. Lift only when needed.

Zustand Patterns

One store per feature domain. Actions own async logic. Reset for tests.

The Selector Rule (Non-Negotiable):

// ✅ Always use selectors
const items = useCartStore((s) => s.items)
const addItem = useCartStore((s) => s.addItem)

// ✅ Multiple fields with useShallow
const { items, total } = useCartStore(
  useShallow((s) => ({ items: s.items, total: s.total }))
)

// ❌ NEVER bare — causes re-render on ANY state change
const store = useCartStore()
const { items, total } = useCartStore()  // Also bad!

Create selector hooks to enforce this pattern:

// In store file — export selector hooks
export const useCartItems = () => useCartStore((s) => s.items)
export const useCartTotal = () => useCartStore((s) => s.items.reduce(...))
export const useCartActions = () => useCartStore(
  useShallow((s) => ({ addItem: s.addItem, removeItem: s.removeItem }))
)

// In components — impossible to use bare store
const items = useCartItems()  // ✅ Always correct

Complete store template: See references/​zustand-store-template.ts

TanStack Query Patterns

Queries for reads. Mutations for writes. Custom hooks wrap both.

// Query hook
export function useProducts(category: string) {
  return useQuery({
    queryKey: ['products', 'list', { category }],
    queryFn: () => api.products.list(category),
  })
}

// Mutation hook
export function useCreateProduct() {
  const queryClient = useQueryClient()
  return useMutation({
    mutationFn: api.products.create,
    onSuccess: () => {
      queryClient.invalidateQueries({ queryKey: ['products'] })
    },
  })
}

Complete patterns: See references/​tanstack-query-patterns.tsx

Error & Loading Strategy

// Page-level boundary
<ErrorBoundary fallback={<PageError />}>
  <ProductsPage />
</​ErrorBoundary>

// Feature-level boundary
<ErrorBoundary fallback={<WidgetError />}>
  <RecommendationsWidget />
</​ErrorBoundary>

Component Hierarchy

src/
├── components/
│   ├── ui/                    # shadcn/​ui primitives
│   ├── [feature]/             # Feature components
│   │   ├── FeatureCard.tsx
│   │   └── __tests__/
│   │       └── FeatureCard.test.tsx
│   └── layout/                # Shell, nav, sidebar
├── stores/                    # Zustand stores
│   └── useCartStore.ts
├── hooks/                     # TanStack Query hooks
│   └── useProducts.ts
└── pages/                     # Route components
    └── ProductsPage.tsx

Common Mistakes

Red Flags — STOP and Fix

If you catch yourself doing any of these, stop and apply the correct pattern:

• const { x, y, z } = useStore() — Bare destructuring. Use selectors or useShallow.
• const store = useStore() — Bare store. Always select specific fields.
• useState for data from API — Use TanStack Query.
• Storing itemCount when you have items — Derive in selector: (s) => s.items.length
• vi.mock on a Zustand store — Use real store, reset in beforeEach.
• Fetching in useEffect — Use TanStack Query's useQuery.
• "Just this once, time is tight" — Patterns take 10 seconds more. Do it right.

Provider Setup

See references/​provider-setup.tsx for the complete app provider stack.

E2E Testing Support

MANDATORY: All interactive elements MUST have data-testid attributes.

Naming convention: {component}-{element}-{action?}

Examples:

<button data-testid="recipe-form-submit">Save</​button>
<input data-testid="recipe-form-title" />
<div data-testid="recipe-card-{id}">...</​div>

For lists/​collections, include the ID:

data-testid="recipe-card-123"
data-testid="shopping-item-456"

Interactive elements requiring data-testid:

• All buttons, links, and clickable elements
• Form inputs (text, select, checkbox, radio)
• Cards and list items that can be interacted with
• Modals, dialogs, and their controls
• Navigation items
• Toast notifications and alerts