/react-patterns

name: react-patterns
description: "React 19 performance patterns and composition architecture for Vite + Cloudflare projects. 50+ rules ranked by impact — eliminating waterfalls, bundle optimisation, re-render prevention, composition over boolean props, server/client boundaries, and React 19 APIs. Use when writing, reviewing, or refactoring React components. Triggers: 'react patterns', 'react review', 'react performance', 'optimise components', 'react best practices', 'composition patterns', 'why is it slow', 'reduce re-renders', 'fix waterfall'."
compatibility: claude-code-only
allowed-tools:
  - Read
  - Glob
  - Grep

React Patterns

Performance and composition patterns for React 19 + Vite + Cloudflare Workers projects. Use as a checklist when writing new components, a review guide when auditing existing code, or a refactoring playbook when something feels slow or tangled.

Rules are ranked by impact. Fix CRITICAL issues before touching MEDIUM ones.

When to Apply

• Writing new React components or pages
• Reviewing code for performance issues
• Refactoring components with too many props or re-renders
• Debugging "why is this slow?" or "why does this re-render?"
• Building reusable component libraries
• Code review before merging

1. Eliminating Waterfalls (CRITICAL)

Sequential async calls where they could be parallel. The #1 performance killer.

| Pattern | Problem | Fix | |---------|---------|-----| | **Await in sequence** | `const a = await getA(); const b = await getB();` | `const [a, b] = await Promise.all([getA(), getB()]);` | | **Fetch in child** | Parent renders, then child fetches, then grandchild fetches | Hoist fetches to the highest common ancestor, pass data down | | **Suspense cascade** | Multiple Suspense boundaries that resolve sequentially | One Suspense boundary wrapping all async siblings | | **Await before branch** | `const data = await fetch(); if (condition) { use(data); }` | Move await inside the branch — don't fetch what you might not use | | **Import then render** | `const Component = await import('./Heavy'); return <Component />` | Use `React.lazy()` + `<Suspense>` — renders fallback instantly |

**How to find them**: Search for `await` in components. Each `await` is a potential waterfall. If two awaits are independent, they should be parallel.

2. Bundle Size (CRITICAL)

Every KB the user downloads is a KB they wait for.

| Pattern | Problem | Fix | |---------|---------|-----| | **Barrel imports** | `import { Button } from '@/components'` pulls the entire barrel file | `import { Button } from '@/components/ui/button'` — direct import | | **No code splitting** | Heavy component loaded on every page | `React.lazy(() => import('./HeavyComponent'))` + `<Suspense>` | | **Third-party at load** | Analytics/tracking loaded before the app renders | Load after hydration: `useEffect(() => { import('./analytics') }, [])` | | **Full library import** | `import _ from 'lodash'` (70KB) | `import debounce from 'lodash/debounce'` (1KB) | | **Lucide tree-shaking** | `import * as Icons from 'lucide-react'` (all icons) | Explicit map: `import { Home, Settings } from 'lucide-react'` | | **Duplicate React** | Library bundles its own React → "Cannot read properties of null" | `resolve.dedupe: ['react', 'react-dom']` in vite.config.ts |

**How to find them**: `npx vite-bundle-visualizer` — shows what's in your bundle.

3. Composition Architecture (HIGH)

How you structure components matters more than how you optimise them.

| Pattern | Problem | Fix | |---------|---------|-----| | **Boolean prop explosion** | `<Card isCompact isClickable showBorder hasIcon isLoading>` | Explicit variants: `<CompactCard>`, `<ClickableCard>` | | **Compound components** | Complex component with 15 props | Split into `<Dialog>`, `<Dialog.Trigger>`, `<Dialog.Content>` with shared context | | **renderX props** | `<Layout renderSidebar={...} renderHeader={...} renderFooter={...}>` | Use children + named slots: `<Layout><Sidebar /><Header /></Layout>` | | **Lift state** | Sibling components can't share state | Move state to parent or context provider | | **Provider implementation** | Consumer code knows about state management internals | Provider exposes interface `{ state, actions, meta }` — implementation hidden | | **Inline components** | `function Parent() { function Child() { ... } return <Child /> }` | Define Child outside Parent — inline components remount on every render |

**The test**: If a component has more than 5 boolean props, it needs composition, not more props.

4. Re-render Prevention (MEDIUM)

Not all re-renders are bad. Only fix re-renders that cause visible jank or wasted computation.

| Pattern | Problem | Fix | |---------|---------|-----| | **Default object/array props** | `function Foo({ items = [] })` → new array ref every render | Hoist: `const DEFAULT = []; function Foo({ items = DEFAULT })` | | **Derived state in effect** | `useEffect(() => setFiltered(items.filter(...)), [items])` | Derive during render: `const filtered = useMemo(() => items.filter(...), [items])` | | **Object dependency** | `useEffect(() => {...}, [config])` fires every render if config is `{}` | Use primitive deps: `useEffect(() => {...}, [config.id, config.type])` | | **Subscribe to unused state** | Component reads `{ user, theme, settings }` but only uses `user` | Split context or use selector: `useSyncExternalStore` | | **State for transient values** | `const [mouseX, setMouseX] = useState(0)` on mousemove | Use `useRef` for values that change frequently but don't need re-render | | **Inline callback props** | `<Button onClick={() => doThing(id)} />` — new function every render | `useCallback` or functional setState: `<Button onClick={handleClick} />` |

**How to find them**: React DevTools Profiler → "Why did this render?" or `<React.StrictMode>` double-renders in dev.

5. React 19 Specifics (MEDIUM)

Patterns that changed or are new in React 19.

| Pattern | Old (React 18) | N