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)	New (React 19)
Form state	useFormState	useActionState — renamed
Ref forwarding	forwardRef((props, ref) => ...)	function Component({ ref, ...props }) — ref is a regular prop
Context	useContext(MyContext)	use(MyContext) — works in conditionals and loops
Pending UI	Manual loading state	useTransition + startTransition for non-urgent updates
Route-level lazy	Works with createBrowserRouter only	Still true — <Route lazy={...}> is silently ignored with <BrowserRouter>
Optimistic updates	Manual state management	useOptimistic hook
Metadata	Helmet or manual <head> management	<title>, <meta>, <link> in component JSX — hoisted to <head> automatically

6. Rendering Performance (MEDIUM)

Pattern	Problem	Fix
Layout shift on load	Content jumps when async data arrives	Skeleton screens matching final layout dimensions
Animate SVG directly	Janky SVG animation	Wrap in <div>, animate the div instead
Large list rendering	1000+ items in a table/list	@tanstack/react-virtual for virtualised rendering
content-visibility	Long scrollable content renders everything upfront	content-visibility: auto on off-screen sections
Conditional render with &&	{count && <Items />} renders 0 when count is 0	Use ternary: {count > 0 ? <Items /> : null}

7. Data Fetching (MEDIUM)

Pattern	Problem	Fix
No deduplication	Same data fetched by 3 components	TanStack Query or SWR — automatic dedup + caching
Fetch on mount	useEffect(() => { fetch(...) }, []) — waterfalls, no caching, no dedup	TanStack Query: useQuery({ queryKey: ['users'], queryFn: fetchUsers })
No optimistic update	User clicks save, waits 2 seconds, then sees change	useMutation with onMutate for instant visual feedback
Stale closure in interval	setInterval captures stale state	useRef for the interval ID and current values
Polling without cleanup	setInterval in useEffect without clearInterval	Return cleanup: useEffect(() => { const id = setInterval(...); return () => clearInterval(id); })

8. Vite + Cloudflare Specifics (MEDIUM)

Pattern	Problem	Fix
import.meta.env in Node scripts	Undefined — only works in Vite-processed files	Use loadEnv() from vite
React duplicate instance	Library bundles its own React	resolve.dedupe + optimizeDeps.include in vite.config.ts
Radix Select empty string	<SelectItem value=""> throws	Use sentinel: <SelectItem value="__any__">
React Hook Form null	{...field} passes null to Input	Spread manually: value={field.value ?? ''}
Env vars at edge	process.env doesn't exist in Workers	Use c.env (Hono context) or import.meta.env (Vite build-time)

Using as a Review Checklist

When reviewing code, go through categories 1-3 (CRITICAL + HIGH) for every PR. Categories 4-8 only when performance is a concern.

/react-patterns [file or component path]

Read the file, check against rules in priority order, report findings as:

file:line — [rule] description of issue