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
