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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') }, []) |
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 |
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 |
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 |
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 && | renders when count is 0 |
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 | captures stale state |
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__"> |
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
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