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