Epic React Patterns

Epic Stack: React Patterns and Guidelines

When to use this skill

Use this skill when you need to:

• Write efficient React components in Epic Stack applications
• Optimize performance and bundle size
• Follow React Router patterns and conventions
• Avoid common React anti-patterns
• Implement proper code splitting
• Optimize re-renders and data fetching
• Use React hooks correctly

Philosophy

Following Epic Web principles:

• Make it work, make it right, make it fast - In that order. First make it functional, then refactor for clarity, then optimize for performance.
• Pragmatism over purity - Choose practical solutions that work well in your context rather than theoretically perfect ones.
• Optimize for sustainable velocity - Write code that's easy to maintain and extend, not just fast to write initially.
• Do as little as possible - Only add complexity when it provides real value.

Patterns and conventions

Data Fetching in React Router

Epic Stack uses React Router loaders for data fetching, not useEffect.

✅ Good - Use loaders:

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// app/routes/users/$username.tsx
export async function loader({ params }: Route.LoaderArgs) {
	const user = await prisma.user.findUnique({
		where: { username: params.username },
	})
	return { user }
}

export default function UserRoute({ loaderData }: Route.ComponentProps) {
	return <div>{loaderData.user.name}</div>
}

❌ Avoid - Don't fetch in useEffect:

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// ❌ Don't do this
export default function UserRoute({ params }: Route.ComponentProps) {
	const [user, setUser] = useState(null)

	useEffect(() => {
		fetch(`/api/users/${params.username}`)
			.then(res => res.json())
			.then(setUser)
	}, [params.username])

	return user ? <div>{user.name}</div> : <div>Loading...</div>
}

Avoid useEffect for Side Effects

You Might Not Need useEffect

Instead of using useEffect, use event handlers, CSS, ref callbacks, or useSyncExternalStore.

✅ Good - Use event handlers:

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function ProductPage({ product, addToCart }: Route.ComponentProps) {
	function buyProduct() {
		addToCart(product)
		showNotification(`Added ${product.name} to cart!`)
	}

	function handleBuyClick() {
		buyProduct()
	}

	function handleCheckoutClick() {
		buyProduct()
		navigate('/checkout')
	}

	return (
		<div>
			<button onClick={handleBuyClick}>Buy Now</button>
			<button onClick={handleCheckoutClick}>Checkout</button>
		</div>
	)
}

❌ Avoid - Side effects in useEffect:

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// ❌ Don't do this
function ProductPage({ product, addToCart }: Route.ComponentProps) {
	useEffect(() => {
		if (product.isInCart) {
			showNotification(`Added ${product.name} to cart!`)
		}
	}, [product])

	function handleBuyClick() {
		addToCart(product)
	}

	// ...
}

✅ Appropriate use of useEffect:

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// ✅ Good - Event listeners are appropriate
useEffect(() => {
	const controller = new AbortController()

	window.addEventListener(
		'keydown',
		(event: KeyboardEvent) => {
			if (event.key !== 'Escape') return
			// handle escape key
		},
		{ signal: controller.signal },
	)

	return () => {
		controller.abort()
	}
}, [])

Code Splitting with React Router

React Router automatically code-splits by route. Use dynamic imports for heavy components.

✅ Good - Dynamic imports:

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// app/routes/admin/dashboard.tsx
import { lazy } from 'react'

const AdminChart = lazy(() => import('#app/components/admin/chart.tsx'))

export default function AdminDashboard() {
	return (
		<Suspense fallback={<div>Loading chart...</div>}>
			<AdminChart />
		</Suspense>
	)
}

Optimizing Re-renders

✅ Good - Memoize expensive computations:

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import { useMemo } from 'react'

function UserList({ users }: { users: User[] }) {
	const sortedUsers = useMemo(() => {
		return [...users].sort((a, b) => a.name.localeCompare(b.name))
	}, [users])

	return (
		<ul>
			{sortedUsers.map(user => (
				<li key={user.id}>{user.name}</li>
			))}
		</ul>
	)
}

✅ Good - Memoize callbacks:

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import { useCallback } from 'react'

function NoteEditor({ noteId, onSave }: { noteId: string; onSave: (note: Note) => void }) {
	const handleSave = useCallback((note: Note) => {
		onSave(note)
	}, [onSave])

	return <Editor onSave={handleSave} />
}

❌ Avoid - Unnecessary memoization:

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// ❌ Don't memoize simple values
const count = useMemo(() => items.length, [items]) // Just use items.length directly

// ❌ Don't memoize simple callbacks
const handleClick = useCallback(() => {
	console.log('clicked')
}, []) // Just define the function normally if it doesn't need memoization

Bundle Size Optimization

✅ Good - Import only what you need:

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// ✅ Import specific functions
import { useSearchParams } from 'react-router'
import { parseWithZod } from '@conform-to/zod'

❌ Avoid - Barrel imports:

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// ❌ Don't import entire libraries if you only need one thing
import * as ReactRouter from 'react-router'
import * as Conform from '@conform-to/zod'

Form Handling with Conform

✅ Good - Use Conform for forms:

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import { useForm, getFormProps } from '@conform-to/react'
import { parseWithZod } from '@conform-to/zod'
import { Form } from 'react-router'

const SignupSchema = z.object({
	email: z.string().email(),
	password: z.string().min(6),
})

export default function SignupRoute({ actionData }: Route.ComponentProps) {
	const [form, fields] = useForm({
		id: 'signup-form',
		lastResult: actionData?.result,
		onValidate({ formData }) {
			return parseWithZod(formData, { schema: SignupSchema })
		},
	})

	return (
		<Form method="POST" {...getFormProps(form)}>
			{/* form fields */}
		</Form>
	)
}

Component Composition

✅ Good - Compose components:

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function UserProfile({ user }: { user: User }) {
	return (
		<Card>
			<UserHeader user={user} />
			<UserDetails user={user} />
			<UserActions userId={user.id} />
		</Card>
	)
}

❌ Avoid - Large monolithic components:

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// ❌ Don't put everything in one component
function UserProfile({ user }: { user: User }) {
	return (
		<div className="card">
			<div className="header">
				<img src={user.avatar} alt={user.name} />
				<h1>{user.name}</h1>
			</div>
			<div className="details">
				<p>{user.email}</p>
				<p>{user.bio}</p>
			</div>
			<div className="actions">
				<button>Edit</button>
				<button>Delete</button>
			</div>
		</div>
	)
}

Error Boundaries

✅ Good - Use error boundaries:

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// app/routes/users/$username.tsx
export function ErrorBoundary() {
	return (
		<GeneralErrorBoundary
			statusHandlers={{
				404: ({ params }) => (
					<p>User "{params.username}" not found</p>
				),
			}}
		/>
	)
}

TypeScript Guidelines

✅ Good - Type props explicitly:

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interface UserCardProps {
	user: {
		id: string
		name: string
		email: string
	}
	onEdit?: (userId: string) => void
}

function UserCard({ user, onEdit }: UserCardProps) {
	return (
		<div>
			<h2>{user.name}</h2>
			<p>{user.email}</p>
			{onEdit && <button onClick={() => onEdit(user.id)}>Edit</button>}
		</div>
	)
}

✅ Good - Use Route types:

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import type { Route } from './+types/users.$username'

export async function loader({ params }: Route.LoaderArgs) {
	// params is type-safe!
	const user = await prisma.user.findUnique({
		where: { username: params.username },
	})
	return { user }
}

export default function UserRoute({ loaderData }: Route.ComponentProps) {
	// loaderData is type-safe!
	return <div>{loaderData.user.name}</div>
}

Loading States

✅ Good - Use React Router's pending states:

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import { useNavigation } from 'react-router'

function NoteForm() {
	const navigation = useNavigation()
	const isSubmitting = navigation.state === 'submitting'

	return (
		<Form method="POST">
			<button type="submit" disabled={isSubmitting}>
				{isSubmitting ? 'Saving...' : 'Save'}
			</button>
		</Form>
	)
}

Preventing Data Fetching Waterfalls

React Router loaders can prevent waterfalls by fetching data in parallel.

❌ Avoid - Sequential data fetching (waterfall):

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// ❌ Don't do this - creates a waterfall
export async function loader({ params }: Route.LoaderArgs) {
	const user = await prisma.user.findUnique({
		where: { username: params.username },
	})
	// Second fetch waits for first to complete
	const notes = await prisma.note.findMany({
		where: { ownerId: user.id },
	})
	return { user, notes }
}

✅ Good - Parallel data fetching:

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// ✅ Fetch data in parallel
export async function loader({ params }: Route.LoaderArgs) {
	const user = await prisma.user.findUnique({
		where: { username: params.username },
		select: { id: true, username: true, name: true },
	})

	// Fetch notes in parallel with user data
	const [notes, stats] = await Promise.all([
		user
			? prisma.note.findMany({
					where: { ownerId: user.id },
					select: { id: true, title: true, updatedAt: true },
				})
			: Promise.resolve([]),
		user
			? prisma.note.count({ where: { ownerId: user.id } })
			: Promise.resolve(0),
	])

	return { user, notes, stats }
}

✅ Good - Nested route parallel loading:

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// Parent route loader
// app/routes/users/$username.tsx
export async function loader({ params }: Route.LoaderArgs) {
	const user = await prisma.user.findUnique({
		where: { username: params.username },
		select: { id: true, username: true, name: true },
	})
	return { user }
}

// Child route loader runs in parallel
// app/routes/users/$username/notes.tsx
export async function loader({ params }: Route.LoaderArgs) {
	const user = await prisma.user.findUnique({
		where: { username: params.username },
		select: { id: true },
	})

	if (!user) {
		throw new Response('Not Found', { status: 404 })
	}

	const notes = await prisma.note.findMany({
		where: { ownerId: user.id },
		select: { id: true, title: true, updatedAt: true },
	})

	return { notes }
}

Server-Side Rendering (SSR) Performance

React Router provides SSR by default. Optimize by:

✅ Good - Selective data fetching:

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export async function loader({ request }: Route.LoaderArgs) {
	// Only fetch what's needed for initial render
	const searchParams = new URL(request.url).searchParams
	const page = Number(searchParams.get('page') || '1')

	const [items, total] = await Promise.all([
		prisma.item.findMany({
			take: 20,
			skip: (page - 1) * 20,
			select: { id: true, title: true }, // Only needed fields
		}),
		prisma.item.count(),
	])

	return { items, total, page }
}

✅ Good - Use caching for expensive operations:

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import { cachified, cache } from '#app/utils/cache.server.ts'

export async function loader({ request }: Route.LoaderArgs) {
	const timings: Timings = {}

	// Cache expensive database queries
	const stats = await cachified({
		key: 'user-stats',
		cache,
		timings,
		getFreshValue: async () => {
			return await prisma.user.aggregate({
				_count: { id: true },
			})
		},
		ttl: 1000 * 60 * 5, // 5 minutes
	})

	return { stats }
}

Rendering Performance

✅ Good - Use React.memo for expensive components:

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import { memo } from 'react'

const ExpensiveChart = memo(function ExpensiveChart({ data }: { data: Data[] }) {
	// Expensive rendering logic
	return <Chart data={data} />
})

// Only re-renders when data changes
export default function Dashboard({ chartData }: { chartData: Data[] }) {
	return <ExpensiveChart data={chartData} />
}

✅ Good - Optimize list rendering:

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import { memo } from 'react'

const UserItem = memo(function UserItem({ user }: { user: User }) {
	return (
		<li>
			<h3>{user.name}</h3>
			<p>{user.email}</p>
		</li>
	)
}, (prev, next) => prev.user.id === next.user.id)

function UserList({ users }: { users: User[] }) {
	return (
		<ul>
			{users.map(user => (
				<UserItem key={user.id} user={user} />
			))}
		</ul>
	)
}

❌ Avoid - Creating new objects/arrays in render:

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// ❌ Don't create new objects on every render
function UserProfile({ user }: { user: User }) {
	return <Card user={{ ...user, fullName: `${user.firstName} ${user.lastName}` }} />
}

// ✅ Good - Compute in loader or memoize
export async function loader({ params }: Route.LoaderArgs) {
	const user = await prisma.user.findUnique({
		where: { username: params.username },
		select: { firstName: true, lastName: true },
	})

	return {
		user: {
			...user,
			fullName: `${user.firstName} ${user.lastName}`,
		},
	}
}

Bundle Size Optimization Strategies

✅ Good - Route-based code splitting: React Router automatically splits code by route. Leverage this:

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// Heavy dependencies are automatically split by route
// app/routes/admin/dashboard.tsx
import { Chart } from 'chart.js' // Only loaded on /admin/dashboard route

✅ Good - Dynamic imports for heavy components:

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import { lazy, Suspense } from 'react'

const HeavyComponent = lazy(() => import('#app/components/heavy-component.tsx'))

export default function Route() {
	return (
		<Suspense fallback={<div>Loading...</div>}>
			<HeavyComponent />
		</Suspense>
	)
}

✅ Good - Tree-shakeable imports:

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// ✅ Tree-shakeable - only imports what you use
import { format } from 'date-fns/format'
import { addDays } from 'date-fns/addDays'

// ❌ Avoid - imports entire library
import * as dateFns from 'date-fns'

React 18+ Features for Performance

✅ Good - Use transitions for non-urgent updates:

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import { useTransition } from 'react'
import { useNavigation } from 'react-router'

function SearchInput() {
	const [isPending, startTransition] = useTransition()
	const navigation = useNavigation()

	function handleSearch(query: string) {
		startTransition(() => {
			// Update search results (non-urgent)
			navigation.navigate(`/search?q=${query}`)
		})
	}

	return (
		<input
			onChange={(e) => handleSearch(e.target.value)}
			placeholder={isPending ? 'Searching...' : 'Search'}
		/>
	)
}

Common mistakes to avoid

• ❌ Fetching data in useEffect: Use React Router loaders instead
• ❌ Overusing useEffect: Prefer event handlers, CSS, or ref callbacks
• ❌ Premature memoization: Only memoize when there's a measurable performance benefit
• ❌ Barrel imports: Import only what you need
• ❌ Ignoring TypeScript types: Use Route types for type safety
• ❌ Not handling loading states: Use React Router's navigation states
• ❌ Large monolithic components: Break components into smaller, focused pieces
• ❌ Not using error boundaries: Always add error boundaries to routes
• ❌ Client-side routing when server-side works: Prefer server-side data fetching
• ❌ Data fetching waterfalls: Use Promise.all() to fetch data in parallel
• ❌ Fetching unnecessary data: Only fetch what's needed for the initial render
• ❌ Creating new objects in render: Compute derived data in loaders or memoize
• ❌ Not using React.memo for expensive lists: Memoize list items for better performance
• ❌ Not leveraging route-based code splitting: React Router splits by route automatically

References

• React Router Documentation
• React Documentation - You Might Not Need useEffect
• Conform Documentation
• Epic Stack Docs
• Epic Web Principles
• app/routes/ - Example routes using these patterns
• .cursor/rules/avoid-use-effect.mdc - Epic Stack rule for avoiding useEffect