React Best Practices

Overview

Apply modern React patterns to build maintainable, performant, and testable applications. This skill covers React 18/19 features including Server Components, hooks best practices, component composition, error boundaries, Suspense, context optimization, and rendering performance. It complements the senior-frontend skill with React-specific depth.

Announce at start: "I'm using the react-best-practices skill for React-specific patterns."

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Phase 1: Analyze Component Requirements

Goal: Understand the component's responsibility and data requirements before coding.

Actions

1. Identify the component's single responsibility
2. Determine data requirements (server vs client data)
3. Choose Server Component (default) or Client Component
4. Identify state management needs
5. Plan error and loading states

Server vs Client Decision Table

Need	Component Type	Reason
Direct data fetching (DB, API)	Server (default)	No client JS, faster
Event handlers (onClick, onChange)	Client ('use client')	Needs browser interactivity
useState / useReducer	Client	State requires client runtime
useEffect / useLayoutEffect	Client	Side effects require client
Browser APIs (window, localStorage)	Client	Server has no browser
Third-party libs using client features	Client	Library requires client
No interactivity needed	Server (default)	Smaller bundle, faster

STOP — Do NOT proceed to Phase 2 until:

• Component responsibility is defined (single purpose)
• Server vs Client decision is made with rationale
• Data requirements are mapped

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Phase 2: Implement with Appropriate Patterns

Goal: Apply the correct React patterns for the component's needs.

Actions

1. Apply appropriate composition pattern
2. Implement hooks correctly
3. Add error boundaries and Suspense
4. Optimize rendering where profiling shows need
5. Write tests that verify behavior

STOP — Do NOT proceed to Phase 3 until:

• Patterns match the component's actual needs
• No unnecessary complexity (no premature optimization)
• Tests cover user-visible behavior

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Phase 3: Test and Verify

Goal: Verify component behavior through tests.

Actions

1. Write tests using accessible queries
2. Test user interactions and outcomes
3. Test error and loading states
4. Verify accessibility

Query Priority (React Testing Library)

Priority	Query	Use For
1st	getByRole	Any element with ARIA role
2nd	getByLabelText	Form fields
3rd	getByPlaceholderText	Fields without labels
4th	getByText	Non-interactive elements
Last	getByTestId	When nothing else works

STOP — Testing complete when:

• User interactions produce expected outcomes
• Error states are tested
• Accessibility checks pass

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Hooks Best Practices

useState

// Functional updates for state based on previous state
setCount(prev => prev + 1);

// Lazy initialization for expensive initial values
const [data, setData] = useState(() => computeExpensiveInitialValue());

// Group related state
const [form, setForm] = useState({ name: '', email: '', role: 'user' });

useEffect

Dependency Array Rules

• Include ALL values from component scope that change over time
• Functions inside effect should be defined inside effect or wrapped in useCallback
• Never lie about dependencies (ESLint: react-hooks/exhaustive-deps)

Cleanup Pattern

useEffect(() => {
  const controller = new AbortController();
  async function fetchData() {
    try {
      const res = await fetch(url, { signal: controller.signal });
      const data = await res.json();
      setData(data);
    } catch (e) {
      if (e.name !== 'AbortError') setError(e);
    }
  }
  fetchData();
  return () => controller.abort();
}, [url]);

When NOT to Use useEffect

Instead of useEffect for...	Use This
Data fetching	React Query, SWR, or Server Components
Transforming data	Compute during render
User events	Event handlers
Syncing external stores	useSyncExternalStore

Custom Hooks Rules

• Name starts with use
• Encapsulate reusable stateful logic
• One hook per concern
• Return object (not array) for > 2 values

function useDebounce<T>(value: T, delay: number): T {
  const [debouncedValue, setDebouncedValue] = useState(value);
  useEffect(() => {
    const timer = setTimeout(() => setDebouncedValue(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);
  return debouncedValue;
}

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Component Composition Patterns

Compound Components

function Tabs({ children, defaultValue }: TabsProps) {
  const [activeTab, setActiveTab] = useState(defaultValue);
  return (
    <TabsContext.Provider value={{ activeTab, setActiveTab }}>
      <div role="tablist">{children}</div>
    </TabsContext.Provider>
  );
}

Tabs.Tab = function Tab({ value, children }: TabProps) {
  const { activeTab, setActiveTab } = useTabsContext();
  return (
    <button role="tab" aria-selected={activeTab === value} onClick={() => setActiveTab(value)}>
      {children}
    </button>
  );
};

Tabs.Panel = function Panel({ value, children }: PanelProps) {
  const { activeTab } = useTabsContext();
  if (activeTab !== value) return null;
  return <div role="tabpanel">{children}</div>;
};

Composition Decision Table

Pattern	Use When	Example
Compound Components	Related components sharing implicit state	Tabs, Accordion, Menu
Slots (Children)	Complex content layout	Card with Header/Body/Footer
Render Props	Child needs parent data for flexible rendering	DataFetcher with custom render
Higher-Order Component	Cross-cutting concerns (legacy)	withAuth, withTheme
Custom Hook	Reusable stateful logic without UI	useDebounce, useLocalStorage

Slots Pattern

// Prefer composition over props for complex content
// Bad
<Card title="Hello" subtitle="World" icon={<Star />} actions={<Button>Edit</Button>} />

// Good
<Card>
  <Card.Header>
    <Card.Icon><Star /></Card.Icon>
    <Card.Title>Hello</Card.Title>
  </Card.Header>
  <Card.Actions>
    <Button>Edit</Button>
  </Card.Actions>
</Card>

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Error Boundaries

Placement Strategy Decision Table

Level	Purpose	Example
Route level	Catch page-level crashes	error.tsx in Next.js
Feature level	Isolate feature failures	Wrap each major section
Data level	Wrap async data components	Around Suspense boundaries
Never leaf level	Too granular, adds noise	Do not wrap individual buttons

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Suspense

// Nested Suspense for granular loading
<Suspense fallback={<PageSkeleton />}>
  <Header />
  <Suspense fallback={<SidebarSkeleton />}>
    <Sidebar />
  </Suspense>
  <Suspense fallback={<ContentSkeleton />}>
    <MainContent />
  </Suspense>
</Suspense>

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Context Optimization

Problem: Context causes unnecessary re-renders

Solution Decision Table

Technique	Use When	Example
Split contexts by frequency	Some values update often, some rarely	ThemeContext (rare) vs UIStateContext (frequent)
Memoize context value	Provider re-renders with same data	useMemo(() => ({ state, dispatch }), [state])
Use selectors (Zustand/Jotai)	Need fine-grained subscriptions	useStore(state => state.user.name)
Lift state up	Only parent needs to re-render	Pass data as props to memoized children

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Rendering Optimization

Memoization Decision Table

Technique	Use When	Do NOT Use When
React.memo	Renders often with same props AND re-render is expensive	Props change every render
useMemo	Expensive computation OR referential equality for deps	Simple calculations
useCallback	Stable function ref for memoized children	Function not passed as prop
None (default)	Always start here	Premature optimization

Rule: Profile BEFORE memoizing. Premature memoization is the most common React anti-pattern.

Virtualization

For lists > 100 items:

import { useVirtualizer } from '@tanstack/react-virtual';

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Server Component Rules

• Cannot use hooks
• Cannot use browser APIs
• Cannot pass functions as props to Client Components
• CAN import and render Client Components
• CAN pass serializable data to Client Components

// Server Component — fetches data directly
async function UserProfile({ userId }: { userId: string }) {
  const user = await db.user.findUnique({ where: { id: userId } });
  return (
    <div>
      <h1>{user.name}</h1>
      <UserActions userId={userId} /> {/* Client Component child */}
    </div>
  );
}

// Client Component — handles interactivity
'use client';
function UserActions({ userId }: { userId: string }) {
  const [isFollowing, setIsFollowing] = useState(false);
  return <Button onClick={() => toggleFollow(userId)}>Follow</Button>;
}

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Anti-Patterns / Common Mistakes

Anti-Pattern	Why It Is Wrong	Correct Approach
useEffect for data fetching	Race conditions, no cache, no dedup	React Query or Server Components
Prop drilling > 2 levels	Tight coupling, maintenance pain	Composition, context, or Zustand
Storing derived state	State that can be computed is unnecessary state	Compute during render
useEffect to sync state from props	Unnecessary effect, stale closures	Derive during render or use key prop
Monolithic components (> 200 lines)	Hard to read, test, maintain	Extract sub-components
Index as key for dynamic lists	Incorrect reconciliation, stale state	Stable unique ID
Direct DOM manipulation	Bypasses React reconciliation	Use refs sparingly, prefer state
Testing state values directly	Implementation detail, breaks on refactor	Test user-visible outcomes
Memoizing everything	Adds complexity, often slower	Profile first, optimize second

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Documentation Lookup (Context7)

Use mcp__context7__resolve-library-id then mcp__context7__query-docs for up-to-date docs. Returned docs override memorized knowledge.

• react — for hooks, context, suspense, server components, or React 19+ changes
• next.js — for App Router patterns, data fetching, or server actions

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Integration Points

Skill	Relationship
senior-frontend	Frontend skill uses React patterns from this skill
testing-strategy	React testing follows the strategy pyramid
clean-code	Component code follows clean code principles
performance-optimization	React rendering optimization follows measurement methodology
webapp-testing	E2E tests validate React component behavior
code-review	Review checks for React anti-patterns
acceptance-testing	UI acceptance criteria drive component tests

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Skill Type

FLEXIBLE — Apply these patterns based on the specific React version, project structure, and team conventions. The principles are consistent, but implementation details may vary. Always profile before optimizing.