2nd/10_Wiki/Topics/Frontend/Re-renders Optimization.md

---
id: wiki-2026-0508-re-renders-optimization
title: Re-renders Optimization
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [React Re-render Optimization, Render Performance]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [frontend, react, performance, memoization, react-compiler]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: TypeScript
  framework: React 19
---

# Re-renders Optimization

## 매 한 줄
> **"매 React 의 component re-render 의 최소화"**. 2026 React 19+ 매 React Compiler (auto-memo) 의 default 로 manual `useMemo`/`useCallback` 의 의미 약화. 매 measure first, optimize where needed.

## 매 핵심

### 매 Re-render Trigger
- State change (`useState`, `useReducer`).
- Parent re-render → child re-render (default).
- Context value change → 매 consumer 전체 re-render.
- Hook 의 internal state.

### 매 Tool (2026)
- **React Compiler** (default in React 19+): 매 auto-memoization, useMemo/useCallback 거의 불필요.
- **React DevTools Profiler**: 매 flame graph 로 re-render cost.
- **why-did-you-render**: 매 dev-time unnecessary re-render 감지.
- **React Scan**: 매 visual highlight (real-time).

### 매 핵심 기법
- Component 분할 (state collocation).
- `React.memo` (props equality).
- `useMemo` / `useCallback` (compiler 가 대부분 자동).
- Context split (frequently-changing 분리).
- Virtualization (TanStack Virtual, react-window).

### 매 응용
1. Large list rendering: virtualization.
2. Form with many fields: react-hook-form `register` (uncontrolled).
3. Heavy chart: memo + Canvas/WebGL.

## 💻 패턴

### React Compiler (default 2026)
```tsx
// React 19 with Compiler — 매 manual memo 거의 불필요
function ExpensiveList({ items, filter }: Props) {
  const filtered = items.filter(i => i.name.includes(filter)); // auto-memoized
  return <ul>{filtered.map(i => <li key={i.id}>{i.name}</li>)}</ul>;
}
```

### 매 State Collocation
```tsx
// Bad: state in App, but only Modal needs it
function App() {
  const [modalOpen, setModalOpen] = useState(false);
  return (
    <>
      <ExpensiveTree /> {/* re-renders on toggle */}
      <Modal open={modalOpen} />
    </>
  );
}

// Good: state in wrapper
function App() {
  return (
    <>
      <ExpensiveTree />
      <ModalWrapper />
    </>
  );
}
function ModalWrapper() {
  const [open, setOpen] = useState(false);
  return <Modal open={open} />;
}
```

### React.memo + useMemo 의 props
```tsx
const Child = React.memo(({ data }: { data: Item[] }) => {
  return <ul>{data.map(i => <li key={i.id}>{i.name}</li>)}</ul>;
});

function Parent({ items, query }: Props) {
  // Without compiler: useMemo necessary
  const filtered = useMemo(() => items.filter(i => i.name.includes(query)), [items, query]);
  return <Child data={filtered} />;
}
```

### Context Split
```tsx
// Bad: single context, every change re-renders all
const AppContext = createContext({ user: null, theme: 'light', cart: [] });

// Good: split by change frequency
const UserContext = createContext<User | null>(null);
const ThemeContext = createContext<Theme>('light');
const CartContext = createContext<Cart>([]);
```

### useSyncExternalStore (selector 의 fine-grained)
```tsx
import { useSyncExternalStore } from 'react';
import { create } from 'zustand';

const useStore = create<State>(() => ({ count: 0, name: 'a' }));

function CountDisplay() {
  // Only re-render on count change
  const count = useStore((s) => s.count);
  return <div>{count}</div>;
}
```

### Virtualization (TanStack Virtual)
```tsx
import { useVirtualizer } from '@tanstack/react-virtual';

function BigList({ items }: { items: Item[] }) {
  const parentRef = useRef<HTMLDivElement>(null);
  const virtualizer = useVirtualizer({
    count: items.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 40,
  });

  return (
    <div ref={parentRef} style={{ height: 400, overflow: 'auto' }}>
      <div style={{ height: virtualizer.getTotalSize(), position: 'relative' }}>
        {virtualizer.getVirtualItems().map(v => (
          <div key={v.key} style={{ position: 'absolute', top: v.start, height: v.size }}>
            {items[v.index].name}
          </div>
        ))}
      </div>
    </div>
  );
}
```

### React Scan (dev only)
```tsx
import { scan } from 'react-scan';
if (process.env.NODE_ENV === 'development') {
  scan({ enabled: true, log: true });
}
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| React 19+ project | React Compiler (자동) |
| Legacy React 18 | manual memo + Profiler |
| Large list (1000+) | Virtualization |
| Complex form | react-hook-form (uncontrolled) |
| Frequently-changing global state | Zustand/Jotai with selector |
| Cross-cutting state | Context (split by frequency) |

**기본값**: 매 React 19 Compiler enable → measure → optimize hot path only.

## 🔗 Graph
- 부모: [[React]] · [[Performance_Optimization]]
- 응용: [[Virtualization]]
- Adjacent: [[React_Compiler]] · [[Concurrent_Rendering]] · [[Suspense]]

## 🤖 LLM 활용
**언제**: Profiler 매 100ms+ render time 의 component, dropped frame, slow input.
**언제 X**: 매 micro-optimization without measurement — 매 premature.

## ❌ 안티패턴
- **모든 곳 useMemo**: 매 dependency 비교 cost > recomputation cost (특히 primitive).
- **Inline object/array as Context value**: 매 every render new reference → consumer re-render.
- **React.memo without checking props**: 매 props 가 항상 다르면 memo 무효 + cost 추가.
- **Optimization without Profiler**: guessing → wasted effort.

## 🧪 검증 / 중복
- Verified (React 19 docs, React Compiler RC, TanStack Virtual 3).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — React Compiler / React Scan modern stack |