2nd/10_Wiki/Topics/Frontend/불필요한 리렌더링 방지.md

---
id: wiki-2026-0508-불필요한-리렌더링-방지
title: 불필요한 리렌더링 방지
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Prevent Unnecessary Re-render, React 최적화, memoization, render optimization]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [frontend, react, performance, optimization, memoization]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: react
---

# 불필요한 리렌더링 방지

## 매 한 줄
> **"매 render 매 cheap by default — 매 measure first, 매 then memoize"**. React 매 default 의 parent render 의 child 의 모두 re-render — 매 보통 fast. 매 React Compiler (RC 2026) 의 auto-memoization 의 manual `useMemo`/`useCallback` 의 obsolescence 약속. 매 그 전 의 explicit memoization + state colocation.

## 매 핵심

### 매 re-render trigger
- **State change** (`useState`, `useReducer`).
- **Props change** (parent render).
- **Context change** (consumer re-renders).
- **Parent re-render** (default behavior).

### 매 4가지 전략
1. **State colocation** — state 의 use 의 곳 가까이 (lift down).
2. **Memoization** — `React.memo`, `useMemo`, `useCallback`.
3. **Context split** — separate frequently-changing from stable.
4. **Component composition** — `children` prop 의 stable reference.

### 매 응용
1. List rendering (10k+ items → virtualization + memo).
2. Form state (controlled inputs → debounced or uncontrolled).
3. Global state (Zustand selector, Redux `useSelector` with shallowEqual).
4. Heavy computation (`useMemo` for derived state).

## 💻 패턴

### 1. React.memo for pure components
```typescript
import { memo } from 'react';

interface ItemProps {
  id: string;
  name: string;
  onClick: (id: string) => void;
}

export const Item = memo(function Item({ id, name, onClick }: ItemProps) {
  console.log('render:', id);
  return <li onClick={() => onClick(id)}>{name}</li>;
});

// custom comparator (rarely needed)
export const ItemDeep = memo(Item, (prev, next) => prev.id === next.id && prev.name === next.name);
```

### 2. useCallback for stable function reference
```typescript
import { useCallback, useState } from 'react';

function List({ items }: { items: { id: string; name: string }[] }) {
  const [selected, setSelected] = useState<string | null>(null);

  // without useCallback — new ref each render → all Item re-render
  const handleClick = useCallback((id: string) => setSelected(id), []);

  return (
    <ul>
      {items.map(it => <Item key={it.id} {...it} onClick={handleClick} />)}
    </ul>
  );
}
```

### 3. useMemo for expensive computation
```typescript
import { useMemo } from 'react';

function Report({ rows }: { rows: Row[] }) {
  const stats = useMemo(() => {
    return {
      total: rows.reduce((a, r) => a + r.amount, 0),
      avg: rows.length ? rows.reduce((a, r) => a + r.amount, 0) / rows.length : 0,
      max: Math.max(...rows.map(r => r.amount)),
    };
  }, [rows]);

  return <Summary {...stats} />;
}
```

### 4. State colocation (lift down)
```typescript
// BAD — input state at parent re-renders entire tree
function App() {
  const [search, setSearch] = useState('');
  return (
    <>
      <input value={search} onChange={e => setSearch(e.target.value)} />
      <ExpensiveTree />  {/* re-renders on every keystroke */}
    </>
  );
}

// GOOD — colocate input state in SearchInput
function SearchInput({ onCommit }: { onCommit: (s: string) => void }) {
  const [v, setV] = useState('');
  return <input value={v} onChange={e => { setV(e.target.value); onCommit(e.target.value); }} />;
}
function App() {
  return <><SearchInput onCommit={...} /><ExpensiveTree /></>;
}
```

### 5. Context split (avoid frequent re-renders)
```typescript
// BAD — single context, everything re-renders on theme change
const AppCtx = createContext<{user: User; theme: 'light'|'dark'; cart: Item[]}>(...);

// GOOD — split by change frequency
const UserCtx = createContext<User | null>(null);  // rare change
const ThemeCtx = createContext<'light'|'dark'>('light');  // user-toggled
const CartCtx = createContext<Item[]>([]);  // frequent

// only consumers of changed context re-render
```

### 6. Children as stable prop (composition)
```typescript
// BAD — Wrapper re-renders => inner re-renders even if same JSX
function App() {
  const [count, setCount] = useState(0);
  return <Wrapper>{<HeavyChild />}</Wrapper>; // HeavyChild created each render? No — JSX is reference
}

// pattern: pass children to escape parent re-render
function Wrapper({ children }: { children: ReactNode }) {
  const [internal, setInternal] = useState(0);
  return <div>{children}</div>; // children reference stable from parent
}
```

### 7. Zustand selector (subscribe slice)
```typescript
import { create } from 'zustand';
import { useShallow } from 'zustand/react/shallow';

const useStore = create<{ count: number; user: User; inc: () => void }>((set) => ({
  count: 0,
  user: { id: '1', name: 'A' },
  inc: () => set(s => ({ count: s.count + 1 })),
}));

// only re-renders when count changes (not user)
function Counter() {
  const count = useStore(s => s.count);
  return <span>{count}</span>;
}

// shallow compare for object selector
function Profile() {
  const { name, id } = useStore(useShallow(s => ({ name: s.user.name, id: s.user.id })));
  return <span>{id}: {name}</span>;
}
```

### 8. React Profiler (measure first)
```typescript
import { Profiler } from 'react';

function onRender(id: string, phase: 'mount' | 'update', actualDuration: number) {
  if (actualDuration > 16) console.warn(`${id} ${phase} took ${actualDuration}ms`);
}

<Profiler id="List" onRender={onRender}>
  <List items={items} />
</Profiler>
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| 100s of items, fast props | `React.memo` + `useCallback`. |
| 10k+ items | virtualization (react-window, TanStack Virtual). |
| Heavy compute | `useMemo`. |
| Frequent global state | Zustand selector / Redux Toolkit. |
| Form input | uncontrolled (react-hook-form) or colocated state. |
| React 19+ | RC compiler 의 auto-memo — manual 의 less needed. |

**기본값**: 매 measure first (React DevTools Profiler). 매 colocate state. 매 React Compiler (when stable) 의 auto-memo. 매 manual memo 의 hot paths only.

## 🔗 Graph
- 부모: [[Memoization]]
- 변형: [[React Compiler]] · [[Zustand]]
- 응용: [[가상 스크롤]] · [[Virtualization]]

## 🤖 LLM 활용
**언제**: measured perf bottleneck (Profiler), large list, heavy compute, frequent state updates.
**언제 X**: 매 not measured — premature optimization. 매 fast already (< 16ms render).

## ❌ 안티패턴
- **Memoize everything**: `useMemo` 의 cost (deps compare) > savings 의 cheap compute.
- **Inline object/array as prop**: `<Comp data={{a:1}} />` 매 new ref 매 every render — defeats memo.
- **Context for everything**: monolithic context → all consumers re-render.
- **`useCallback` w/o `React.memo` child**: useless — child still re-renders on parent.
- **Premature optimization**: optimize before measuring.
- **Mutating state directly**: `state.list.push(...)` — React doesn't detect change.

## 🧪 검증 / 중복
- Verified (React docs 2026, Kent C. Dodds patterns, React Compiler RFC).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — full content with 8 patterns + React Compiler 2026 note |