2nd/10_Wiki/Topics/Frontend/Hydration-Mismatch-and-SSR-Debugging.md

---
id: wiki-2026-0508-hydration-mismatch-and-ssr-debug
title: Hydration Mismatch and SSR Debugging
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Hydration Mismatch, SSR Debug, Hydration Error]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [react, ssr, hydration, debugging]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: react-next
---

# Hydration Mismatch and SSR Debugging

## 매 한 줄
> **"매 server-rendered HTML 과 매 client first render 가 매 다르면 매 React 가 매 throw — 매 일관된 input 이 매 핵심."**. 매 typical 원인: Date.now / Math.random / window 접근 / locale / 매 third-party DOM 조작. React 19 부터 매 error message 가 매 어떤 attribute 가 매 mismatch 인지 매 정확히 표시.

## 매 핵심

### 매 원인 분류
- **Time / random**: server 의 매 시각 ≠ client 의 매 시각.
- **Locale / timezone**: Intl.DateTimeFormat 의 매 다른 결과.
- **Window / document**: server 에 매 없음.
- **User-agent branching**: useragent 의 매 다른 처리.
- **Third-party script**: AdSense, Cookiebot 의 매 DOM 변경.
- **Browser extension**: Grammarly, Dark Reader 의 매 inject.
- **Conditional rendering on `typeof window`**: 매 안티패턴.

### 매 React 의 매 행동 (19)
- 매 mismatch detection — 매 server HTML 을 매 polluted attribute 로 매 표시.
- 매 partial recovery — 매 mismatch boundary 만 매 client re-render.
- 매 보존되는 attribute (data-, aria-, custom): 매 dev warning.

### 매 Debug 도구
- **Next.js**: `?_rsc_no_cache` 로 매 server payload 확인. `next dev` 의 매 colored diff.
- **React DevTools**: Profiler — hydration milestone.
- **Browser**: 매 view-source vs DOM inspect 비교.

### 매 응용
1. e-commerce — 매 product price (locale), cart count (cookie).
2. Auth — 매 logged-in / logged-out 분기.
3. A/B test — 매 server vs client variant 의 매 일치.

## 💻 패턴

### Detect mismatch source (Next.js 15)
```tsx
// Bad — Date.now() differs
function Now() { return <span>{Date.now()}</span>; }

// Good — pass server time as prop, render same on both
function Now({ serverTime }: { serverTime: number }) {
  return <span suppressHydrationWarning>{new Date(serverTime).toISOString()}</span>;
}
```

### useEffect for client-only side effects
```tsx
function ClientGreeting() {
  const [name, setName] = useState<string | null>(null);
  useEffect(() => setName(localStorage.getItem('name')), []);
  if (!name) return null;          // server returns null, client fills after mount
  return <p>Hi {name}</p>;
}
```

### dynamic() with ssr: false (Next.js)
```tsx
import dynamic from 'next/dynamic';
const Chart = dynamic(() => import('./Chart'), { ssr: false });
// Renders nothing on server, only on client
```

### Avoid Math.random in render
```tsx
// Bad
function Card() { return <div data-id={Math.random()}>...</div>; }

// Good — useId
function Card() { const id = useId(); return <div id={id}>...</div>; }
```

### suppressHydrationWarning (last resort)
```tsx
<time dateTime={iso} suppressHydrationWarning>
  {new Intl.DateTimeFormat('ko').format(new Date(iso))}
</time>
```

### Diagnose third-party DOM mutation
```tsx
useEffect(() => {
  const obs = new MutationObserver(records => {
    for (const r of records) {
      console.warn('mutation', r.target, r.attributeName, r.addedNodes);
    }
  });
  obs.observe(document.body, { subtree: true, childList: true, attributes: true });
  return () => obs.disconnect();
}, []);
```

### Locale-stable rendering
```tsx
// Server and client must agree — pass timezone explicitly
const fmt = new Intl.DateTimeFormat('ko-KR', { timeZone: 'Asia/Seoul' });
return <span>{fmt.format(new Date(props.iso))}</span>;
```

### Replay server HTML offline
```bash
# Capture server HTML
curl -s https://app.example.com/page > server.html
# Open in fresh browser, compare with hydrated DOM via outerHTML diff
```

### React 19 onRecoverableError
```tsx
hydrateRoot(document, <App />, {
  onRecoverableError(err, info) {
    fetch('/log/hydration-error', {
      method: 'POST',
      body: JSON.stringify({ msg: String(err), digest: info.digest }),
    });
  },
});
```

## 매 결정 기준
| 원인 | Fix |
|---|---|
| 매 time / random | useId / passed-down server value |
| 매 locale / TZ | 매 explicit Intl 의 매 timeZone |
| 매 window / localStorage | 매 useEffect 후 setState |
| 매 SSR 무가치 component | 매 dynamic ssr:false |
| 매 third-party inject | 매 suppressHydrationWarning + 매 root 외부 |
| 매 extension (Grammarly) | 매 무시 — 매 알려진 false positive |

**기본값**: 매 server / client 를 매 동일 input 으로 매 강제. 매 last resort suppressHydrationWarning.

## 🔗 Graph
- 부모: [[Hydration]] · [[SSR]]
- 변형: [[Selective Hydration]] · [[Streaming SSR]]
- 응용: [[Remix]] · [[Astro]]
- Adjacent: [[useEffect]]

## 🤖 LLM 활용
**언제**: 매 hydration error stack 분석, 매 SSR 코드 review, 매 timezone bug 진단.
**언제 X**: 매 pure CSR — 매 hydration 자체 X.

## ❌ 안티패턴
- **`typeof window !== 'undefined'` in render**: 매 server / client output 차이 — 매 mismatch 보장.
- **Date.now() in render**: 매 항상 mismatch.
- **suppressHydrationWarning 남발**: 매 진짜 bug 숨김.
- **try/catch around hydrateRoot 만**: 매 root cause 추적 X.

## 🧪 검증 / 중복
- Verified (React 19 docs — Hydration Errors, Next.js 15 docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — 매 mismatch 원인 + Next.js 15 debug |