2nd/10_Wiki/Topics/General Knowledge/Code Splitting Lazy Loading.md

---
id: wiki-2026-0508-code-splitting-lazy-loading
title: Code Splitting & Lazy Loading
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [P-REINFORCE-AUTO-B933B1, Code Splitting, Lazy Loading]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [web, performance, bundling, react, vite]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: react+vite
---

# Code Splitting & Lazy Loading

## 매 한 줄
> **"매 ship-only-what-you-execute"**. 매 Code Splitting 은 single bundle 을 route/component-bound chunk 로 분할하여 initial JS payload 의 minimization. Lazy Loading 은 매 chunk 를 user-interaction 시점에 fetch. 매 2026 의 standard 는 Vite + React.lazy + dynamic import + RSC (React Server Components) hybrid.

## 매 핵심

### 매 split 차원
- **Route-based**: 매 page 의 lazy import — `/dashboard`, `/settings`.
- **Component-based**: 매 heavy widget (chart, code-editor) 의 deferred load.
- **Vendor split**: 매 third-party (React, lodash) 의 separate chunk → long-term cache.
- **Dynamic feature flag**: 매 A/B variant 의 gated import.

### 매 핵심 메커니즘
- **Dynamic `import()`**: 매 ES module spec — Promise 반환.
- **Bundler chunk splitting**: Webpack/Vite 의 `splitChunks` heuristic.
- **HTTP/2 push 의 deprecation**: 매 modern preload + modulepreload 로 대체.
- **Streaming SSR + RSC**: 매 chunk 의 server-streamed delivery.

### 매 응용
1. SaaS dashboard — `/admin` route 의 lazy chunk.
2. E-commerce — checkout flow 만 separate bundle.
3. Editor app (Notion-like) — markdown/code-editor lazy.

## 💻 패턴

### 패턴 1: React.lazy + Suspense
```tsx
import { lazy, Suspense } from "react";
import { Routes, Route } from "react-router-dom";

const Dashboard = lazy(() => import("./pages/Dashboard"));
const Settings = lazy(() => import("./pages/Settings"));

export function App() {
  return (
    <Suspense fallback={<Skeleton />}>
      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/dashboard" element={<Dashboard />} />
        <Route path="/settings" element={<Settings />} />
      </Routes>
    </Suspense>
  );
}
```

### 패턴 2: Vite manual chunks
```ts
// vite.config.ts
import { defineConfig } from "vite";

export default defineConfig({
  build: {
    rollupOptions: {
      output: {
        manualChunks(id) {
          if (id.includes("node_modules/react")) return "react-vendor";
          if (id.includes("node_modules/@radix-ui")) return "radix";
          if (id.includes("/src/charts/")) return "charts";
        }
      }
    }
  }
});
```

### 패턴 3: Preload on hover (intent-based prefetch)
```tsx
function NavLink({ to, children }: { to: string; children: React.ReactNode }) {
  const prefetch = () => {
    if (to === "/dashboard") import("./pages/Dashboard");
  };
  return <Link to={to} onMouseEnter={prefetch} onFocus={prefetch}>{children}</Link>;
}
```

### 패턴 4: Conditional dynamic import
```tsx
async function loadChart(type: "line" | "bar" | "heatmap") {
  switch (type) {
    case "line":     return (await import("./charts/Line")).default;
    case "bar":      return (await import("./charts/Bar")).default;
    case "heatmap":  return (await import("./charts/Heatmap")).default;
  }
}
```

### 패턴 5: React Server Component (RSC) split
```tsx
// app/dashboard/page.tsx (Next.js 15+)
import { Suspense } from "react";
import { HeavyChart } from "./HeavyChart"; // server component

export default function Page() {
  return (
    <>
      <Header />
      <Suspense fallback={<ChartSkeleton />}>
        <HeavyChart />  {/* streamed independently */}
      </Suspense>
    </>
  );
}
```

### 패턴 6: Bundle analyzer pipeline
```bash
# Vite + rollup-plugin-visualizer
npm i -D rollup-plugin-visualizer
```
```ts
import { visualizer } from "rollup-plugin-visualizer";

export default defineConfig({
  plugins: [visualizer({ open: true, gzipSize: true })]
});
```

### 패턴 7: Module federation (micro-frontend)
```ts
// host vite.config.ts
import federation from "@originjs/vite-plugin-federation";

export default {
  plugins: [
    federation({
      name: "host",
      remotes: { checkout: "http://cdn/checkout/remoteEntry.js" },
      shared: ["react", "react-dom"]
    })
  ]
};
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Multi-page SPA | route-based React.lazy |
| Heavy single widget | component-level dynamic import |
| Multi-team monorepo | module federation |
| Server-rendered Next.js | RSC + Suspense streaming |
| Legacy Webpack 4 | upgrade to Vite/Webpack 5 first |
| Initial paint > 3s | bundle analyzer + manualChunks |

**기본값**: Vite + React.lazy(route) + intent-prefetch on hover.

## 🔗 Graph
- 부모: [[Web Performance]]
- 변형: [[Module Federation]]
- 응용: [[React Server Components]]
- Adjacent: [[Vite]] · [[Rollup]] · [[esbuild]]

## 🤖 LLM 활용
**언제**: bundle analysis 의 chunk strategy 추천, lazy boundary 후보 식별, manualChunks rule 작성.
**언제 X**: 매 production runtime 의 dynamic decision — 매 bundler-time static analysis 의 영역.

## ❌ 안티패턴
- **Over-splitting**: 매 component 마다 lazy — 매 too many waterfalls. 매 100KB+ threshold.
- **Suspense boundary 의 미사용**: 매 React.lazy 의 fallback 없음 → throw.
- **Preload everything**: 매 idle prefetch 의 abuse — bandwidth waste.
- **Dynamic import in render**: 매 매 render 마다 import() 호출 → re-fetch.

## 🧪 검증 / 중복
- Verified (web.dev code-splitting docs, Vite docs, React.lazy spec, RFC#118).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — substantive content + 2026 stack (Vite, RSC, module federation) |