---
id: wiki-2026-0508-javascriptcore
title: JavaScriptCore
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [JSC, WebKit JSC, Nitro]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [javascript, engine, webkit, jit]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: C++
  framework: WebKit
---

# JavaScriptCore

## 매 한 줄
> **"매 Apple WebKit 의 JavaScript 엔진 — 4-tier JIT pipeline (LLInt → Baseline → DFG → FTL/B3)"**. Safari, iOS WebView, React Native (Hermes 도입 전), Bun runtime의 core. 매 V8 대비 startup 빠름, peak performance 비슷 — 매 mobile-first 최적화.

## 매 핵심

### 매 4-Tier JIT Pipeline
- **LLInt** (Low-Level Interpreter): 매 첫 실행, profiling 시작.
- **Baseline JIT**: 매 hot 코드 → simple machine code, type feedback 수집.
- **DFG** (Data Flow Graph): 매 speculative optimization, OSR exit 가능.
- **FTL** (Faster Than Light) / B3 backend: 매 최상위 tier, LLVM-style IR.

### 매 GC
- **Riptide**: 매 generational + concurrent mark-and-sweep.
- **Eden GC**: 매 young objects, frequent.
- **Full GC**: 매 entire heap, infrequent.

### 매 응용
1. Safari / WKWebView (iOS, macOS).
2. Bun runtime — 매 Node.js 대안, JSC 채택 (V8 X).
3. React Native (legacy iOS, Hermes 이전).
4. GNOME / GTK WebKitGTK.

## 💻 패턴

### Bun runtime — JSC embed
```javascript
// bun runs JS via JSC (not V8)
// startup: ~10ms vs Node ~30ms
import { serve } from "bun";

serve({
  port: 3000,
  fetch(req) {
    return new Response("Hello from JSC");
  },
});
```

### Inline cache observation (JSC IR dump)
```bash
# JSC bytecode dump
JSC_dumpBytecodesAfterGeneration=true \
  /System/Library/Frameworks/JavaScriptCore.framework/Helpers/jsc script.js

# DFG/FTL 단계 확인
JSC_dumpDFGGraphAtEachPhase=true jsc script.js
```

### Hidden class friendly object
```javascript
// JSC structure (V8의 hidden class 와 동일 개념)
function Point(x, y) {
  this.x = x; // structure transition T0 → T1
  this.y = y; // T1 → T2
}
// T2 가 stable → inline cache hit
```

### Type feedback (monomorphic 유지)
```javascript
function add(a, b) { return a + b; }
add(1, 2); // int+int → DFG int specialization
add(1.5, 2.5); // float+float → polymorphic, slower

// 매 monomorphic 유지 — 매 동일 type 만 호출
```

### OffHeap typed array (Bun)
```javascript
// JSC 의 ArrayBuffer 는 GC 외부 — large data 적합
const buf = new ArrayBuffer(1024 * 1024 * 100); // 100MB
const view = new Uint8Array(buf);
// 매 GC pressure X
```

### WeakRef (JSC 1.16+)
```javascript
const cache = new WeakRef(largeObject);
// 매 GC 가 collect 가능 — memory pressure 시 free
const ref = cache.deref();
if (ref) { /* still alive */ }
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| iOS native app + JS | JSC (WKWebView 또는 JavaScriptCore.framework) |
| Cross-platform server | V8 (Node.js) — 매 ecosystem 큼 |
| Fast startup CLI | Bun (JSC) — 매 startup 압도적 |
| RN new architecture | Hermes — 매 mobile 최적화 |

**기본값**: server는 V8/Node, mobile WebView는 JSC, fast CLI는 Bun.

## 🔗 Graph
- 부모: [[WebKit]]
- 변형: [[V8]] · [[Hermes]]
- 응용: [[React Native]]
- Adjacent: [[Garbage Collection]]

## 🤖 LLM 활용
**언제**: iOS WebView 성능 분석, Bun 코드 최적화, JSC-specific bug 진단.
**언제 X**: Node.js / Chrome 환경 — 매 V8 별도 문서.

## ❌ 안티패턴
- **Polymorphic call sites**: 매 inline cache miss → DFG bailout.
- **delete operator**: 매 hidden class transition 깨짐 — 매 undefined 할당으로 대체.
- **arguments object 남용**: 매 modern JSC 는 rest params 가 빠름.

## 🧪 검증 / 중복
- Verified (WebKit Wiki, JSC source — github.com/WebKit/WebKit).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — JSC 4-tier pipeline + Bun 응용 |