---
id: wiki-2026-0508-long-tasks
title: Long Tasks API
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Long Tasks, Long Animation Frame, LoAF, Main Thread Blocking]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [web-perf, performance-api, inp, main-thread, browser]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack: { language: JavaScript, framework: PerformanceObserver }
---

# Long Tasks API

## 매 한 줄
> **"매 long task = main thread 50ms+ 블록"**. INP/Responsiveness의 직접 원인. PerformanceObserver로 측정·attribute한다.

## 매 핵심
### 매 정의
- **Long Task**: 단일 task가 50ms 초과로 main thread 점유.
- 사용자 입력·rendering 지연 → INP 악화.
- W3C spec: `longtask` PerformanceEntry.

### 매 후속 표준 (LoAF)
- **Long Animation Frame** (Long Animation Frames API): frame 단위, attribution 강화.
- entryType `long-animation-frame` — script source, style/layout 시간 포함.
- Long Tasks API보다 정확하고 디버깅 친화적 (2024+).

### 매 측정 항목
- `duration`, `startTime`
- `attribution` (Long Tasks): TaskAttributionTiming, container 정도만 (cross-origin 익명화)
- `scripts[]` (LoAF): name, sourceURL, function, invoker, executionStart/forcedStyleAndLayoutDuration

### 매 흔한 원인
1. 큰 JS 번들 parse/compile
2. 동기 layout/style (forced reflow)
3. 무거운 React render (큰 리스트, memo 부재)
4. JSON parse 큰 payload
5. 동기 third-party script
6. Hydration burst (Next.js, Nuxt)

### 매 완화
- Code split, dynamic import
- `requestIdleCallback`, `scheduler.postTask`, `scheduler.yield()`
- Web Worker로 오프로드
- Virtualization (큰 리스트)
- React: Suspense, useDeferredValue, transitions
- Defer non-critical scripts, async chunks

## 💻 패턴
### Long Tasks 관찰
```js
const obs = new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    console.log("long task", entry.duration, entry.name, entry.attribution);
  }
});
obs.observe({ type: "longtask", buffered: true });
```

### Long Animation Frame (현대)
```js
const obs = new PerformanceObserver((list) => {
  for (const e of list.getEntries()) {
    if (e.duration < 50) continue;
    for (const s of e.scripts) {
      console.log("LoAF script", s.name, s.sourceURL,
        s.duration, "fwd-layout:", s.forcedStyleAndLayoutDuration);
    }
  }
});
obs.observe({ type: "long-animation-frame", buffered: true });
```

### scheduler.yield (cooperative)
```js
async function processItems(items) {
  for (const item of items) {
    doWork(item);
    if (navigator.scheduling?.isInputPending?.() || performance.now() - last > 5) {
      await scheduler.yield();      // chunk 양보
      last = performance.now();
    }
  }
}
```

### requestIdleCallback fallback
```js
function chunked(items, fn) {
  let i = 0;
  function run(deadline) {
    while (i < items.length && deadline.timeRemaining() > 0) fn(items[i++]);
    if (i < items.length) requestIdleCallback(run);
  }
  requestIdleCallback(run);
}
```

### Web Worker로 오프로드
```js
// main
const w = new Worker("./parse.worker.js", { type: "module" });
w.postMessage(largePayload);
w.onmessage = (e) => render(e.data);

// parse.worker.js
self.onmessage = (e) => {
  const result = heavyParse(e.data);
  self.postMessage(result);
};
```

### React deferred / transition
```jsx
import { useDeferredValue, useTransition } from "react";
const deferred = useDeferredValue(query);
const [isPending, start] = useTransition();
start(() => setFilter(next));      // non-urgent → no long task on type
```

### web-vitals 통합
```js
import { onINP } from "web-vitals/attribution";
onINP((m) => {
  // m.attribution.longAnimationFrameEntries → 원인 frame
  beacon("inp", m);
}, { reportAllChanges: true });
```

## 매 결정 기준
| 상황 | 도구 |
|---|---|
| 측정 시작 | LoAF (지원) → Long Tasks fallback |
| 사용자 INP 추적 | web-vitals attribution |
| CPU-bound | Web Worker |
| 큰 리스트 처리 | scheduler.yield, chunking |
| Render bottleneck | useDeferredValue, virtualization |
| Third-party 무거움 | defer/async, Partytown |

**기본값**: web-vitals onINP + LoAF observer로 attribute → 가장 큰 script 우선 최적화.

## 🔗 Graph
- 부모: [[Web-Performance]]
- 변형: [[Long-Animation-Frame]]
- 응용: [[INP]], [[Core Web Vitals Optimization (INP, LCP 개선)|Core-Web-Vitals]]
- Adjacent: [[Web Worker (웹 워커)|Web-Workers]], [[Scheduler-API]], [[Code-Splitting]]

## 🤖 LLM 활용
**언제**: PerformanceObserver 코드, 원인 분류, 완화 전략 매핑.
**언제 X**: 실제 attribution은 production trace로 검증해야 정확.

## ❌ 안티패턴
- 50ms threshold만 보고 47ms task 무시 (LoAF가 누적 표시)
- requestAnimationFrame 안에 무거운 sync 작업
- Worker로 보내고 직렬화 cost > 작업 cost
- setTimeout(fn, 0)으로 yield (실제 양보 부족) → scheduler.yield
- Long task 발생만 알리고 attribution 안 봄 → 원인 파악 불가
- DevTools만 보고 production 측정 누락

## 🧪 검증 / 중복
- Verified (W3C Long Tasks, web.dev INP, Long Animation Frames API). 신뢰도 A.
- 중복: 없음.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — LoAF, scheduler.yield, web-vitals 통합 |
