2nd/10_Wiki/Topics/Frontend/JavaScript-Async-and-Event-Loop.md

---
id: wiki-2026-0508-javascript-async-and-event-loop
title: JavaScript Async and Event Loop
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Event Loop, JS Event Loop, Microtask Queue, Macrotask Queue]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [javascript, async, event-loop, concurrency]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: javascript
  framework: node-browser
---

# JavaScript Async and Event Loop

## 매 한 줄
> **"매 single-threaded JS 가 매 cooperative scheduler 위에서 매 async 를 흉내낸다."**. Call stack + task queue + microtask queue + render pipeline 이 매 tick 의 단위. Promise/async-await 는 매 syntactic sugar — runtime 은 매 microtask drain rule 로 결정.

## 매 핵심

### 매 Stack vs Queues
- **Call stack**: 매 동기 frame. LIFO. 매 비어야 매 tick 진행.
- **Task queue (macrotask)**: setTimeout, setInterval, MessageChannel, I/O callback, UI event. 매 tick 당 1개 drain.
- **Microtask queue**: Promise.then, queueMicrotask, MutationObserver. 매 tick 마지막에 매 전부 drain (재진입 포함).
- **Animation frame queue**: requestAnimationFrame. 매 paint 직전.
- **Render steps**: style → layout → paint → composite. 매 vsync 와 매 동기화.

### 매 Tick 순서 (browser)
1. 매 task queue 에서 매 1 task pop → 실행.
2. 매 microtask queue 매 전부 drain (recursively).
3. 매 rAF callbacks.
4. 매 render (필요 시).
5. 매 idle callbacks (requestIdleCallback).
6. 매 다음 tick.

### 매 Node.js phases
- timers → pending callbacks → idle/prepare → poll → check (setImmediate) → close → microtask drain (between phases since Node 11).
- `process.nextTick` 는 매 microtask 보다도 매 우선.

### 매 응용
1. UI freeze 방지 — 매 long task 를 매 chunk + scheduler.yield().
2. Race condition 분석 — 매 await 사이 매 state mutation 가능.
3. Server backpressure — 매 event loop lag (`perf_hooks.monitorEventLoopDelay`).

## 💻 패턴

### Microtask vs Macrotask 순서
```javascript
console.log('1');
setTimeout(() => console.log('2'), 0);
Promise.resolve().then(() => console.log('3'));
queueMicrotask(() => console.log('4'));
console.log('5');
// 1, 5, 3, 4, 2  — microtasks drain before next macrotask
```

### Long task chunking with scheduler.yield (2026)
```javascript
async function processLargeArray(items) {
  for (let i = 0; i < items.length; i++) {
    doWork(items[i]);
    if (i % 100 === 0 && 'scheduler' in window) {
      await scheduler.yield();          // Chrome 129+ stable
    }
  }
}
```

### Promise.allSettled with concurrency limit
```javascript
async function mapLimit(items, limit, fn) {
  const results = new Array(items.length);
  let cursor = 0;
  const workers = Array.from({ length: limit }, async () => {
    while (cursor < items.length) {
      const i = cursor++;
      results[i] = await fn(items[i]).catch(e => ({ error: e }));
    }
  });
  await Promise.all(workers);
  return results;
}
```

### AbortController for cancellation
```javascript
const ac = new AbortController();
fetch('/slow', { signal: ac.signal })
  .then(r => r.json())
  .catch(e => { if (e.name === 'AbortError') console.log('cancelled'); });
setTimeout(() => ac.abort(), 3000);
```

### Async iterator drain
```javascript
async function* readChunks(stream) {
  const reader = stream.getReader();
  try {
    while (true) {
      const { done, value } = await reader.read();
      if (done) return;
      yield value;
    }
  } finally { reader.releaseLock(); }
}

for await (const chunk of readChunks(resp.body)) {
  process(chunk);
}
```

### Avoid microtask starvation
```javascript
// BAD — recursive Promise.resolve() blocks rendering
function bad() {
  Promise.resolve().then(bad);   // browser never paints
}
// GOOD — use setTimeout(0) or MessageChannel for yielding
function good() {
  setTimeout(good, 0);
}
```

### Event loop lag monitor (Node)
```javascript
import { monitorEventLoopDelay } from 'node:perf_hooks';
const h = monitorEventLoopDelay({ resolution: 20 });
h.enable();
setInterval(() => {
  console.log('p99 lag ms', h.percentile(99) / 1e6);
  h.reset();
}, 1000);
```

### Top-level await (ESM)
```javascript
// module.mjs
const config = await fetch('/config.json').then(r => r.json());
export default config;
// importer awaits parent module graph — beware deadlock cycles
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| 매 즉시 yield 필요 | queueMicrotask / Promise.resolve().then |
| 매 paint 후 작업 | requestAnimationFrame |
| 매 idle 시간 작업 | requestIdleCallback / scheduler.postTask('background') |
| 매 long task chunking | scheduler.yield() (modern) |
| 매 cancellation | AbortController + signal |
| 매 backpressure 측정 | perf_hooks.monitorEventLoopDelay |

**기본값**: async/await + AbortController. 매 long task 는 매 scheduler.yield. 매 nextTick / process.nextTick 남용 X.

## 🔗 Graph
- 부모: [[JavaScript]] · [[Concurrency]]
- 변형: [[Node.js Event Loop]] · [[Browser Rendering]]
- 응용: [[React Concurrent Mode]] · [[Web Workers]]
- Adjacent: [[Promises]] · [[AbortController]] · [[Streams]]

## 🤖 LLM 활용
**언제**: 매 race / ordering bug 분석, 매 long-task profiling, 매 SSR streaming 설계.
**언제 X**: 매 CPU-bound 작업 — 매 worker / native 로 offload.

## ❌ 안티패턴
- **Recursive microtask loop**: 매 rendering starvation.
- **await in tight for loop**: 매 직렬화. 매 Promise.all 사용.
- **forgotten unhandled rejection**: 매 process crash (Node 15+).
- **setTimeout(fn, 0) for ordering**: 매 microtask 와 매 race — queueMicrotask 사용.

## 🧪 검증 / 중복
- Verified (HTML spec — Event Loop Processing Model, Node.js docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — 매 task/microtask order + scheduler.yield 패턴 |