2nd/10_Wiki/Topic_JavaScript/JavaScript_Async.md

---
id: javascript-async
title: "JavaScript Async"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["Async programming", "JS async basics", "Synchronous vs asynchronous", "Async execution order", "Non-blocking code"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.87
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["javascript", "js", "web", "frontend", "w3schools", "asynchronous", "settimeout"]
raw_sources: ["https://www.w3schools.com/js/js_async.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Async]]

## 🎯 한 줄 통찰 (One-line insight)
JavaScript runs line by line by default, but async code can start a long-running task and keep working — so the visible order of results may differ from the source order. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Default execution is top-to-bottom, left-to-right** — JavaScript normally executes one line at a time in source order. [S1]
- **Async reorders work** — async code lets a program start a long-running task (like fetching data from a file) and continue with other tasks before the first finishes, which can change the execution order. [S1]
- **The result-not-ready problem** — reading an async result too early yields `undefined` because the async code has not finished yet. [S1]
- **Async is not parallel** — parallel runs multiple things simultaneously on different processors; async switches between tasks without necessarily running them at the same time. [S1]
- **Six building blocks** — synchronous flow, timers, callbacks, events, promises, and async/await. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Order swap by call order** — calling `mySecond()` before `myFirst()` simply changes which output appears first; ordinary synchronous calls obey source order. [S1]
- **Deferred call via setTimeout** — wrapping a call in `setTimeout(fn, ms)` pushes it after the synchronous lines, yielding A, C, B. [S1]
- **Event-driven callback** — an HTML attribute like `onclick` wires user interaction to a function that runs later. [S1]

## 📖 세부 내용 (Details)
**Synchronous calls run in order**
Plain calls produce output A, B, C: [S1]
```javascript
myDisplayer("A");
myDisplayer("B");
myDisplayer("C");
```

**Function order matters**
Defining two functions and calling them in order: [S1]
```javascript
function myFirst() {
  myDisplayer("Hello");
}

function mySecond() {
  myDisplayer("Goodbye");
}

myFirst();
mySecond();
```
Swapping the call order swaps the output: [S1]
```javascript
function myFirst() {
  myDisplayer("Hello");
}

function mySecond() {
  myDisplayer("Goodbye");
}

mySecond();
myFirst();
```

**Asynchronous flow with setTimeout**
The deferred `B` runs after the synchronous `C`, so the order is A, C, B: [S1]
```javascript
myDisplayer("A");

setTimeout(function() {
  myDisplayer("B");
}, 1000);

myDisplayer("C");
```

**The result-not-ready trap**
The result is `undefined` because the async code has not finished yet: [S1]
```javascript
let result;

setTimeout(function() {
  result = 5;
}, 1000);

// What is result here?
```

**JavaScript events**
Events wire callbacks to user interaction: [S1]
```javascript
<button onclick="displayDate()">The time is?</button>
```

**Core async concepts**
[S1]

| Concept | Description |
|---------|-------------|
| Synchronous | The JavaScript standard flow is executing line by line |
| Timers | Allows code to run while other code is waiting |
| Callbacks | Callbacks were the first solution for async JavaScript |
| Events | Stores a callback function waiting to be executed |
| Promises | Tools to handle asynchronous operations cleanly |
| Async/Await | The clean and modern way to handle async code |

**Asynchronous vs parallel**
Parallel means doing multiple things at the same time on different processors. Asynchronous means switching between tasks, not necessarily running them simultaneously. [S1]

**Restaurant analogy**
Place your order (async call); sit down and do other things while the chef makes it; the server brings the food (the callback). [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets — ordered/reordered function calls, the deferred A/C/B `setTimeout`, the `undefined` result trap, and the `onclick` event button — are the canonical applied examples. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Deferred execution that reorders output (language: JavaScript):
```javascript
myDisplayer("A");

setTimeout(function() {
  myDisplayer("B");
}, 1000);

myDisplayer("C");
```
Event-driven callback:
```javascript
<button onclick="displayDate()">The time is?</button>
```

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source.

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.87
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Asynchronous]], [[JavaScript Async Timeouts]], [[JavaScript Async Callbacks]], [[JavaScript Promise]]
- **참조 맥락:** The foundational lesson establishing why execution order matters before introducing timeouts, callbacks, and promises.

## 📚 출처 (Sources)
- [S1] W3Schools — JavaScript Async — https://www.w3schools.com/js/js_async.asp

## 📝 변경 이력 (Change history)
- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Async" page (Astra wiki-curation, P-Reinforce v3.1 format).