---
id: javascript-asynchronous
title: "JavaScript Asynchronous"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["Async JavaScript", "Asynchronous programming", "JS async overview", "Async study path", "Non-blocking JavaScript"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.86
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["javascript", "js", "web", "frontend", "w3schools", "asynchronous", "concurrency"]
raw_sources: ["https://www.w3schools.com/js/js_asynchronous.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Asynchronous]]

## 🎯 한 줄 통찰 (One-line insight)
Asynchronous code lets a program start a long-running task and continue with other work before that task finishes, instead of executing strictly one blocking line at a time. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Default flow is synchronous** — JavaScript executes code one line at a time, and each line must finish before the next can run. [S1]
- **Async changes the order** — async code allows a program to start a long-running task (like fetching data from a file) and continue with other tasks before the first one finishes. [S1]
- **A learning progression** — the page frames async as a curriculum to "Learn Asynchronous JavaScript in the Right Order," moving from timeouts → callbacks → promises → async/await → fetch → debugging. [S1]
- **Async ≠ parallel** — parallel means doing multiple things at the same time on different processors; asynchronous means switching between tasks, not necessarily running them simultaneously. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Defer-and-continue** — schedule work to run later (e.g. via a timer) so the current call stack can finish first, producing an out-of-order result such as A, C, B. [S1]
- **Result-not-ready trap** — reading the result of an async operation immediately yields `undefined` because the async code has not finished yet. [S1]
- **Restaurant model** — place the order (async call), do other things while it is prepared, and the server brings the food (the callback). [S1]

## 📖 세부 내용 (Details)
**What is asynchronous code?**
Async code allows a program to start a long-running task (like fetching data from a file) and continue with other tasks before the first one finishes. By default JavaScript executes from top to bottom, one line at a time; each line must finish before the next can run. Async programming can change this execution order. [S1]

**Synchronous flow**
Calls run in source order, producing output A, B, C: [S1]
```javascript
myDisplayer("A");
myDisplayer("B");
myDisplayer("C");
```

**Asynchronous flow with setTimeout**
When a timer defers a call, the output order changes to A, C, B — the deferred `B` runs after the synchronous `C`: [S1]
```javascript
myDisplayer("A");

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

myDisplayer("C");
```

**The "result is not ready yet" challenge**
Beginners often expect async results immediately. Here 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?
```

**Core async concepts**
The module covers these building blocks: [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 itself is an index/study path; its illustrative snippets (synchronous A/B/C, deferred A/C/B, and the `undefined` result trap) are the canonical applied examples. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Deferring work so later lines run first (language: JavaScript):
```javascript
myDisplayer("A");

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

myDisplayer("C");
```
Demonstrating the not-yet-ready result:
```javascript
let result;

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

// What is result here?
```

## ⚖️ 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:** 0.86
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Async Timeouts]], [[JavaScript Async Callbacks]], [[JavaScript Promise]], [[JavaScript Async Await]], [[JavaScript Async Fetch]]
- **참조 맥락:** The conceptual entry point for the entire asynchronous JavaScript study path.

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

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