2nd/10_Wiki/Topic_JavaScript/JavaScript_Async_Await.md

---
id: javascript-async-await
title: "JavaScript Async Await"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["async await", "await keyword", "async function", "try catch await", "Promise.all"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.89
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["javascript", "js", "web", "frontend", "w3schools", "async-await", "promises"]
raw_sources: ["https://www.w3schools.com/js/js_async_await.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Async Await]]

## 🎯 한 줄 통찰 (One-line insight)
`async` and `await` make promises easier by letting you write asynchronous code like normal step-by-step code, with familiar `try...catch` error handling. [S1]

## 🧠 핵심 개념 (Core concepts)
- **They simplify promises** — `async` and `await` make promises easier by letting code be written like normal step-by-step code. [S1]
- **`async` returns a promise** — an `async` function returns a promise; its return value is what the promise resolves to. [S1]
- **`await` unwraps a promise** — `await` pauses inside the async function until the awaited promise resolves, giving back the value directly. [S1]
- **Error handling with try...catch** — wrap awaited calls in `try { ... } catch (error) { ... }` to handle rejections. [S1]
- **Sequential vs parallel** — awaiting one call after another runs them in sequence; `Promise.all([...])` runs them together and awaits all. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Linearize a promise chain** — replace `.then()` chaining with successive `await` calls inside an `async function`. [S1]
- **try/catch around await** — catch a rejected promise (e.g. `await fail()`) like a synchronous error. [S1]
- **Promise.all for parallelism** — start promises first, then `await Promise.all([p1, p2])` to collect results together. [S1]

## 📖 세부 내용 (Details)
**Promises approach (before)**
[S1]
```javascript
// Three functions to run in steps
function step1() {
  return Promise.resolve("A");
}
function step2(value) {
  return Promise.resolve(value + "B");
}
function step3(value) {
  return Promise.resolve(value + "C");
}

// Run the three functions in steps
step1()
.then(function(value) {
  return step2(value);
})
.then(function(value) {
  return step3(value);
})
.then(function(value) {
  myDisplayer(value);
});
```

**async/await approach (after)**
The same logic reads like sequential code: [S1]
```javascript
// Function to run the three functions in steps
async function run() {
  let v1 = await step1();
  let v2 = await step2(v1);
  let v3 = await step3(v2);
  myDisplayer(v3);
}

run();
```

**The async keyword**
An `async` function returns a promise: [S1]
```javascript
async function myFunction() {
  return "Hello";
}
myFunction().then(
  function(value) {myDisplayer(value);}
);
```

**The await keyword**
`await` waits for a promise and returns its resolved value: [S1]
```javascript
function step1() {
  return Promise.resolve("A");
}

async function run() {
  let value = await step1();
  myDisplayer(value);
}

run();
```

**Error handling with try...catch**
[S1]
```javascript
function fail() {
  return Promise.reject("Failed");
}

async function run() {
  try {
    let value = await fail();
    console.log(value);
  } catch (error) {
    console.log(error);
  }
}

run();
```

**Sequential execution**
[S1]
```javascript
async function run() {
  let a = await step1();
  let b = await step2();
  console.log(a, b);
}
```

**Parallel with Promise.all()**
Start the promises first, then await them together: [S1]
```javascript
async function run() {
  let p1 = step1();
  let p2 = step2();
  let values = await Promise.all([p1, p2]);
  console.log(values);
}
```

**fetch() with async/await**
[S1]
```javascript
async function loadData() {
  try {
    let response = await fetch("data.json");
    let data = await response.json();
    console.log(data);
  } catch (error) {
    console.log(error);
  }
}

loadData();
```

**Basic syntax with a Promise**
Awaiting a constructed promise: [S1]
```javascript
async function myDisplay() {
  let myPromise = new Promise(function(resolve, reject) {
    resolve("I love You !!");
  });
  document.getElementById("demo").innerHTML = await myPromise;
}

myDisplay();
```
Without the reject parameter: [S1]
```javascript
async function myDisplay() {
  let myPromise = new Promise(function(resolve) {
    resolve("I love You !!");
  });
  document.getElementById("demo").innerHTML = await myPromise;
}

myDisplay();
```

**Waiting for a timeout**
[S1]
```javascript
async function myDisplay() {
  let myPromise = new Promise(function(resolve) {
    setTimeout(function() {resolve("I love You !!");}, 3000);
  });
  document.getElementById("demo").innerHTML = await myPromise;
}

myDisplay();
```

## 🛠️ 적용 사례 (Applied in summary)
The page's snippets — the `run()` function linearizing step1/step2/step3, the `try...catch` around `await fail()`, and `await Promise.all([p1, p2])` — are the canonical applied examples. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Linearize a chain with await (language: JavaScript):
```javascript
async function run() {
  let v1 = await step1();
  let v2 = await step2(v1);
  let v3 = await step3(v2);
  myDisplayer(v3);
}

run();
```
Error handling and parallelism:
```javascript
async function run() {
  try {
    let values = await Promise.all([step1(), step2()]);
    console.log(values);
  } catch (error) {
    console.log(error);
  }
}
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
- **Promise `.then()` chaining** — explicit, but reads as nested handler functions and requires returning each step. [S1]
- **async/await** — the same logic written like normal step-by-step code, with `try...catch` for errors; recommended for readability. Use sequential `await` for dependent steps and `Promise.all` for independent ones. [S1]

## ⚖️ 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:** 0.89
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Promise]], [[JavaScript Async Fetch]], [[JavaScript Async Callbacks]], [[JavaScript Async Debug]]
- **참조 맥락:** The syntactic sugar over promises that most modern async data-fetching code uses.

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

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