2nd/10_Wiki/Topic_JavaScript/JavaScript_Async_Fetch.md

---
id: javascript-async-fetch
title: "JavaScript Async Fetch"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["fetch", "fetch API", "response.json", "response.ok", "HTTP request JavaScript", "XMLHttpRequest vs fetch"]
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", "fetch", "asynchronous", "http"]
raw_sources: ["https://www.w3schools.com/js/js_async_fetch.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Async Fetch]]

## 🎯 한 줄 통찰 (One-line insight)
`fetch()` is the modern, asynchronous way to request data from a server — it returns a promise for a Response, and you must read the body (and check `response.ok`) to get the actual data. [S1]

## 🧠 핵심 개념 (Core concepts)
- **fetch returns a promise** — `fetch()` is the modern way to request data from a server; it is asynchronous and returns a promise that becomes a response later. [S1]
- **Response is not the data** — the result is a `Response` object, not the JSON data; to get JSON you must read the body with `response.json()`, which itself returns a promise. [S1]
- **async/await is recommended** — `async` and `await` make fetch code easier to read; this is the recommended way for beginners. [S1]
- **404/500 do not reject** — fetch only rejects on network errors; a 404 response is not a rejected promise, so you must check `response.ok`. [S1]
- **Network errors reject** — a network error (offline mode, DNS errors) happens when the request cannot be completed and does reject the promise. [S1]
- **Forgetting await yields a promise** — omitting `await` on `response.json()` logs a promise instead of the data. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **fetch → json promise chain** — `fetch(url).then(r => r.json()).then(data => ...)`. [S1]
- **async/await fetch** — `let response = await fetch(url); let data = await response.json();`. [S1]
- **Guard with response.ok** — check `if (!response.ok)` to handle HTTP errors that fetch will not reject. [S1]
- **try/catch for network errors** — wrap the await calls to catch true network failures. [S1]

## 📖 세부 내용 (Details)
**fetch returns a promise, not the data**
`fetch()` does not return the data; it returns a promise that becomes a response later. The result is a `Response` object, not the JSON data: [S1]
```javascript
fetch("data.json")
.then(function(response) {
  console.log(response);
});
```

**Reading the body**
To get JSON you must read the response body; `response.json()` returns a promise. The following is a promise chain: [S1]
```javascript
fetch("data.json")
.then(function(response) {
  return response.json();
})
.then(function(data) {
  console.log(data);
});
```

**async/await version (recommended)**
`async` and `await` make fetch code easier to read; this is the recommended way for beginners: [S1]
```javascript
async function loadData() {
  let response = await fetch("data.json");
  let data = await response.json();
  console.log(data);
}

loadData();
```

**Checking response.ok (HTTP errors)**
A common beginner mistake is expecting fetch to fail on 404 or 500. Fetch only rejects on network errors; a 404 response is not a rejected promise — you must check `response.ok`. The following handles HTTP errors correctly: [S1]
```javascript
async function loadData() {
  let response = await fetch("missing.json");

  if (!response.ok) {
    console.log("HTTP Error:", response.status);
    return;
  }

  let data = await response.json();
  console.log(data);
}

loadData();
```

**Network errors with try...catch**
A network error happens when the request cannot be completed (offline mode, DNS errors); network errors reject the promise: [S1]
```javascript
async function loadData() {
  try {
    let response = await fetch("data.json");
    let data = await response.json();
    console.log(data);
  } catch (error) {
    console.log("Network error");
  }
}
```

**Forgetting await on response.json()**
Forgetting `await` gives you a promise instead of data; the following logs a promise — you must use `await` to get the JSON: [S1]
```javascript
async function loadData() {
  let response = await fetch("data.json");
  let data = response.json();
  console.log(data);
}
```
Correct version: [S1]
```javascript
async function loadData() {
  let response = await fetch("data.json");
  let data = await response.json();
  console.log(data);
}
```

**Callback (XMLHttpRequest) vs fetch**
Callback approach with XMLHttpRequest: [S1]
```javascript
function loadFile(callback) {
  let xhr = new XMLHttpRequest();
  xhr.open("GET", "data.json", true);
  xhr.onload = function() {
    if (xhr.status === 200) {
      callback(null, JSON.parse(xhr.responseText));
    } else {
      callback("HTTP Error: " + xhr.status, null);
    }
  };
  xhr.onerror = function() {
    callback("Network Error", null);
  };
  xhr.send();
}

loadFile(function(error, data) {
  if (error) {
    console.log(error);
    return;
  }
  console.log(data);
});
```
Fetch approach: [S1]
```javascript
async function loadFile() {
  try {
    let response = await fetch("data.json");

    if (!response.ok) {
      throw new Error("HTTP Error: " + response.status);
    }

    let data = await response.json();
    console.log(data);
  } catch (error) {
    console.log(error);
  }
}

loadFile();
```

**Callback vs Promise/async-await comparison**
[S1]

| Callback | Promise / async-await |
|----------|----------------------|
| You pass a function to run later | You wait for a promise |
| Manual error-first pattern | Built-in error flow |
| Can become nested | Reads like normal code |
| Hard to chain steps | Easy to chain steps |

**Debugging hint**
If fetch is not working, check the console, then the Network tab: is the file path correct, is the status code 200, is the response JSON. Most fetch bugs are not JavaScript bugs — they are path and response problems. [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's snippets — the `fetch().then(r => r.json())` chain, the `await fetch` + `response.ok` guard, and the XMLHttpRequest-vs-fetch comparison — are the canonical applied examples. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Recommended async/await fetch with HTTP-error guard (language: JavaScript):
```javascript
async function loadData() {
  let response = await fetch("missing.json");

  if (!response.ok) {
    console.log("HTTP Error:", response.status);
    return;
  }

  let data = await response.json();
  console.log(data);
}

loadData();
```
Promise-chain form:
```javascript
fetch("data.json")
.then(function(response) { return response.json(); })
.then(function(data) { console.log(data); });
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
- **Callback (XMLHttpRequest)** — pass a function to run later; manual error-first pattern; can become nested and is hard to chain. [S1]
- **Promise / async-await (fetch)** — wait for a promise; built-in error flow; reads like normal code and is easy to chain. Recommended for beginners. [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 Async Await]], [[JavaScript Promise]], [[JavaScript Async Debug]], [[JavaScript Async Callbacks]]
- **참조 맥락:** The practical application of promises and async/await for retrieving server data.

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

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