docs(10_Wiki): W3Schools 위키화 — HTML/CSS/JavaScript(core)

W3Schools 튜토리얼을 P-Reinforce v3.1 포맷으로 위키화(영어 본문, 한/영 섹션 헤더). - Topic_HTML: 59문서 (튜토리얼+예제, 레퍼런스/메타 제외) - Topic_CSS: 190문서 (메인 + Advanced/Flexbox/Grid/RWD 전체) - Topic_JavaScript: 120문서 (코어 언어; Temporal/DOM상세/BOM/WebAPI/AJAX/jQuery/Graphics 등은 후속) 각 폴더 00_INDEX.md(MOC) 포함. 코드 verbatim, 미확인분은 "Not found in source" 표기. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
2026-06-23 19:21:18 +09:00
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+---
+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).