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>

10_Wiki/Topic_JavaScript/JavaScript_undefined.md

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+---
+id: javascript-undefined
+title: "JavaScript undefined"
+category: "Frontend"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["undefined", "JS undefined", "undefined value", "typeof undefined", "unassigned variable"]
+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", "undefined", "data-types"]
+raw_sources: ["https://www.w3schools.com/js/js_undefined.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[JavaScript undefined]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+`undefined` is the value a variable automatically receives when it has been declared but no value has been assigned — a distinct JavaScript value, separate from `null` and from an empty string. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **Declared-but-unassigned** — A variable declared without a value automatically has the value `undefined`. [S1]
+- **`typeof` reports it** — You can use `typeof` to check if a variable is undefined. [S1]
+- **Empty string is not undefined** — An empty string has a value and a type; it is not the same as `undefined`. [S1]
+- **Missing object property** — Accessing a non-existing object property returns `undefined`. [S1]
+- **No-return function** — A function without a return value returns `undefined`. [S1]
+- **Emptying an object** — Objects can be emptied by setting the value to `undefined` (or alternatively to `null`). [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **Detect unset state with `typeof`** — Use `typeof x` to distinguish a declared-but-unassigned variable from one holding a value. [S1]
+- **`undefined` as a default signal** — Missing object properties and value-less functions both surface as `undefined`, making it the de facto "absence of value" marker. [S1]
+- **Empty vs. undefined** — An empty string `""` is a real value with type `"string"`, distinct from `undefined`. [S1]
+
+## 📖 세부 내용 (Details)
+**Undefined Variables**
+A variable declared without a value will automatically have the value `undefined`. [S1]
+```javascript
+let car;
+
+document.getElementById("demo").innerHTML = car;
+```
+
+**The Type of undefined**
+You can use `typeof` to check if a variable is undefined. [S1]
+```javascript
+let car;
+
+document.getElementById("demo").innerHTML = typeof car;
+```
+
+**Empty Values**
+An empty string has a value and a type. It is not the same as `undefined`. [S1]
+```javascript
+let text = "";
+
+document.getElementById("demo").innerHTML =
+text + "<br>" + typeof text;
+```
+
+**Objects and undefined**
+Accessing a non-existing object property returns `undefined`. [S1]
+```javascript
+const person = {firstName:"John", lastName:"Doe"};
+
+document.getElementById("demo").innerHTML = person.age;
+```
+
+**Functions and undefined**
+A function without a return value returns `undefined`. [S1]
+```javascript
+function myFunction() {
+  let x = 5;
+}
+
+document.getElementById("demo").innerHTML = myFunction();
+```
+
+**You Can Empty an Object**
+Objects can be emptied by setting the value to `undefined`. Objects can also be emptied by setting the value to `null`. [S1]
+```javascript
+const person = {firstName:"John", lastName:"Doe"};
+
+person = undefined;
+```
+
+**Note**
+"`undefined` is a JavaScript value. It means that a variable has been declared, but no value has been assigned." [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+The page's own snippets are the canonical applied examples — reading an unassigned `car`, checking `typeof car`, and accessing the missing `person.age`. No external project/commit applications found in the source.
+
+## 💻 코드 패턴 (Code patterns)
+Check whether a variable is undefined (language: JavaScript):
+```javascript
+let car;
+document.getElementById("demo").innerHTML = typeof car;
+```
+Detect a missing object property:
+```javascript
+const person = {firstName:"John", lastName:"Doe"};
+document.getElementById("demo").innerHTML = person.age;
+```
+
+## ⚖️ 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 NaN]], [[JavaScript Type Conversion]], [[JavaScript Object Types Note]], [[JavaScript Introduction]]
+- **참조 맥락:** Referenced whenever checking for missing values, optional object properties, or void function returns.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — JavaScript undefined — https://www.w3schools.com/js/js_undefined.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript undefined" page (Astra wiki-curation, P-Reinforce v3.1 format).