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_Symbols.md

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+---
+id: javascript-symbols
+title: "JavaScript Symbols"
+category: "Frontend"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["JS symbols", "Symbol()", "Symbol.for", "Symbol.iterator", "unique identifier"]
+duplicate_of: ""
+source_trust_level: "B"
+confidence_score: 0.87
+created_at: 2026-06-23
+updated_at: 2026-06-23
+review_reason: ""
+merge_history: []
+tags: ["javascript", "js", "web", "frontend", "w3schools", "symbol", "data-types", "unique-identifier"]
+raw_sources: ["https://www.w3schools.com/js/js_datatypes_symbol.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[JavaScript Symbols]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+A Symbol is a primitive that represents a unique identifier — every Symbol value is distinct even with identical descriptions — making it ideal for hidden, collision-free object property keys. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **Symbol = unique identifier** — a Symbol represents a unique identifier; every Symbol value is distinct, even when created with the same description. [S1]
+- **Hidden identifiers** — Symbols act as hidden identifiers that no code can accidentally access, preventing property name conflicts when multiple developers work with shared objects. [S1]
+- **Optional description** — `Symbol("id")` attaches a description (label) without affecting uniqueness. [S1]
+- **`Symbol.for()` is global/shared** — symbols created with `Symbol.for(key)` are looked up in a global registry, so two `Symbol.for("id")` calls return the same symbol. [S1]
+- **`typeof` a symbol is `"symbol"`** — Symbols have their own primitive type. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **Symbol as object key** — use `person[id] = 123;` (with `id` a Symbol) to add a property that won't collide with string keys. [S1]
+- **Hidden from enumeration/serialization** — Symbol properties are excluded from `for...in` loops and ignored by `JSON.stringify()`. [S1]
+- **`Symbol.iterator` makes objects iterable** — define a `[Symbol.iterator]()` method returning a `next()`-based iterator to support `for...of`. [S1]
+
+## 📖 세부 내용 (Details)
+**JavaScript Symbols**
+A Symbol represents a unique identifier. Symbols are hidden identifiers that no code can accidentally access, which helps prevent accidental property conflicts. Every Symbol value is distinct, even with identical descriptions. [S1]
+
+**Creating Symbols** [S1]
+```javascript
+const id1 = Symbol();
+const id2 = Symbol();
+```
+
+**Symbol Descriptions** [S1]
+```javascript
+const id = Symbol("id");
+```
+
+**Uniqueness** — two symbols with the same description are still different: [S1]
+```javascript
+const id1 = Symbol("id");
+const id2 = Symbol("id");
+let result = (id1 === id2);
+```
+
+**Symbol as Object Property Key** [S1]
+```javascript
+const id = Symbol("id");
+const person = {
+  firstName: "John",
+  lastName: "Doe"
+};
+person[id] = 123;
+```
+
+**Type Checking** — `typeof` of a symbol is `"symbol"`: [S1]
+```javascript
+const id = Symbol("id");
+let type = typeof id;
+```
+
+**Global Symbols** — `Symbol.for()` reuses a shared symbol from a global registry: [S1]
+```javascript
+const id1 = Symbol.for("id");
+const id2 = Symbol.for("id");
+let result = (id1 === id2);
+```
+
+**`Symbol.iterator` Implementation** — define a custom iterator so the object works with `for...of`: [S1]
+```javascript
+const myObject = {
+  data: ["A", "B", "C"],
+  [Symbol.iterator]() {
+    let index = 0;
+    let data = this.data;
+    return {
+      next() {
+        if (index < data.length) {
+          return {value:data[index++], done:false};
+        } else {
+          return {done:true};
+        }
+      }
+    };
+  }
+};
+let text = "";
+for (const x of myObject) {
+  text += x + "<br>";
+}
+```
+
+**Notes on behavior**
+- **`for...in` loops** — Symbol properties are excluded from `for...in` iteration, maintaining their hidden nature. [S1]
+- **JSON serialization** — `JSON.stringify()` ignores Symbol properties entirely. [S1]
+- **When to use** — recommended for unique property names, hidden properties, custom iterables, and special behaviors, but cautioned against routine use in everyday code. [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+The page's own snippets are the canonical applied examples — creating symbols, using one as a `person[id]` key, the `Symbol.for()` global lookup, and the `Symbol.iterator` custom iterable. No external project/commit applications found in the source.
+
+## 💻 코드 패턴 (Code patterns)
+Use a Symbol as a collision-free property key (language: JavaScript):
+```javascript
+const id = Symbol("id");
+person[id] = 123;
+```
+Shared global symbol:
+```javascript
+const id1 = Symbol.for("id");
+const id2 = Symbol.for("id");
+let result = (id1 === id2);
+```
+
+## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
+- **`Symbol()` vs `Symbol.for()`** — `Symbol()` always creates a brand-new, unique symbol (even with the same description); `Symbol.for(key)` returns a shared symbol from the global registry, so repeated calls with the same key are equal. Use `Symbol()` for truly private keys and `Symbol.for()` when the same symbol must be retrievable across code. [S1]
+- **Symbol key vs string key** — Symbol keys are hidden from `for...in` and `JSON.stringify()`, unlike string keys; choose symbols when a property should not be enumerated or serialized, but prefer ordinary keys for everyday data. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+No contradictions found in the source. The page advises against routine use of Symbols in everyday code. [S1]
+
+## ✅ 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:** 0.87
+- **중복 검사 결과:** 신규 생성 (New discovery)
+
+## 🔗 지식 그래프 (Knowledge Graph)
+- **상위/루트:** [[JavaScript Tutorial]]
+- **관련 개념:** [[JavaScript Data Types]], [[JavaScript Iterables]], [[JavaScript Iterators]], [[JavaScript typeof]]
+- **참조 맥락:** Referenced when defining hidden/unique object keys or implementing `Symbol.iterator` for custom iterables.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — JavaScript Symbols — https://www.w3schools.com/js/js_datatypes_symbol.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Symbols" page (Astra wiki-curation, P-Reinforce v3.1 format).