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_Variables.md

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+---
+id: javascript-variables
+title: "JavaScript Variables"
+category: "Frontend"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["JS variables", "var let const", "identifiers", "assignment operator", "data containers"]
+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", "variables", "var-let-const"]
+raw_sources: ["https://www.w3schools.com/js/js_variables.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[JavaScript Variables]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+Variables are containers for storing data, declared in four ways (`var`, `let`, `const`, or automatically) — and the modern guidance is to prefer `const`, fall back to `let`, and avoid `var`. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **Variables are data containers** — they store data values. [S1]
+- **Four ways to declare** — automatically, using `var`, using `let`, or using `const`. [S1]
+- **Identifier rules** — names can contain letters, digits, underscores, and dollar signs; must begin with a letter, `$`, or `_`; are case-sensitive; reserved words cannot be used. [S1]
+- **`$` is treated like a letter** — the dollar sign is treated as a letter in identifiers and is often used as an alias for the main function in JavaScript libraries; `_` is sometimes used to denote "private" variables. [S1]
+- **Assignment vs equality** — the `=` operator assigns a value (it does not mean "equal to"). [S1]
+- **`+` is overloaded** — with numbers it adds; with strings it concatenates. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **const-first selection** — always declare with `const` if the value should not change (including new Arrays, Objects, Functions); use `let` when the value may change; avoid `var`. [S1]
+- **One statement, many variables** — declare several variables in one statement, separated by commas, optionally across multiple lines. [S1]
+- **String-vs-number `+` evaluation** — once a string appears in a `+` chain, subsequent operands are concatenated, so order and types matter. [S1]
+
+## 📖 세부 내용 (Details)
+**Variables = Data Containers** — Variables are containers for storing data (storing data values). [S1]
+
+A variable can be declared four ways. Using `let`: [S1]
+```javascript
+let x = 5;
+let y = 6;
+let z = x + y;
+```
+Using `const`: [S1]
+```javascript
+const x = 5;
+const y = 6;
+const z = x + y;
+```
+
+**JavaScript Identifiers** — All JavaScript variables must be identified with unique names. These unique names are called identifiers. The general rules for constructing names for variables (unique identifiers) are: names can contain letters, digits, underscores, and dollar signs; names must begin with a letter, `$`, or `_`; names are case-sensitive (`y` and `Y` are different variables); and reserved words (like JavaScript keywords) cannot be used as names. [S1]
+
+**JavaScript Underscore (`_`)** — Since JavaScript treats the underscore as a letter, identifiers containing `_` are valid variable names. Some programmers like to use underscores to denote "private (hidden)" variables. [S1]
+
+**JavaScript Dollar Sign (`$`)** — Since JavaScript treats the dollar sign as a letter, identifiers containing `$` are valid variable names. Using the dollar sign is not very common in JavaScript, but professional programmers often use it as an alias for the main function in a JavaScript library. [S1]
+
+**When to Use var, let, or const?** — The source's guidance: always declare variables; always use `const` if the value should not be changed; always use `const` if the type should not be changed (Arrays and Objects); only use `let` if you can't use `const`; only use `var` if you MUST support old browsers. [S1]
+
+**One Statement, Many Variables** — You can declare many variables in one statement. Start the statement with `let` and separate the variables by comma: [S1]
+```javascript
+let person = "John Doe", carName = "Volvo", price = 200;
+```
+A declaration can span multiple lines: [S1]
+```javascript
+let person = "John Doe",
+carName = "Volvo",
+price = 200;
+```
+
+**The Assignment Operator** — In JavaScript, the `=` sign is an "assignment" operator, not an "equal to" operator. [S1]
+
+**JavaScript Arithmetic** — You can do arithmetic with JavaScript variables, using operators like `=` and `+`. The value of `x` differs depending on whether operands are numbers or strings: [S1]
+```javascript
+let x = 5 + 2 + 3;
+```
+```javascript
+let x = "John" + " " + "Doe";
+```
+If you put a number in quotes, the rest of the numbers will be treated as strings and concatenated: [S1]
+```javascript
+let x = "5" + 2 + 3;
+```
+```javascript
+let x = 2 + 3 + "5";
+```
+
+## 🛠️ 적용 사례 (Applied in summary)
+The page's own snippets are the canonical applied examples — `let`/`const` declarations computing `x + y`, the comma-separated multi-variable statement, and the `+` arithmetic vs concatenation cases. No external project/commit applications found in the source.
+
+## 💻 코드 패턴 (Code patterns)
+Declare and compute:
+```javascript
+let x = 5;
+let y = 6;
+let z = x + y;
+```
+Many variables, one statement:
+```javascript
+let person = "John Doe", carName = "Volvo", price = 200;
+```
+
+## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
+The source gives explicit selection rules among the declaration keywords: [S1]
+
+| Keyword | Use when |
+|---------|----------|
+| `const` | The value (or type, for Arrays/Objects) should not change — the default choice |
+| `let` | You cannot use `const` because the value will change |
+| `var` | Only if you MUST support old browsers |
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+No contradictions found in the source. (The page notes `var` was the only option before 2015 (ES6) but is now discouraged — an update in guidance, not a contradiction.)
+
+## ✅ 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 Let]], [[JavaScript Const]], [[JavaScript Syntax]]
+- **참조 맥락:** The foundation for all data handling; refined by the dedicated Let and Const pages.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — JavaScript Variables — https://www.w3schools.com/js/js_variables.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Variables" page (Astra wiki-curation, P-Reinforce v3.1 format).