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>
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id: javascript-arrow-functions
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title: "JavaScript Arrow Functions"
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category: "Frontend"
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status: "draft"
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verification_status: "conceptual"
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canonical_id: ""
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aliases: ["JS arrow functions", "arrow function syntax", "fat arrow", "=> functions", "lexical this", "concise functions"]
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duplicate_of: ""
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source_trust_level: "B"
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confidence_score: 0.88
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created_at: 2026-06-23
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updated_at: 2026-06-23
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review_reason: ""
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merge_history: []
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tags: ["javascript", "js", "web", "frontend", "w3schools", "functions", "arrow-functions", "this"]
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raw_sources: ["https://www.w3schools.com/js/js_arrow_function.asp"]
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applied_in: []
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github_commit: ""
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---
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# [[JavaScript Arrow Functions]]
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## 🎯 한 줄 통찰 (One-line insight)
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Arrow functions give a shorter syntax for function expressions and do not have their own `this` — they inherit it from the surrounding code, which makes them great for callbacks but poor as object methods. [S1]
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## 🧠 핵심 개념 (Core concepts)
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- **Shorter function expressions** — arrow functions let you skip the `function` keyword, the `return` keyword, and the curly brackets. [S1]
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- **Always an expression** — an arrow function uses `=>` and is always written as a function expression. [S1]
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- **Implicit return** — a single-statement body returns its value automatically without `return` or braces. [S1]
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- **Parentheses rules** — one parameter can omit parentheses; zero or multiple parameters require them. [S1]
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- **No own `this`** — arrow functions do not have their own `this`; they inherit it from the surrounding code, so they are unsuitable as object methods. [S1]
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- **Not hoisted** — arrow functions are expressions, must be assigned to a variable, and cannot be used before they are defined. [S1]
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## 🧩 추출된 패턴 (Extracted patterns)
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- **Concise one-liner** — `const multiply = (a, b) => a * b;` collapses an expression into a single statement with implicit return. [S1]
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- **Single-param shorthand** — drop the parentheses for exactly one parameter: `const square = x => x * x;`. [S1]
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- **Always-keep-braces habit** — because implicit return only works for a single statement, keeping `{ return ... }` avoids accidental `undefined`. [S1]
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## 📖 세부 내용 (Details)
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Arrow functions allow a shorter syntax for function expressions. You can skip the `function` keyword, the `return` keyword, and the curly brackets. [S1]
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**Arrow Function Syntax**
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An arrow function uses the `=>` symbol and is always written as a function expression. [S1]
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```javascript
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const add = (a, b) => {
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return a + b;
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};
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```
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**Shorter Syntax**
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If the function body contains only one statement, you can remove the word `function`, the curly brackets, and the `return` keyword. Before arrow: [S1]
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```javascript
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const multiply = function(a, b) {return a * b}
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```
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With arrow: [S1]
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```javascript
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const multiply = (a, b) => a * b;
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```
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With arrow: [S1]
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```javascript
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const hello = () => "Hello World!";
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```
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Before arrow: [S1]
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```javascript
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const hello = function() {return "Hello World!";}
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```
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**Arrow Functions with One Parameter**
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If a function has only one parameter, you can omit the parentheses. With parentheses: [S1]
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```javascript
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const square = (x) => x * x;
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```
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Without parentheses: [S1]
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```javascript
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const square = x => x * x;
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```
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With parentheses: [S1]
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```javascript
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const hello = (val) => "Hello " + val;
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```
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Without parentheses: [S1]
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```javascript
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const hello = val => "Hello " + val;
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```
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**Arrow Functions Return Value by Default**
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If the function has only one statement that returns a value, you can remove the brackets and the `return` keyword. This works only if the function has only one statement. [S1]
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```javascript
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const hello = () => "Hello World!";
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```
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**Arrow Function Parameters**
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If you have parameters, you pass them inside the parentheses: [S1]
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```javascript
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const hello = (val) => "Hello " + val;
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```
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If you have only one parameter, you can skip the parentheses as well: [S1]
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```javascript
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const hello = val => "Hello " + val;
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```
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**Arrow Functions with No Parameters**
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If there are no parameters, parentheses are required: [S1]
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```javascript
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const hello = () => "Hello World!";
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```
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You can only omit the `return` keyword and the curly brackets if the function is a single statement. Because of this, it might be a good habit to always keep them: [S1]
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```javascript
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// This will return undefined
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const myFunction = (x, y) => { x * y };
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// This will return undefined
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const myFunction = (x, y) => return x * y;
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// This will return the expected result
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const myFunction = (x, y) => { return x * y };
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```
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**Arrow Functions Are Not Declarations**
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Arrow functions are always expressions and must be assigned to a variable. They cannot be used before they are defined: [S1]
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```javascript
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hello(); // Error
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const hello = () => "Hello";
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```
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Arrow functions must be defined before they are used. [S1]
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**Arrow Functions and the this Keyword**
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Arrow functions do not have their own `this` value. They inherit `this` from the surrounding code. [S1]
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```javascript
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const person = {
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name: "John",
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greet: function() {
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return this.name;
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}
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};
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```
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Using an arrow function as a method often gives unexpected results: [S1]
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```javascript
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const person = {
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name: "John",
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greet: () => {
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return this.name;
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}
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};
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```
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In this case, `this` does not refer to the person object. [S1]
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**When to Use Arrow Functions**
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For short functions; for callbacks and array methods; when you do not need your own `this`. [S1]
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**When Not to Use Arrow Functions**
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As object methods; when you need your own `this`; when using function declarations. [S1]
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**Common Mistakes**
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Forgetting parentheses rules (parentheses are required for zero or multiple parameters); using arrow functions as methods (arrow functions do not bind `this`); expecting hoisting (arrow functions are not hoisted). [S1]
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## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
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| Situation | Use arrow function? | Reason |
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| --- | --- | --- |
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| Short functions, callbacks, array methods | Yes | Concise syntax; no own `this` needed | [S1]
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| Object methods | No | Arrow functions do not bind `this` to the object | [S1]
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| Need your own `this` | No | Arrow `this` is inherited from surrounding code | [S1]
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| Function declarations | No | Arrow functions are always expressions, not declarations | [S1]
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## 🛠️ 적용 사례 (Applied in summary)
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The page's own snippets are the canonical applied examples — `multiply`, `square`, and `hello` concise forms, plus the `person.greet` method comparison demonstrating the `this` pitfall. No external project/commit applications found in the source.
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## 💻 코드 패턴 (Code patterns)
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Concise single-expression arrow:
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```javascript
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const multiply = (a, b) => a * b;
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```
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Single-parameter without parentheses:
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```javascript
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const square = x => x * x;
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```
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Explicit-return habit (avoids accidental undefined):
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```javascript
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const myFunction = (x, y) => { return x * y };
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```
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## ⚖️ 모순 및 업데이트 (Contradictions & updates)
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No contradictions found in the source.
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## ✅ 검증 상태 및 신뢰도
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- **상태:** draft
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- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
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- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
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- **신뢰 점수:** 0.88
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- **중복 검사 결과:** 신규 생성 (New discovery)
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## 🔗 지식 그래프 (Knowledge Graph)
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- **상위/루트:** [[JavaScript Tutorial]]
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- **관련 개념:** [[JavaScript Function Expressions]], [[JavaScript Function Arguments]], [[JavaScript Object this]]
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- **참조 맥락:** Referenced when writing concise callbacks and when reasoning about `this` binding in object methods.
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## 📚 출처 (Sources)
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- [S1] W3Schools — JavaScript Arrow Functions — https://www.w3schools.com/js/js_arrow_function.asp
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## 📝 변경 이력 (Change history)
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- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Arrow Functions" page (Astra wiki-curation, P-Reinforce v3.1 format).
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