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-function-expressions
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title: "JavaScript Function Expressions"
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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 function expressions", "anonymous functions", "function declaration vs expression", "hoisting", "named function expression", "callbacks"]
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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", "expressions", "hoisting"]
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raw_sources: ["https://www.w3schools.com/js/js_function_expressions.asp"]
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applied_in: []
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github_commit: ""
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---
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# [[JavaScript Function Expressions]]
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## 🎯 한 줄 통찰 (One-line insight)
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A function expression is a function stored in a variable — it behaves like a value (great for callbacks) but, unlike a function declaration, it is not hoisted and cannot be called before it is defined. [S1]
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## 🧠 핵심 개념 (Core concepts)
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- **A function expression is a function stored in a variable.** [S1]
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- **Often anonymous** — functions stored in variables do not need names; the variable name is used to call the function. They can still be named. [S1]
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- **It is a statement** — a function expression usually ends with a semicolon. [S1]
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- **Used as a value** — because it is stored in a variable, it can be passed to other functions (callbacks). [S1]
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- **Hoisting difference** — function declarations are hoisted and can be called before they are defined; function expressions are not hoisted and cannot. [S1]
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## 🧩 추출된 패턴 (Extracted patterns)
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- **Store-then-call** — assign a function to a `const`, then invoke through the variable name (`sayHello()`). [S1]
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- **Callback passing** — pass a function expression as an argument so another function can invoke it (`run(sayHello)`). [S1]
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- **Define-before-use ordering** — because expressions are not hoisted, declare them above the point of first call. [S1]
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## 📖 세부 내용 (Details)
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**What is a Function Expression?**
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A function expression is a function stored in a variable. A standard function: [S1]
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```javascript
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function multiply(a, b) {
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return a * b;
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}
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```
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A function expression: [S1]
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```javascript
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const multiply = function(a, b) {
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return a * b;
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};
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```
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After storing in a variable, the variable can be used as a function: [S1]
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```javascript
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let z = multiply(4, 3);
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```
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**Anonymous Functions**
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Function expressions commonly create anonymous functions. The function is actually a function without a name; functions stored in variables do not need names — the variable name is used to call the function. However, function expressions can also be named: [S1]
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```javascript
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const add = function add(a, b) {return a + b;};
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```
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**Function Expressions Use Semicolons**
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A function expression is a JavaScript statement. That is why it usually ends with a semicolon. [S1]
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```javascript
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const add = function(a, b) {
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return a + b;
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};
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```
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**Functions Stored in Variables**
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Because a function expression is stored in a variable, it can be used like a value. This is useful when passing functions to other functions (callbacks). [S1]
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```javascript
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function run(fn) {
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return fn();
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}
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const sayHello = function() {
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return "Hello";
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};
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run(sayHello);
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```
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**Functions vs. Function Expressions**
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JavaScript functions are defined with the `function` keyword and can be defined in different ways. They both work the same when you call them; the difference is when they become available in your code. [S1]
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**Function Declarations vs Function Expression**
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A function declaration uses the `function` keyword, has a function name, parameters in parentheses, and a code block in brackets: [S1]
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```javascript
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function add(a, b) {return a + b;}
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```
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A function expression uses the `function` keyword, parameters in parentheses, and a code block in brackets: [S1]
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```javascript
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const add = function(a, b) {return a + b;};
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```
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Example: [S1]
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```javascript
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const sayHello = function() {
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return "Hello World";
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};
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sayHello();
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```
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The function above is stored in the variable `sayHello`. To run it, you call `sayHello()`. [S1]
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**Hoisting**
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Function declarations can be called before they are defined. Function expressions can not be called before they are defined. Function declarations are hoisted: [S1]
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```javascript
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let sum = add(2, 3); // Will work
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function add(a, b) {return a + b;}
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```
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Function expressions are not hoisted: [S1]
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```javascript
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let sum = add(2, 3); // ⛔ Will generate error
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const add = function (a, b) {return a + b;};
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```
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**Key Differences**
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Syntax: function declarations require a name; function expressions can be anonymous. Hoisting: function declarations are hoisted, function expressions are not. Flexibility: function declarations offer more flexibility in how and where they are used. [S1]
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**When to Use Each**
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Use function declarations for general-purpose functions; use function expressions when assigning functions to variables; use function expressions in callbacks and event handlers. [S1]
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**Later in this Tutorial**
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Function expressions enable several advanced techniques: arrow functions (modern concise `=>` syntax for function expressions), callbacks (passing functions as arguments), closures (accessing variables from the containing scope), and IIFEs (functions that execute immediately upon definition). [S1]
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**Common Mistakes**
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Forgetting the semicolon (expressions are statements and should end with semicolons); expecting hoisting (expressions are not hoisted, cannot call before definition); confusing reference and call (`sayHello` is the function, `sayHello()` calls it); confusing function names and variables (in expressions, the variable name provides the function reference). [S1]
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## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
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| Aspect | Function Declaration | Function Expression |
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| --- | --- | --- |
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| Name | Required | Can be anonymous | [S1]
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| Hoisting | Hoisted (callable before definition) | Not hoisted (must define first) | [S1]
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| Flexibility | More flexible in how/where used | Used as a value (callbacks, variables) | [S1]
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| When to use | General-purpose functions | Variable assignment, callbacks, event handlers | [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`/`add` expressions, the `run(sayHello)` callback pattern, and the hoisting error/success comparison. No external project/commit applications found in the source.
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## 💻 코드 패턴 (Code patterns)
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Function expression stored in a variable:
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```javascript
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const multiply = function(a, b) {
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return a * b;
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};
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```
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Passing a function expression as a callback:
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```javascript
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function run(fn) {
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return fn();
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}
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const sayHello = function() {
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return "Hello";
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};
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run(sayHello);
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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 Arrow Functions]], [[JavaScript Function Arguments]], [[JavaScript Objects]]
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- **참조 맥락:** Referenced when choosing between declaration and expression syntax, and as the foundation for arrow functions, callbacks, and closures.
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## 📚 출처 (Sources)
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- [S1] W3Schools — JavaScript Function Expressions — https://www.w3schools.com/js/js_function_expressions.asp
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## 📝 변경 이력 (Change history)
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- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Function Expressions" page (Astra wiki-curation, P-Reinforce v3.1 format).
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