2nd/10_Wiki/Topic_JavaScript/JavaScript_Function_Expressions.md

---
id: javascript-function-expressions
title: "JavaScript Function Expressions"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["JS function expressions", "anonymous functions", "function declaration vs expression", "hoisting", "named function expression", "callbacks"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.88
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["javascript", "js", "web", "frontend", "w3schools", "functions", "expressions", "hoisting"]
raw_sources: ["https://www.w3schools.com/js/js_function_expressions.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Function Expressions]]

## 🎯 한 줄 통찰 (One-line insight)
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]

## 🧠 핵심 개념 (Core concepts)
- **A function expression is a function stored in a variable.** [S1]
- **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]
- **It is a statement** — a function expression usually ends with a semicolon. [S1]
- **Used as a value** — because it is stored in a variable, it can be passed to other functions (callbacks). [S1]
- **Hoisting difference** — function declarations are hoisted and can be called before they are defined; function expressions are not hoisted and cannot. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Store-then-call** — assign a function to a `const`, then invoke through the variable name (`sayHello()`). [S1]
- **Callback passing** — pass a function expression as an argument so another function can invoke it (`run(sayHello)`). [S1]
- **Define-before-use ordering** — because expressions are not hoisted, declare them above the point of first call. [S1]

## 📖 세부 내용 (Details)
**What is a Function Expression?**
A function expression is a function stored in a variable. A standard function: [S1]
```javascript
function multiply(a, b) {
  return a * b;
}
```
A function expression: [S1]
```javascript
const multiply = function(a, b) {
  return a * b;
};
```
After storing in a variable, the variable can be used as a function: [S1]
```javascript
let z = multiply(4, 3);
```

**Anonymous Functions**
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]
```javascript
const add = function add(a, b) {return a + b;};
```

**Function Expressions Use Semicolons**
A function expression is a JavaScript statement. That is why it usually ends with a semicolon. [S1]
```javascript
const add = function(a, b) {
  return a + b;
};
```

**Functions Stored in Variables**
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]
```javascript
function run(fn) {
  return fn();
}

const sayHello = function() {
  return "Hello";
};

run(sayHello);
```

**Functions vs. Function Expressions**
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]

**Function Declarations vs Function Expression**
A function declaration uses the `function` keyword, has a function name, parameters in parentheses, and a code block in brackets: [S1]
```javascript
function add(a, b) {return a + b;}
```
A function expression uses the `function` keyword, parameters in parentheses, and a code block in brackets: [S1]
```javascript
const add = function(a, b) {return a + b;};
```
Example: [S1]
```javascript
const sayHello = function() {
  return "Hello World";
};

sayHello();
```
The function above is stored in the variable `sayHello`. To run it, you call `sayHello()`. [S1]

**Hoisting**
Function declarations can be called before they are defined. Function expressions can not be called before they are defined. Function declarations are hoisted: [S1]
```javascript
let sum = add(2, 3); // Will work

function add(a, b) {return a + b;}
```
Function expressions are not hoisted: [S1]
```javascript
let sum = add(2, 3); // ⛔ Will generate error

const add = function (a, b) {return a + b;};
```

**Key Differences**
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]

**When to Use Each**
Use function declarations for general-purpose functions; use function expressions when assigning functions to variables; use function expressions in callbacks and event handlers. [S1]

**Later in this Tutorial**
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]

**Common Mistakes**
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]

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
| Aspect | Function Declaration | Function Expression |
| --- | --- | --- |
| Name | Required | Can be anonymous | [S1]
| Hoisting | Hoisted (callable before definition) | Not hoisted (must define first) | [S1]
| Flexibility | More flexible in how/where used | Used as a value (callbacks, variables) | [S1]
| When to use | General-purpose functions | Variable assignment, callbacks, event handlers | [S1]

## 🛠️ 적용 사례 (Applied in summary)
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.

## 💻 코드 패턴 (Code patterns)
Function expression stored in a variable:
```javascript
const multiply = function(a, b) {
  return a * b;
};
```
Passing a function expression as a callback:
```javascript
function run(fn) {
  return fn();
}

const sayHello = function() {
  return "Hello";
};

run(sayHello);
```

## ⚖️ 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:** 0.88
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Arrow Functions]], [[JavaScript Function Arguments]], [[JavaScript Objects]]
- **참조 맥락:** Referenced when choosing between declaration and expression syntax, and as the foundation for arrow functions, callbacks, and closures.

## 📚 출처 (Sources)
- [S1] W3Schools — JavaScript Function Expressions — https://www.w3schools.com/js/js_function_expressions.asp

## 📝 변경 이력 (Change history)
- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Function Expressions" page (Astra wiki-curation, P-Reinforce v3.1 format).