2nd/10_Wiki/Topic_JavaScript/JavaScript_Hoisting.md

---
id: javascript-hoisting
title: "JavaScript Hoisting"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["hoisting", "variable hoisting", "temporal dead zone", "declaration hoisting", "var hoisting"]
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", "hoisting", "var", "let", "const", "scope"]
raw_sources: ["https://www.w3schools.com/js/js_hoisting.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Hoisting]]

## 🎯 한 줄 통찰 (One-line insight)
Hoisting is JavaScript's default behavior of moving all *declarations* (not initializations) to the top of the current scope — so a `var` can be used before it is declared, but `let`/`const` cannot. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Hoisting moves declarations up** — JavaScript moves all declarations to the top of the current scope (the top of the current script or the current function). [S1]
- **`var` is hoisted and usable early** — a `var` variable can be referenced before its declaration line without error. [S1]
- **`let` and `const` are hoisted but not initialized** — using them before declaration throws a `ReferenceError` because of the "temporal dead zone"; using `const` before declaration is a syntax error. [S1]
- **Only declarations hoist, not initializations** — the assignment (`= value`) stays where it is written, so an early-referenced variable reads as `undefined`. [S1]
- **Best practice** — always declare all variables at the beginning of every scope; strict mode does not allow undeclared variables. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Declare at the top of scope** — to avoid hoisting-related bugs, put all declarations first. [S1]
- **Never rely on `let`/`const` before declaration** — they live in the temporal dead zone until their declaration line runs. [S1]
- **Remember: declaration ≠ initialization** — an early read returns `undefined`, not the assigned value. [S1]

## 📖 세부 내용 (Details)
**What is Hoisting?**
Hoisting is JavaScript's default behavior of moving all declarations to the top of the current scope (to the top of the current script or the current function). [S1]

**`var` Declarations are Hoisted**
A variable can be used before it has been declared. The following example gives the same result as declaring `x` first: [S1]
```javascript
x = 5; // Assign 5 to x

elem = document.getElementById("demo"); // Find an element
elem.innerHTML = x;                     // Display x in the element

var x; // Declare x
```
This is equivalent to: [S1]
```javascript
var x; // Declare x
x = 5; // Assign 5 to x

elem = document.getElementById("demo"); // Find an element
elem.innerHTML = x;                     // Display x in the element
```

**`let` and `const` Hoisting**
Variables defined with `let` and `const` are hoisted to the top of the block, but they are *not* initialized. Using a `let` variable before it is declared results in a `ReferenceError`: [S1]
```javascript
carName = "Volvo";
let carName;
```
Using a `const` variable before it is declared is a syntax error, so the code will not run: [S1]
```javascript
carName = "Volvo";
const carName;
```

**JavaScript Initializations are Not Hoisted**
JavaScript hoists *declarations*, not *initializations*. Because only the declaration of `y` is hoisted (and not its assignment), `y` is `undefined` when it is first used: [S1]
```javascript
var x = 5; // Initialize x

elem = document.getElementById("demo"); // Find an element
elem.innerHTML = x + " " + y;           // Display x and y

var y = 7; // Initialize y
```

**Declare Your Variables At the Top**
To avoid bugs, always declare all variables at the beginning of every scope. Since this is how JavaScript interprets the code, it is a good rule. JavaScript in strict mode does not allow variables to be used if they are not declared. [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's examples are the applied cases: showing `var x` usable before its declaration line, the `let`/`const` `ReferenceError`/syntax-error cases, and the `y` value reading as `undefined` because its initialization did not hoist. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
`var` is hoisted (usable before declaration) (language: JavaScript):
```javascript
x = 5;
var x; // Declaration hoisted to top of scope
```
`let` before declaration throws:
```javascript
carName = "Volvo";
let carName; // ReferenceError
```
Initialization is not hoisted (reads undefined):
```javascript
elem.innerHTML = x + " " + y; // y is undefined here
var y = 7;
```

## ⚖️ 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:** 0.87
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript var let const]], [[JavaScript Scope]], [[JavaScript Strict Mode]], [[JavaScript Code Blocks]]
- **참조 맥락:** Explains why variable order matters and motivates declaring variables at the top of each scope.

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

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