---
id: javascript-let
title: "JavaScript Let"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["let keyword", "block scope", "let vs var", "let hoisting", "ES6 let"]
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", "let", "block-scope"]
raw_sources: ["https://www.w3schools.com/js/js_let.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Let]]

## 🎯 한 줄 통찰 (One-line insight)
The `let` keyword (ES6, 2015) introduced block scope to JavaScript — `let` variables are confined to their `{ }` block, cannot be redeclared in the same scope, and are hoisted but not initialized. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Block scope (new in ES6)** — before ES6 (2015) JavaScript had no block scope; variables declared with `let` inside a `{ }` block cannot be accessed outside it. [S1]
- **Function scope** — variables declared with `var`, `let`, or `const` inside a function cannot be accessed outside it. [S1]
- **`var` is global/function scoped** — a `var` declared inside a block can still be used outside the block. [S1]
- **Cannot be redeclared in the same scope** — `let` variables cannot be redeclared in the same scope (whereas `var` can). [S1]
- **Hoisted but not initialized** — `let` variables are hoisted to the top of their block but not initialized; using one before its declaration causes a `ReferenceError`. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Block-local shadowing** — declaring `let x` inside a block shadows an outer `x` without altering it, unlike `var` which leaks back out. [S1]
- **Redeclare across different blocks** — the same `let` name may be redeclared in different blocks, just not twice in the same scope. [S1]
- **Declare-before-use discipline** — because `let` is not initialized when hoisted, always declare before using. [S1]

## 📖 세부 내용 (Details)
**Block Scope** — Before ES6 (2015), JavaScript had only Global Scope and Function Scope. ES6 introduced two new JavaScript keywords: `let` and `const`. These two keywords provided Block Scope in JavaScript. Variables declared inside a `{ }` block cannot be accessed from outside the block: [S1]
```javascript
{
  let x = 2;
}
// x can NOT be used here
```

**Function Scope** — Variables declared with `var`, `let`, and `const` are quite similar when declared inside a function — they all have Function Scope: [S1]
```javascript
function myfunction() {
  var x = 1;
  let y = 2;
  const z = 3;
}
//x can NOT be used here
//y can NOT be used here
//z can NOT be used here
```

**Global Scope (var)** — Variables declared with `var` inside a block can be accessed from outside the block: [S1]
```javascript
{
  var x = 2;
}
// x CAN be used here
```

**Cannot be Redeclared** — Variables defined with `let` can not be redeclared. You can not accidentally redeclare a variable declared with `let`: [S1]
```javascript
let x = "John Doe";
let x = 0;  // Error
```
Variables defined with `var` can be redeclared: [S1]
```javascript
var x = "John Doe";
var x = 0;  // Allowed
```

**Redeclaring Variables** — Redeclaring a variable using the `var` keyword can impose problems. Redeclaring a variable inside a block will also redeclare the variable outside the block: [S1]
```javascript
var x = 10;
// Here x is 10

{
  var x = 2;
  // Here x is 2
}

// Here x is 2
```
Redeclaring a variable using the `let` keyword can solve this problem. Redeclaring a variable inside a block will not redeclare the variable outside the block: [S1]
```javascript
let x = 10;
// Here x is 10

{
  let x = 2;
  // Here x is 2
}

// Here x is 10
```

**Redeclaring (across different blocks)** — Redeclaring a `let` variable in different blocks is allowed: [S1]
```javascript
let x = 2;   // Allowed

{
  let x = 3;   // Allowed
}

{
  let x = 4;    // Allowed
}
```

**Let Hoisting** — Variables defined with `let` are hoisted to the top of the block, but not initialized. Using a `let` variable before it is declared will result in a `ReferenceError`: [S1]
```javascript
carName = "Saab";
let carName = "Volvo";  // ReferenceError
```
By contrast, variables defined with `var` are hoisted to the top and can be used before declaration: [S1]
```javascript
carName = "Volvo";
var carName;  // OK
```

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets are the canonical applied examples — block-scoped `let x`, the `var`-leaks vs `let`-contained redeclaration pair, redeclaring `let` across separate blocks, and the hoisting `ReferenceError` contrast. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Block-scoped variable:
```javascript
{
  let x = 2;
}
// x can NOT be used here
```
Safe shadowing:
```javascript
let x = 10;
{
  let x = 2;
}
// Here x is 10
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
The source compares the three declaration keywords: [S1]

| Feature | var | let | const |
|---------|-----|-----|-------|
| Scope | Function or global | Block-scope `{ }` | Block-scope `{ }` |
| Reassignment | Can be updated | Can be updated | Cannot be updated |
| Redeclaration | Can be redeclared | Cannot be redeclared | Cannot be redeclared |
| Hoisting | Initialized as `undefined` | Hoisted, not initialized | Hoisted, not initialized |

Choose `let` when you need a reassignable variable scoped to a block; choose `const` when the value will not change; avoid `var` to prevent unintentional global/function-scope leakage. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source. (The page frames `let`/`const` as a 2015 update over the older `var`-only model — an evolution, 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:** 0.88
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Variables]], [[JavaScript Const]], [[JavaScript Statements]]
- **참조 맥락:** The block-scoped declaration keyword, contrasted against `var` and paired with `const`.

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

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