2nd/10_Wiki/Topic_JavaScript/JavaScript_Var_Let_Const.md

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

id	title	category	status	verification_status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	created_at	updated_at	review_reason	merge_history	tags	raw_sources	applied_in	github_commit
javascript-var-let-const	JavaScript var let const	Frontend	draft	conceptual		var let const variable declarations block scope redeclaration const reassignment		B	0.86	2026-06-23	2026-06-23			javascript js web frontend w3schools var let const scope variables	https://www.w3schools.com/js/js_varletconst.asp

JavaScript var let const

🎯 한 줄 통찰 (One-line insight)

var, let, and const differ in scope, redeclaration, and reassignment — modern best practice is const by default, let when you must reassign, and avoid var. [S1]

🧠 핵심 개념 (Core concepts)

• Three declaration keywords — var, let, and const declare variables but with different scoping and mutation rules. [S1]
• Scope — var has function or global scope; let and const are confined to the nearest pair of curly braces { } (block scope). [S1]
• Reassignment — var and let can be updated (reassigned); const cannot be updated with a new value. [S1]
• Redeclaration — var can be redeclared; let and const cannot be redeclared in the same scope. [S1]
• const is "const reference," not "const value" — you cannot reassign a const variable to a completely new value, but you can still change the properties of an object or array assigned to it. [S1]
• Hoisting — var is hoisted and initialized to undefined; let and const are also hoisted but not initialized, creating a temporal dead zone where access throws a ReferenceError. [S1]

🧩 추출된 패턴 (Extracted patterns)

• const by default — use const unless you know you must reassign. [S1]
• let only when reassigning — switch to let only when the variable's value must change after initialization. [S1]
• Avoid var — its function-scope and redeclaration rules cause bugs; modern code avoids it. [S1]
• Mutate, don't reassign, a const object/array — change properties/elements in place rather than rebinding the variable. [S1]

📖 세부 내용 (Details)

The three keywords JavaScript declares variables with var, let, or const. They differ in scope, in whether they can be redeclared, and in whether they can be reassigned. [S1]

Comparison of scope, reassignment, and redeclaration [S1]

Keyword	Scope	Reassignment	Redeclaration
var	Function or global scope	Can be updated	Can be redeclared
let	Block scope { }	Can be updated	Cannot be redeclared
const	Block scope { }	Cannot be updated	Cannot be redeclared

Block scope (let) Both let and const are confined strictly to the nearest pair of curly braces { }. A variable declared with let inside a block is not accessible outside it: [S1]

{
  let x = 10;
  // x is accessible here
}
// x is NOT accessible here

var can be redeclared A variable declared with var can be redeclared in the same scope without error. (let cannot be redeclared in the same scope.) [S1]

const cannot be reassigned, but objects/arrays can be mutated You cannot reassign a const variable with a completely new value, but you can still change the properties of an object — or the elements of an array — declared with const. [S1]

Note (W3Schools): On this page these behaviors are demonstrated through "Try it Yourself" examples (linked to tryit.asp) and the comparison table above. The exact full source of the individual "Try it Yourself" boxes is not rendered inline on the page — Not found in source (verbatim per-example code beyond the snippets shown here). [S1]

Hoisting Variables declared with var are hoisted to the top of their scope and initialized as undefined, so they can be referenced before their declaration line. let and const are also hoisted but not initialized, creating a Temporal Dead Zone where accessing them before declaration causes a ReferenceError. [S1]

Recommendation Use const by default, and only use let when you have to reassign the variable after initialization. Avoid using var. Modern JavaScript standards recommend avoiding var to minimize bugs related to its scoping and redeclaration rules. [S1]

🛠️ 적용 사례 (Applied in summary)

The page applies these rules to everyday declarations: choosing const for values that never rebind, let for counters/accumulators that change, and avoiding var entirely. No external project/commit applications found in the source.

💻 코드 패턴 (Code patterns)

Block scope of let (language: JavaScript):

{
  let x = 10;
}
// x is NOT accessible here

⚖️ 비교 및 선택 기준 (Comparison & decision criteria)

• Scope: var → function/global; let/const → block { }. Prefer block scope to limit visibility. [S1]
• Reassignment: var/let can be updated; const cannot. Use let only when reassignment is required. [S1]
• Redeclaration: var can be redeclared; let/const cannot — making accidental redeclaration an early error. [S1]
• Overall default: const first, let when you must reassign, avoid var. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

No contradictions found in the source. The page notes that let and const (block scope) are ES6 (2015) features, superseding the older var-only model.

✅ 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: 0.86
• 중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

• 상위/루트: JavaScript Tutorial
• 관련 개념: JavaScript Scope, JavaScript Hoisting, JavaScript Code Blocks, JavaScript Const
• 참조 맥락: The reference for choosing a declaration keyword in any JavaScript code.

📚 출처 (Sources)

• [S1] W3Schools — JavaScript var let const — https://www.w3schools.com/js/js_varletconst.asp

📝 변경 이력 (Change history)

• 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript var let const" page (Astra wiki-curation, P-Reinforce v3.1 format).