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title

JavaScript Array Const

🎯 한 줄 통찰 (One-line insight)

A const array cannot be reassigned, but its elements can be changed, added, or removed — const protects the binding, not the contents. [S1]

🧠 핵심 개념 (Core concepts)

const is common practice for arrays — ES2015 (ES6) made declaring arrays with const standard practice. [S1]
A const array cannot be reassigned — assigning a new array to the same const name is an ERROR. [S1]
Const array elements CAN change — you can change, add (push), or remove elements of a const array. [S1]
Const must be assigned when declared — declaring without assignment is a syntax error. [S1]
Const has block scope — a const declared inside a block is not the same variable as one outside the block. [S1]
Const cannot be redeclared — you cannot redeclare or reassign a const in the same scope, but you can declare a new const with the same name in a different block scope. [S1]

🧩 추출된 패턴 (Extracted patterns)

Mutate-not-rebind pattern — use const for arrays you will mutate in place (cars[0] = ..., cars.push(...)) but never reassign. [S1]
Block-scoped shadowing — declare a same-named const in a nested block to safely shadow the outer one. [S1]
Declare-and-initialize — always assign a const at declaration; deferred assignment is illegal. [S1]

📖 세부 내용 (Details)

Const arrays — It is a common practice to declare arrays using const: [S1]

const cars = ["Saab", "Volvo", "BMW"];

Cannot be Reassigned — an array declared with const cannot be reassigned: [S1]

const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"];    // ERROR

Elements Can be Reassigned — you can change the elements of a constant array: [S1]

// You can create a constant array:
const cars = ["Saab", "Volvo", "BMW"];

// You can change an element:
cars[0] = "Toyota";

// You can add an element:
cars.push("Audi");

Assigned When Declared — JavaScript const variables must be assigned a value when they are declared. This means an array declared with const must be initialized when it is declared. Declaring without initializing is a syntax error: [S1]

const cars;
cars = ["Saab", "Volvo", "BMW"];

This is not how var works. With var, you can use the variable before it is declared: [S1]

cars = ["Saab", "Volvo", "BMW"];
var cars;

Const Block Scope — a variable declared with const has block scope. A const declared in a block {} is not the same as a variable declared outside the block: [S1]

const cars = ["Saab", "Volvo", "BMW"];
// Here cars[0] is "Saab"
{
  const cars = ["Toyota", "Volvo", "BMW"];
  // Here cars[0] is "Toyota"
}
// Here cars[0] is "Saab"

Redeclaring a var array with var, in another block, in the same program, is allowed and behaves differently (no block scope): [S1]

var cars = ["Saab", "Volvo", "BMW"];
// Here cars[0] is "Saab"
{
  var cars = ["Toyota", "Volvo", "BMW"];
  // Here cars[0] is "Toyota"
}
// Here cars[0] is "Toyota"

Redeclaring Arrays — Redeclaring a var array is allowed anywhere in a program: [S1]

var cars = ["Volvo", "BMW"];   // Allowed
var cars = ["Toyota", "BMW"];  // Allowed
cars = ["Volvo", "Saab"];      // Allowed

Redeclaring or reassigning an existing var or const array to const, in the same scope or block, is not allowed: [S1]

var cars = ["Volvo", "BMW"];     // Allowed
const cars = ["Volvo", "BMW"];   // Not allowed
{
  var cars = ["Volvo", "BMW"];   // Allowed
  const cars = ["Volvo", "BMW"]; // Not allowed
}

Redeclaring or reassigning an existing const array, in the same scope or block, is not allowed: [S1]

const cars = ["Volvo", "BMW"];   // Allowed
const cars = ["Volvo", "BMW"];   // Not allowed
var cars = ["Volvo", "BMW"];     // Not allowed
cars = ["Volvo", "BMW"];         // Not allowed

{
  const cars = ["Volvo", "BMW"]; // Allowed
  const cars = ["Volvo", "BMW"]; // Not allowed
  var cars = ["Volvo", "BMW"];   // Not allowed
  cars = ["Volvo", "BMW"];       // Not allowed
}

Redeclaring a const array, in another scope or block, is allowed: [S1]

const cars = ["Volvo", "BMW"];   // Allowed
{
  const cars = ["Volvo", "BMW"]; // Allowed
}
{
  const cars = ["Volvo", "BMW"]; // Allowed
}

🛠️ 적용 사례 (Applied in summary)

The page's own snippets are the canonical applied examples — declaring a cars array with const, mutating its elements via index assignment and push, and demonstrating block-scope shadowing. No external project/commit applications found in the source.

💻 코드 패턴 (Code patterns)

Declare and mutate (allowed):

const cars = ["Saab", "Volvo", "BMW"];
cars[0] = "Toyota";
cars.push("Audi");

Reassign (error):

const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"];    // ERROR

⚖️ 비교 및 선택 기준 (Comparison & decision criteria)

const vs var for arrays — const has block scope, must be initialized at declaration, and cannot be reassigned or redeclared in the same scope; var lacks block scope, can be used before declaration, and can be freely redeclared. const is the recommended common practice. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

No contradictions found in the source. The source notes that declaring arrays with const became common practice with ES2015 (ES6). [S1]

✅ 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: 0.89
중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

상위/루트: JavaScript Tutorial
관련 개념: JavaScript Arrays, JavaScript Array Iteration, JavaScript Const
참조 맥락: Referenced when deciding how to declare arrays and what mutability const guarantees.

📚 출처 (Sources)

[S1] W3Schools — JavaScript Array Const — https://www.w3schools.com/js/js_array_const.asp

📝 변경 이력 (Change history)

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

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