2nd/10_Wiki/Topic_JavaScript/JavaScript_Variables.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-variables	JavaScript Variables	Frontend	draft	conceptual		JS variables var let const identifiers assignment operator data containers		B	0.87	2026-06-23	2026-06-23			javascript js web frontend w3schools variables var-let-const	https://www.w3schools.com/js/js_variables.asp

JavaScript Variables

🎯 한 줄 통찰 (One-line insight)

Variables are containers for storing data, declared in four ways (var, let, const, or automatically) — and the modern guidance is to prefer const, fall back to let, and avoid var. [S1]

🧠 핵심 개념 (Core concepts)

• Variables are data containers — they store data values. [S1]
• Four ways to declare — automatically, using var, using let, or using const. [S1]
• Identifier rules — names can contain letters, digits, underscores, and dollar signs; must begin with a letter, $, or _; are case-sensitive; reserved words cannot be used. [S1]
• $ is treated like a letter — the dollar sign is treated as a letter in identifiers and is often used as an alias for the main function in JavaScript libraries; _ is sometimes used to denote "private" variables. [S1]
• Assignment vs equality — the = operator assigns a value (it does not mean "equal to"). [S1]
• + is overloaded — with numbers it adds; with strings it concatenates. [S1]

🧩 추출된 패턴 (Extracted patterns)

• const-first selection — always declare with const if the value should not change (including new Arrays, Objects, Functions); use let when the value may change; avoid var. [S1]
• One statement, many variables — declare several variables in one statement, separated by commas, optionally across multiple lines. [S1]
• String-vs-number + evaluation — once a string appears in a + chain, subsequent operands are concatenated, so order and types matter. [S1]

📖 세부 내용 (Details)

Variables = Data Containers — Variables are containers for storing data (storing data values). [S1]

A variable can be declared four ways. Using let: [S1]

let x = 5;
let y = 6;
let z = x + y;

Using const: [S1]

const x = 5;
const y = 6;
const z = x + y;

JavaScript Identifiers — All JavaScript variables must be identified with unique names. These unique names are called identifiers. The general rules for constructing names for variables (unique identifiers) are: names can contain letters, digits, underscores, and dollar signs; names must begin with a letter, $, or _; names are case-sensitive (y and Y are different variables); and reserved words (like JavaScript keywords) cannot be used as names. [S1]

JavaScript Underscore (_) — Since JavaScript treats the underscore as a letter, identifiers containing _ are valid variable names. Some programmers like to use underscores to denote "private (hidden)" variables. [S1]

JavaScript Dollar Sign ($) — Since JavaScript treats the dollar sign as a letter, identifiers containing $ are valid variable names. Using the dollar sign is not very common in JavaScript, but professional programmers often use it as an alias for the main function in a JavaScript library. [S1]

When to Use var, let, or const? — The source's guidance: always declare variables; always use const if the value should not be changed; always use const if the type should not be changed (Arrays and Objects); only use let if you can't use const; only use var if you MUST support old browsers. [S1]

One Statement, Many Variables — You can declare many variables in one statement. Start the statement with let and separate the variables by comma: [S1]

let person = "John Doe", carName = "Volvo", price = 200;

A declaration can span multiple lines: [S1]

let person = "John Doe",
carName = "Volvo",
price = 200;

The Assignment Operator — In JavaScript, the = sign is an "assignment" operator, not an "equal to" operator. [S1]

JavaScript Arithmetic — You can do arithmetic with JavaScript variables, using operators like = and +. The value of x differs depending on whether operands are numbers or strings: [S1]

let x = 5 + 2 + 3;

let x = "John" + " " + "Doe";

If you put a number in quotes, the rest of the numbers will be treated as strings and concatenated: [S1]

let x = "5" + 2 + 3;

let x = 2 + 3 + "5";

🛠️ 적용 사례 (Applied in summary)

The page's own snippets are the canonical applied examples — let/const declarations computing x + y, the comma-separated multi-variable statement, and the + arithmetic vs concatenation cases. No external project/commit applications found in the source.

💻 코드 패턴 (Code patterns)

Declare and compute:

let x = 5;
let y = 6;
let z = x + y;

Many variables, one statement:

let person = "John Doe", carName = "Volvo", price = 200;

⚖️ 비교 및 선택 기준 (Comparison & decision criteria)

The source gives explicit selection rules among the declaration keywords: [S1]

Keyword	Use when
const	The value (or type, for Arrays/Objects) should not change — the default choice
let	You cannot use const because the value will change
var	Only if you MUST support old browsers

⚖️ 모순 및 업데이트 (Contradictions & updates)

No contradictions found in the source. (The page notes var was the only option before 2015 (ES6) but is now discouraged — an update in guidance, 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: 0.87
• 중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

• 상위/루트: JavaScript Tutorial
• 관련 개념: JavaScript Let, JavaScript Const, JavaScript Syntax
• 참조 맥락: The foundation for all data handling; refined by the dedicated Let and Const pages.

📚 출처 (Sources)

• [S1] W3Schools — JavaScript Variables — https://www.w3schools.com/js/js_variables.asp

📝 변경 이력 (Change history)

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