2nd/10_Wiki/Topic_JavaScript/JavaScript_Loop_For.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-for-loop	JavaScript For Loop	Frontend	draft	conceptual		for loop JS for loop for statement loop expressions for loop scope		B	0.89	2026-06-23	2026-06-23			javascript js web frontend w3schools for-loop loops iteration	https://www.w3schools.com/js/js_loop_for.asp

JavaScript For Loop

🎯 한 줄 통찰 (One-line insight)

The for statement creates a loop with three optional expressions — initialize, test, update — letting a code block run a controlled number of times. [S1]

🧠 핵심 개념 (Core concepts)

• Three optional expressions — the for statement creates a loop with three optional expressions. [S1]
• exp1 — initializer — sets a variable before the loop starts (let i = 0), and can be omitted if initialization happens beforehand. [S1]
• exp2 — condition — defines the condition for the loop to run; it is optional, but if omitted you must break inside the loop or it runs forever. [S1]
• exp3 — update — increases a value (i++) after the code block executes; it is optional and can do other operations. [S1]
• Loop scope matters — let scopes the counter to the loop; var redeclares the outer variable. [S1]

🧩 추출된 패턴 (Extracted patterns)

• Index-driven array traversal — cache cars.length once, then iterate i from 0 to that length. [S1]
• Omit-and-supply elsewhere — any of the three expressions can be moved out of the for(...) header (initialize before, update inside the body). [S1]
• Prefer let for the counter — keeps i private to the loop. [S1]

📖 세부 내용 (Details)

The for statement [S1] The for statement creates a loop with 3 optional expressions, allowing the code block to execute multiple times.

• exp1 sets a variable before the loop starts (let i = 0).
• exp2 defines the condition for the loop to run (i must be less than 5).
• exp3 increases a value (i++) after the code block has been executed.

The first expression can be omitted if initialization occurs before the loop. The second expression is optional but requires a break statement inside the loop to prevent infinite execution. The third expression is optional and can perform various operations like negative increment or custom calculations.

Basic for loop [S1]

for (let i = 0; i < 5; i++) {
  text += "The number is " + i + "<br>";
}

Iterating through an array [S1]

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let len = cars.length;
let text = "";
for (let i = 0; i < len; i++) {
  text += cars[i];
}

Omitting exp1 [S1]

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let len = cars.length;
let i = 2;
let text = "";
for (; i < len; i++) {
  text += cars[i] + "<br>";
}

Omitting exp3 [S1]

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let len = cars.length;
let i = 0;
let text = "";
for (; i < len; ) {
  text += cars[i] + "<br>";
  i++;
}

Loop scope [S1] Using var in a loop, the loop variable redeclares the outer variable:

var i = 5;
for (var i = 0; i < 10; i++) {
  // some code
}
// Here i is 10

Using let in a loop, the variable is scoped only within the loop. When let is used to declare the i variable, the i variable will only be visible within the loop:

let i = 5;
for (let i = 0; i < 10; i++) {
  // some code
}
// Here i is 5

🛠️ 적용 사례 (Applied in summary)

The page's own snippets are the canonical applied examples — counting 0–4, traversing a cars array (with len cached), and demonstrating exp1/exp3 omission and let/var scoping. No external project/commit applications found in the source.

💻 코드 패턴 (Code patterns)

Standard counted loop:

for (let i = 0; i < 5; i++) {
  text += "The number is " + i + "<br>";
}

Array traversal with cached length:

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let len = cars.length;
for (let i = 0; i < len; i++) {
  text += cars[i];
}

⚖️ 비교 및 선택 기준 (Comparison & decision criteria)

• let vs var for the counter — let keeps i visible only inside the loop (i is 5 afterward in the example); var leaks/redeclares it to the outer scope (i is 10 afterward). Prefer let. [S1]

⚖️ 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: 0.89
• 중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

• 상위/루트: JavaScript Tutorial
• 관련 개념: JavaScript Loops, JavaScript While Loop, JavaScript Break, JavaScript For In
• 참조 맥락: The primary counted-iteration construct; detailed expansion of the for loop introduced in JavaScript Loops.

📚 출처 (Sources)

• [S1] W3Schools — JavaScript For Loop — https://www.w3schools.com/js/js_loop_for.asp

📝 변경 이력 (Change history)

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