---
id: javascript-loops
title: "JavaScript Loops"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["loops", "JS loops", "for loop", "while loop", "do while", "iteration"]
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", "loops", "iteration", "control-flow"]
raw_sources: ["https://www.w3schools.com/js/js_loops.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Loops]]

## 🎯 한 줄 통찰 (One-line insight)
Loops let you run the same block of code many times, each time with a different value — replacing long repetitive sequences with a compact iteration. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Loops repeat a block of code** — they execute a code block multiple times, often with a different value each pass. [S1]
- **`for` loop** — uses three expressions: an initializer, a condition, and an update run after each iteration. [S1]
- **`while` loop** — runs as long as a specified condition is true. [S1]
- **`do/while` loop** — a variant that runs the block at least once before checking the condition. [S1]
- **Loop scope** — variables declared with `let` inside a loop are visible only within the loop, unlike `var`. [S1]
- **Infinite-loop hazard** — forgetting to update the condition variable means the loop never ends and crashes the browser. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Replace repetition with iteration** — instead of writing the same statement for each array index, loop over `cars.length`. [S1]
- **Index accumulation** — append each iteration's value into a result string (`text += ...`). [S1]
- **Always advance the condition variable** — increment (e.g. `i++`) so the condition eventually becomes false. [S1]

## 📖 세부 내용 (Details)
**Why loops** [S1]
Loops are handy if you want to run the same code over and over again, each time with a different value. Instead of writing the same statement many times:
```javascript
text += cars[0] + "<br>";
text += cars[1] + "<br>";
text += cars[2] + "<br>";
text += cars[3] + "<br>";
text += cars[4] + "<br>";
text += cars[5] + "<br>";
```
You can write:
```javascript
for (let i = 0; i < cars.length; i++) {
  text += cars[i] + "<br>";
}
```

**The `for` loop** [S1]
The `for` loop has the syntax `for (expr1; expr2; expr3) { // code block to be executed }`.
```javascript
for (let i = 0; i < 5; i++) {
  text += "The number is " + i + "<br>";
}
```

**Loop scope — `var` vs `let`** [S1]
Using `var` in a loop, the loop variable redeclares the outer variable:
```javascript
let i = 5;

for (i = 0; i < 10; i++) {
  // some code
}

// Here i is 10
```
Using `let` in a loop, the loop variable is scoped only to the loop:
```javascript
let i = 5;

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

// Here i is 5
```

**The `while` loop** [S1]
The `while` loop has the syntax `while (condition) { // code block to be executed }`.
```javascript
while (i < 10) {
  text += "The number is " + i;
  i++;
}
```
Note: If you forget to increase the variable used in the condition, the loop will never end. This will crash your browser.

**The `do/while` loop** [S1]
The `do/while` loop has the syntax `do { // code block to be executed } while (condition);`.
```javascript
do {
  text += "The number is " + i;
  i++;
}
while (i < 10);
```
Note: The `do while` runs at least once, even if the condition is false from the start.

**Different kinds of loops** [S1]
JavaScript supports the `for` loop (loops through a block of code a number of times), the `for/in` loop (loops through the properties of an object), the `for/of` loop (loops through the values of an iterable object), the `while` loop (loops through a block of code while a condition is true), and the `do/while` loop (also loops while a condition is true).

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets are the canonical applied examples — replacing repeated `text += cars[n]` statements with a `for` loop over `cars.length`, and accumulating numbers in `for`/`while`/`do-while` loops. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Iterate an array:
```javascript
for (let i = 0; i < cars.length; i++) {
  text += cars[i] + "<br>";
}
```
While loop with advancing counter:
```javascript
while (i < 10) {
  text += "The number is " + i;
  i++;
}
```
Do-while (runs at least once):
```javascript
do {
  text += "The number is " + i;
  i++;
}
while (i < 10);
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
- **`for` vs `while`** — use `for` when the number of iterations / the counter is known up front (init, condition, update in one place); use `while` when looping purely on a condition. [S1]
- **`while` vs `do/while`** — use `do/while` when the block must run at least once even if the condition is initially false. [S1]
- **`let` vs `var` for the loop variable** — `let` keeps the counter scoped to the loop; `var` leaks it to the surrounding scope. [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:** 0.88
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript For Loop]], [[JavaScript While Loop]], [[JavaScript Break]], [[JavaScript Booleans]]
- **참조 맥락:** The overview entry point for all iteration constructs; specific loop types are detailed in their own pages.

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

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