---
id: javascript-iterators
title: "JavaScript Iterators"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["JS iterators", "iterator protocol", "next()", "Iterator helpers", "Iterator.from"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.87
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["javascript", "js", "web", "frontend", "w3schools", "iterators", "iterator-helpers", "es2025"]
raw_sources: ["https://www.w3schools.com/js/js_iterators.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Iterators]]

## 🎯 한 줄 통찰 (One-line insight)
An iterator is an object that provides a standard way to access elements sequentially through a `next()` method; ES2025 adds iterator helper methods (`map`, `filter`, `reduce`, `take`, `drop`, etc.) and `Iterator.from()` that bring iteration directly into the core language. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Iterator = sequential access object** — an iterator is an object that provides a standard way to access elements one after another. [S1]
- **The iterator protocol** — an iterator must implement a `next()` method that returns an object with `value` (the next value) and `done` (`false` while more elements exist, otherwise `true`). [S1]
- **`for...of` consumes iterables** — the `for...of` statement loops through the elements of an iterable object; iterables must implement `Symbol.iterator`. Built-in iterables are Strings, Arrays, Typed Arrays, Sets, and Maps. [S1]
- **Iterator helper methods (ES2025)** — new helpers let you transform and consume iterators lazily: `drop`, `every`, `filter`, `find`, `flatMap`, `forEach`, `from`, `map`, `reduce`, `some`, `take`. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **`Iterator.from(iterable)`** — wrap any iterable into an iterator object so the helper methods become available. [S1]
- **Lazy chaining** — methods like `filter`, `map`, `take`, and `drop` return a new iterator (not an array), enabling pipeline-style transformation of sequences. [S1]
- **Terminal reducers** — `every`, `some`, `find`, `reduce`, and `forEach` consume the iterator to produce a single value or side effect. [S1]

## 📖 세부 내용 (Details)
**JavaScript Iterators**
An iterator is an object that provides a standard way to access elements sequentially. Iterators must adhere to the iterator protocol by implementing a `next()` method. [S1]

**The `next()` Method**
The `next()` method returns an object with two properties: `value` holds the next value in the iteration sequence, and `done` returns `false` if there are more elements, otherwise `true`. [S1]

**The `for...of` Loop**
The JavaScript `for...of` statement loops through the elements of an iterable object. Iterables must implement the `Symbol.iterator` method. In JavaScript the following are iterables: Strings, Arrays, Typed Arrays, Sets, and Maps — their prototypes have a `Symbol.iterator` method. [S1]

**`Iterator.from()`** — creates an iterator object from an iterable: [S1]
```javascript
const myIterator = Iterator.from([1, 2, 3]);
let text = "";
for (const x of myIterator) {
  text += x;
}
```

**`drop()`** — returns an iterator that skips a specified number of elements before yielding the rest: [S1]
```javascript
const myIterator = Iterator.from([1, 2, 3, 4, 5, 6]);
const firstFive = myIterator.drop(5);
```

**`every()`** — returns `true` if all elements satisfy a test function: [S1]
```javascript
const myIterator = Iterator.from("123456789");
let result = myIterator.every(x => x > 7);
```

**`filter()`** — returns an iterator containing elements that satisfy a filter function: [S1]
```javascript
const myIterator = Iterator.from([32, 33, 16, 40]);
const filteredIterator = myIterator.filter(x => x > 18);
```

**`find()`** — returns the first element that satisfies a test function: [S1]
```javascript
const myIterator = Iterator.from([3, 10, 18, 30, 20]);
let result = myIterator.find(x => x > 18);
```

**`flatMap()`** — returns an iterator by mapping each element and then flattening the results: [S1]
```javascript
const myIterator = Iterator.from([1, 2, 3, 4, 5, 6]);
const mappedIterator = myIterator.flatMap(x => [x, x * 10]);
```

**`forEach()`** — executes a function once for each element in the iterator: [S1]
```javascript
const myIterator = Iterator.from("123456789");
let text = "";
myIterator.forEach (x => text += x);
```

**`map()`** — returns an iterator with all elements transformed by a map function: [S1]
```javascript
const myIterator = Iterator.from("123456789");
const mappedIterator = myIterator.map(x => x * 2);
```

**`reduce()`** — applies a reducer function against each element to reduce it to a single value: [S1]
```javascript
const myIterator = Iterator.from([175, 50, 25]);
let result = myIterator.reduce(myFunc);
```

**`some()`** — returns `true` if at least one element satisfies a test function: [S1]
```javascript
const myIterator = Iterator.from("123456789");
let result = myIterator.some(x => x > 7);
```

**`take()`** — returns an iterator that yields a specified number of elements: [S1]
```javascript
const myIterator = Iterator.from([1, 2, 3, 4, 5, 6]);
const firstFive = myIterator.take(5);
```

**Iterator Helper Methods (ES2025)** [S1]

| Function | Description |
|----------|-------------|
| `drop()` | Returns an iterator that skips a specified number of elements before yielding the rest |
| `every()` | Returns `true` if all elements satisfy a test function |
| `filter()` | Returns an iterator containing elements that satisfy a filter function |
| `find()` | Returns the first element that satisfies a test function |
| `flatMap()` | Returns an iterator by mapping each element and then flattening the results |
| `forEach()` | Executes a function once for each element in the iterator |
| `from()` | Creates an iterator object from an iterable |
| `map()` | Returns an iterator with all elements transformed by a map function |
| `reduce()` | Applies a reducer function against each element to reduce it to a single value |
| `some()` | Returns `true` if at least one element satisfies a test function |
| `take()` | Returns an iterator that yields a specified number of elements |

Iterators bring the iteration concept directly into the core JavaScript language and provide a mechanism for customizing the behavior of `for...of`. [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's own `Iterator.from(...)` snippets are the canonical applied examples — wrapping arrays and strings into iterators and applying `filter`, `map`, `reduce`, `take`, `drop`, and the predicate helpers. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Wrap an iterable, then transform it (language: JavaScript):
```javascript
const myIterator = Iterator.from([32, 33, 16, 40]);
const filteredIterator = myIterator.filter(x => x > 18);
```
Reduce to a single value:
```javascript
const myIterator = Iterator.from([175, 50, 25]);
let result = myIterator.reduce(myFunc);
```

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
The iterator helper methods (`map`, `filter`, `reduce`, `take`, `drop`, etc.) and `Iterator.from()` are an ES2025 addition; availability depends on the runtime supporting that version. [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:** 0.87
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Iterables]], [[JavaScript Generators]], [[JavaScript Symbols]]
- **참조 맥락:** Referenced when consuming sequences with `for...of` or composing lazy data pipelines with the ES2025 iterator helpers.

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

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