docs(10_Wiki): W3Schools 위키화 — HTML/CSS/JavaScript(core)
W3Schools 튜토리얼을 P-Reinforce v3.1 포맷으로 위키화(영어 본문, 한/영 섹션 헤더). - Topic_HTML: 59문서 (튜토리얼+예제, 레퍼런스/메타 제외) - Topic_CSS: 190문서 (메인 + Advanced/Flexbox/Grid/RWD 전체) - Topic_JavaScript: 120문서 (코어 언어; Temporal/DOM상세/BOM/WebAPI/AJAX/jQuery/Graphics 등은 후속) 각 폴더 00_INDEX.md(MOC) 포함. 코드 verbatim, 미확인분은 "Not found in source" 표기. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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id: javascript-map-methods
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title: "JavaScript Map Methods"
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category: "Frontend"
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status: "draft"
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verification_status: "conceptual"
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canonical_id: ""
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aliases: ["Map methods", "Map.set", "Map.get", "Map.has", "Map.size", "Map.groupBy", "objects as keys"]
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duplicate_of: ""
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source_trust_level: "B"
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confidence_score: 0.9
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created_at: 2026-06-23
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updated_at: 2026-06-23
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review_reason: ""
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merge_history: []
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tags: ["javascript", "js", "web", "frontend", "w3schools", "map", "methods"]
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raw_sources: ["https://www.w3schools.com/js/js_map_methods.asp"]
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applied_in: []
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github_commit: ""
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---
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# [[JavaScript Map Methods]]
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## 🎯 한 줄 통찰 (One-line insight)
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Map methods — `set`, `get`, `has`, `delete`, `clear`, `forEach`, `entries`, `keys`, `values`, plus `size` — manage and iterate key-value pairs, and crucially Maps can use objects as keys; ES2024 also adds the static `Map.groupBy()`. [S1]
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## 🧠 핵심 개념 (Core concepts)
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- **`get()` retrieves**, **`set()` adds or updates** a key's value, **`has()` tests** key existence, **`delete()` removes** one element, **`clear()` empties** the Map, **`size`** returns the count. [S1]
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- **Three iteration methods** — `forEach()` (callback per pair), `entries()` (`[key, value]` iterator), `keys()` and `values()` (key/value iterators). [S1]
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- **Objects can be Map keys** — an important Map feature; a string key won't match an object key, so `get("apples")` returns `undefined` when the key is an object. [S1]
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- **`Map.groupBy()` (ES2024)** groups elements of an iterable by a callback's return value, producing a Map. (`Object.groupBy()` groups into an object; `Map.groupBy()` groups into a Map.) [S1]
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## 🧩 추출된 패턴 (Extracted patterns)
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- **Iterate-and-accumulate** — `for...of` over `entries()`, `keys()`, or `values()` to build text or sum totals. [S1]
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- **delete-then-check** — `delete()` followed by `has()` to confirm removal. [S1]
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- **Object keys for identity** — store per-object data by using the object itself as the key. [S1]
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- **Group by predicate** — `Map.groupBy(iterable, callback)` to bucket items. [S1]
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## 📖 세부 내용 (Details)
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**The get() Method** — gets the value of a key in a Map: [S1]
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```javascript
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// Create a Map
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const fruits = new Map([
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["apples", 500],
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["bananas", 300],
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["oranges", 200]
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]);
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```
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```javascript
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fruits.get("apples");
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```
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**The set() Method** — adds or changes the value of a key: [S1]
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```javascript
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// Create a Map
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const fruits = new Map();
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// Set Map Values
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fruits.set("apples", 500);
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fruits.set("bananas", 300);
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fruits.set("oranges", 200);
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```
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```javascript
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fruits.set("apples", 500);
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```
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**The size Property** — returns the number of elements: [S1]
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```javascript
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fruits.size;
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```
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**The delete() Method** — removes a Map element specified by a key: [S1]
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```javascript
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fruits.delete("apples");
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```
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**The clear() Method** — removes all elements: [S1]
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```javascript
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fruits.clear();
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```
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**The has() Method** — returns `true` if a key exists: [S1]
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```javascript
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fruits.has("apples");
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```
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```javascript
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fruits.delete("apples");
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fruits.has("apples");
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```
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**The forEach() Method** — invokes a callback for each key/value pair: [S1]
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```javascript
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// List all entries
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let text = "";
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fruits.forEach (function(value, key) {
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text += key + ' = ' + value;
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})
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```
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**The entries() Method** — returns an iterator with the `[key, value]` pairs: [S1]
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```javascript
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// List all entries
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let text = "";
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for (const x of fruits.entries()) {
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text += x;
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}
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```
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**The keys() Method** — returns an iterator with the keys: [S1]
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```javascript
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// List all keys
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let text = "";
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for (const x of fruits.keys()) {
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text += x;
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}
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```
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**The values() Method** — returns an iterator with the values: [S1]
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```javascript
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// List all values
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let text = "";
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for (const x of fruits.values()) {
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text += x;
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}
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```
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You can use the `values()` method to sum the values in a Map: [S1]
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```javascript
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// Sum all values
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let total = 0;
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for (const x of fruits.values()) {
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total += x;
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}
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```
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**Objects as Keys** — being able to use objects as keys is an important Map feature: [S1]
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```javascript
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// Create Objects
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const apples = {name: 'Apples'};
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const bananas = {name: 'Bananas'};
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const oranges = {name: 'Oranges'};
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// Create a Map
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const fruits = new Map();
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// Add new Elements to the Map
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fruits.set(apples, 500);
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fruits.set(bananas, 300);
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fruits.set(oranges, 200);
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```
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Remember: the key is an object (`apples`), not a string (`"apples"`): [S1]
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```javascript
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fruits.get("apples"); // Returns undefined
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```
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**JavaScript Map.groupBy()** — the ES2024 `Map.groupBy()` method groups elements of an iterable into a Map according to a callback's return value. The difference between `Object.groupBy()` and `Map.groupBy()` is output type: the former groups elements into a JavaScript object, the latter into a Map object: [S1]
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```javascript
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// Create an Array
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const fruits = [
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{name:"apples", quantity:300},
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{name:"bananas", quantity:500},
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{name:"oranges", quantity:200},
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{name:"kiwi", quantity:150}
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];
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// Callback function to Group Elements
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function myCallback({ quantity }) {
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return quantity > 200 ? "ok" : "low";
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}
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// Group by Quantity
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const result = Map.groupBy(fruits, myCallback);
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```
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**Map Methods and Properties** [S1]
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| Method | Description |
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|--------|-------------|
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| new Map() | Creates a new Map |
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| set() | Sets the value for a key in a Map |
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| get() | Gets the value for a key in a Map |
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| has() | Returns true if a key exists in a Map |
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| delete() | Removes a Map element specified by a key |
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| clear() | Removes all the elements from a Map |
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| forEach() | Calls a function for each key/value pair in a Map |
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| entries() | Returns an iterator object with the [key, value] pairs in a Map |
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| keys() | Returns an iterator object with the keys in a Map |
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| values() | Returns an iterator object of the values in a Map |
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| size | Returns the number of elements in a Map |
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## 🛠️ 적용 사례 (Applied in summary)
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The page's own snippets are the canonical applied examples — building a `fruits` Map, reading `size`, removing/checking with `delete()`+`has()`, iterating via `forEach()`/`entries()`/`keys()`/`values()`, summing values, using objects as keys, and grouping an array with `Map.groupBy()`. No external project/commit applications found in the source.
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## 💻 코드 패턴 (Code patterns)
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Sum all values in a Map (language: JavaScript):
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```javascript
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let total = 0;
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for (const x of fruits.values()) {
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total += x;
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}
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```
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Group an array into a Map by a callback:
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```javascript
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const result = Map.groupBy(fruits, myCallback);
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```
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## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
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Both `Object.groupBy()` and `Map.groupBy()` group an iterable by a callback's return value; choose by desired output type: `Object.groupBy()` produces a plain JavaScript object, while `Map.groupBy()` produces a Map object. [S1]
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## ⚖️ 모순 및 업데이트 (Contradictions & updates)
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`Map.groupBy()` is an ECMAScript 2024 addition, so older environments may need a polyfill. A common pitfall noted by the source: object keys are matched by identity, so `get("apples")` (a string) returns `undefined` when the key was the object `apples`. No contradictions found in the source. [S1]
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## ✅ 검증 상태 및 신뢰도
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- **상태:** draft
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- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
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- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
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- **신뢰 점수:** 0.90
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- **중복 검사 결과:** 신규 생성 (New discovery)
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## 🔗 지식 그래프 (Knowledge Graph)
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- **상위/루트:** [[JavaScript Tutorial]]
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- **관련 개념:** [[JavaScript Maps]], [[JavaScript Set Methods]], [[JavaScript Sets]]
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- **참조 맥락:** Referenced whenever managing, iterating, or grouping the contents of a Map.
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## 📚 출처 (Sources)
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- [S1] W3Schools — JavaScript Map Methods — https://www.w3schools.com/js/js_map_methods.asp
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## 📝 변경 이력 (Change history)
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- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Map Methods" page (Astra wiki-curation, P-Reinforce v3.1 format).
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