2nd/10_Wiki/Topic_JavaScript/JavaScript_Map_Methods.md

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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-map-methods	JavaScript Map Methods	Frontend	draft	conceptual		Map methods Map.set Map.get Map.has Map.size Map.groupBy objects as keys		B	0.9	2026-06-23	2026-06-23			javascript js web frontend w3schools map methods	https://www.w3schools.com/js/js_map_methods.asp

JavaScript Map Methods

🎯 한 줄 통찰 (One-line insight)

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]

🧠 핵심 개념 (Core concepts)

• 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]
• Three iteration methods — forEach() (callback per pair), entries() ([key, value] iterator), keys() and values() (key/value iterators). [S1]
• 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]
• 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]

🧩 추출된 패턴 (Extracted patterns)

• Iterate-and-accumulate — for...of over entries(), keys(), or values() to build text or sum totals. [S1]
• delete-then-check — delete() followed by has() to confirm removal. [S1]
• Object keys for identity — store per-object data by using the object itself as the key. [S1]
• Group by predicate — Map.groupBy(iterable, callback) to bucket items. [S1]

📖 세부 내용 (Details)

The get() Method — gets the value of a key in a Map: [S1]

// Create a Map
const fruits = new Map([
  ["apples", 500],
  ["bananas", 300],
  ["oranges", 200]
]);

fruits.get("apples");

The set() Method — adds or changes the value of a key: [S1]

// Create a Map
const fruits = new Map();

// Set Map Values
fruits.set("apples", 500);
fruits.set("bananas", 300);
fruits.set("oranges", 200);

fruits.set("apples", 500);

The size Property — returns the number of elements: [S1]

fruits.size;

The delete() Method — removes a Map element specified by a key: [S1]

fruits.delete("apples");

The clear() Method — removes all elements: [S1]

fruits.clear();

The has() Method — returns true if a key exists: [S1]

fruits.has("apples");

fruits.delete("apples");
fruits.has("apples");

The forEach() Method — invokes a callback for each key/value pair: [S1]

// List all entries
let text = "";
fruits.forEach (function(value, key) {
  text += key + ' = ' + value;
})

The entries() Method — returns an iterator with the [key, value] pairs: [S1]

// List all entries
let text = "";
for (const x of fruits.entries()) {
  text += x;
}

The keys() Method — returns an iterator with the keys: [S1]

// List all keys
let text = "";
for (const x of fruits.keys()) {
  text += x;
}

The values() Method — returns an iterator with the values: [S1]

// List all values
let text = "";
for (const x of fruits.values()) {
  text += x;
}

You can use the values() method to sum the values in a Map: [S1]

// Sum all values
let total = 0;
for (const x of fruits.values()) {
  total += x;
}

Objects as Keys — being able to use objects as keys is an important Map feature: [S1]

// Create Objects
const apples = {name: 'Apples'};
const bananas = {name: 'Bananas'};
const oranges = {name: 'Oranges'};

// Create a Map
const fruits = new Map();

// Add new Elements to the Map
fruits.set(apples, 500);
fruits.set(bananas, 300);
fruits.set(oranges, 200);

Remember: the key is an object (apples), not a string ("apples"): [S1]

fruits.get("apples");  // Returns undefined

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]

// Create an Array
const fruits = [
  {name:"apples", quantity:300},
  {name:"bananas", quantity:500},
  {name:"oranges", quantity:200},
  {name:"kiwi", quantity:150}
];

// Callback function to Group Elements
function myCallback({ quantity }) {
  return quantity > 200 ? "ok" : "low";
}

// Group by Quantity
const result = Map.groupBy(fruits, myCallback);

Map Methods and Properties [S1]

Method	Description
new Map()	Creates a new Map
set()	Sets the value for a key in a Map
get()	Gets the value for a key in a Map
has()	Returns true if a key exists in a Map
delete()	Removes a Map element specified by a key
clear()	Removes all the elements from a Map
forEach()	Calls a function for each key/value pair in a Map
entries()	Returns an iterator object with the [key, value] pairs in a Map
keys()	Returns an iterator object with the keys in a Map
values()	Returns an iterator object of the values in a Map
size	Returns the number of elements in a Map

🛠️ 적용 사례 (Applied in summary)

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.

💻 코드 패턴 (Code patterns)

Sum all values in a Map (language: JavaScript):

let total = 0;
for (const x of fruits.values()) {
  total += x;
}

Group an array into a Map by a callback:

const result = Map.groupBy(fruits, myCallback);

⚖️ 비교 및 선택 기준 (Comparison & decision criteria)

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]

⚖️ 모순 및 업데이트 (Contradictions & updates)

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]

✅ 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: 0.90
• 중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

• 상위/루트: JavaScript Tutorial
• 관련 개념: JavaScript Maps, JavaScript Set Methods, JavaScript Sets
• 참조 맥락: Referenced whenever managing, iterating, or grouping the contents of a Map.

📚 출처 (Sources)

• [S1] W3Schools — JavaScript Map Methods — https://www.w3schools.com/js/js_map_methods.asp

📝 변경 이력 (Change history)

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