Map

Baseline *

20157

Map

Map API

const map = new Map();

map.set("a", 1);
map.set("b", 2);
map.set("c", 3);

console.log(map.get("a"));
// : 1

map.set("a", 97);

console.log(map.get("a"));
// : 97

console.log(map.size);
// : 3

map.delete("b");

console.log(map.size);
// : 2

Map Map Map Map for...of [, ] 2 set() set()

O(N) O(1) O(log(N))

SameValueZero SameValue 0 -0 NaN NaN NaN !== NaN === Map

Object Map

Object Map Object Map

Map

	Map	Object
	`Map`	`Object` : ES5 `Object.create(null)`
	`Map`	`Object` `null`-prototype
	`Map`	`Object`
	`Map` `Map`	`Object` ECMAScript 2015 ECMAScript 2020 `for-in` `Object.keys` `Object.getOwnPropertyNames` `Object.getOwnPropertySymbols` `Symbol`
	`Map` `size`	`Object` `Object.keys()` `length`
	`Map`	`Object` `Object` JavaScript for...of : `Object.keys` `Object.entries` for...in

	replacer `JSON.stringify()` reviver `JSON.parse()` `Map` Stack Overflow How do you JSON.stringify an ES6 Map?	`Object` JSON `JSON.stringify()` JSON `Object` `JSON.parse()`

Map

const wrongMap = new Map();
wrongMap["bla"] = "blaa";
wrongMap["bla2"] = "blaaa2";

console.log(wrongMap); // Map { bla: 'blaa', bla2: 'blaaa2' }

Map 'bla' Map

wrongMap.has("bla"); // false
wrongMap.delete("bla"); // false
console.log(wrongMap); // Map { bla: 'blaa', bla2: 'blaaa2' }

Map set(key, value)

const contacts = new Map();
contacts.set("Jessie", { phone: "213-555-1234", address: "123 N 1st Ave" });
contacts.has("Jessie"); // true
contacts.get("Hilary"); // undefined
contacts.set("Hilary", { phone: "617-555-4321", address: "321 S 2nd St" });
contacts.get("Jessie"); // {phone: "213-555-1234", address: "123 N 1st Ave"}
contacts.delete("Raymond"); // false
contacts.delete("Jessie"); // true
console.log(contacts.size); // 1

Map API

Map (Map ) Map Web API

Map Map Map Map

IDL RTCStatsReport Map IDL

webidl

interface RTCStatsReport {
  readonly maplike<DOMString, object>;
};

Map ( IDL readonly )

Map size entries(), forEach(), get(), has(), keys(), values(), Symbol.iterator()
Map clear(), delete(), set()

Map

Map(): Map

Map[Symbol.species]

Map.groupBy(): Map

Map.prototype Map

Map.prototype.constructor: Map Map
Map.prototype.size: Map
Map.prototype[Symbol.toStringTag]: [Symbol.toStringTag] "Map" Object.prototype.toString()

Map.prototype.clear(): Map
Map.prototype.delete(): Map
Map.prototype.entries(): Map [key, value] 2
Map.prototype.forEach(): Map callbackFn thisArg forEach this
Map.prototype.get(): Map undefined
Map.prototype.getOrInsert(): Map
Map.prototype.getOrInsertComputed(): Map
Map.prototype.has(): Map
Map.prototype.keys(): Map
Map.prototype.set(): Map
Map.prototype.values(): Map
Map.prototype[Symbol.iterator](): Map [key, value]

Map

const myMap = new Map();

const keyString = "";
const keyObj = {};
const keyFunc = () => {};

// 
myMap.set(keyString, "'' ");
myMap.set(keyObj, "keyObj ");
myMap.set(keyFunc, "keyFunc ");

console.log(myMap.size); // 3

// 
console.log(myMap.get(keyString)); // "'' "
console.log(myMap.get(keyObj)); // "keyObj "
console.log(myMap.get(keyFunc)); // "keyFunc "

console.log(myMap.get("")); // "'' " keyString === '' 
console.log(myMap.get({})); // undefined, keyObj !== {} 
console.log(myMap.get(() => {})); // undefined, keyFunc !== () => {}

NaN Map

NaN NaN NaN !== NaN NaN

const myMap = new Map();
myMap.set(NaN, "not a number");

myMap.get(NaN);
// "not a number"

const otherNaN = Number("foo");
myMap.get(otherNaN);
// "not a number"

for..of Map

for..of

const myMap = new Map();
myMap.set(0, "zero");
myMap.set(1, "one");

for (const [key, value] of myMap) {
  console.log(`${key} = ${value}`);
}
// 0 = zero
// 1 = one

for (const key of myMap.keys()) {
  console.log(key);
}
// 0
// 1

for (const value of myMap.values()) {
  console.log(value);
}
// zero
// one

for (const [key, value] of myMap.entries()) {
  console.log(`${key} = ${value}`);
}
// 0 = zero
// 1 = one

forEach() Map

forEach()

myMap.forEach((value, key) => {
  console.log(`${key} = ${value}`);
});
// 0 = zero
// 1 = one

Array

const kvArray = [
  ["1", "1"],
  ["2", "2"],
];

//  Map  2 
const myMap = new Map(kvArray);

console.log(myMap.get("1")); // "1"

//  2 
console.log(Array.from(myMap)); // kvArray  Array 

// 
console.log([...myMap]);

//  keys()  values() 
console.log(Array.from(myMap.keys())); // ["key1", "key2"]

Map

Array Map

const original = new Map([[1, "one"]]);

const clone = new Map(original);

console.log(clone.get(1)); // one
console.log(original === clone); // false

: Map

const first = new Map([
  [1, "one"],
  [2, "two"],
  [3, "three"],
]);

const second = new Map([
  [1, "uno"],
  [2, "dos"],
]);

// 2 
//  Map  Array 
const merged = new Map([...first, ...second]);

console.log(merged.get(1)); // uno
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three

Map Array

const first = new Map([
  [1, "one"],
  [2, "two"],
  [3, "three"],
]);

const second = new Map([
  [1, "uno"],
  [2, "dos"],
]);

// 
const merged = new Map([...first, ...second, [1, "un"]]);

console.log(merged.get(1)); // un
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three


ECMAScript 2027 LanguageSpecification # sec-map-objects

MDN

MDN 2026521

GitHub