--- title: Map slug: Web/JavaScript/Reference/Global_Objects/Map translation_of: Web/JavaScript/Reference/Global_Objects/Map ---

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and {{glossary("Primitive", "primitive values")}}) may be used as either a key or a value.

Description

A Map object iterates its elements in insertion order — a {{jsxref("Statements/for...of", "for...of")}} loop returns an array of [key, value] for each iteration.

Key equality

Key equality is based on the sameValueZero algorithm.
{{jsxref("NaN")}} is considered the same as NaN (even though NaN !== NaN) and all other values are considered equal according to the semantics of the === operator.
In the current ECMAScript specification, -0 and +0 are considered equal, although this was not so in earlier drafts. See "Value equality for -0 and 0" in the Browser compatibility table for details.

Objects vs. Maps

{{jsxref("Object")}} is similar to Map—both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. For this reason (and because there were no built-in alternatives), Objects have been used as Maps historically.

However, there are important differences that make Map preferable in certain cases:

	Map	Object
Accidental Keys	A `Map` does not contain any keys by default. It only contains what is explicitly put into it.	An `Object` has a prototype, so it contains default keys that could collide with your own keys if you're not careful. Note: As of ES5, this can be bypassed by using {{jsxref("Object.create", "Object.create(null)")}}, but this is seldom done.
Key Types	A `Map`'s keys can be any value (including functions, objects, or any primitive).	The keys of an `Object` must be either a {{jsxref("String")}} or a {{jsxref("Symbol")}}.
Key Order	The keys in `Map` are ordered. Thus, when iterating over it, a `Map` object returns keys in order of insertion.	The keys of an `Object` are not ordered. Note: Since ECMAScript 2015, objects do preserve creation order for string and `Symbol` keys. In JavaScript engines that comply with the ECMAScript 2015 spec, iterating over an object with only string keys will yield the keys in order of insertion.
Size	The number of items in a `Map` is easily retrieved from its {{jsxref("Map.prototype.size", "size")}} property.	The number of items in an `Object` must be determined manually.
Iteration	A `Map` is an iterable, so it can be directly iterated.	Iterating over an `Object` requires obtaining its keys in some fashion and iterating over them.
Performance	Performs better in scenarios involving frequent additions and removals of key-value pairs.	Not optimized for frequent additions and removals of key-value pairs.

Setting object properties

Setting Object properties works for Map objects as well, and can cause considerable confusion.

Therefore, this appears to work in a way:

let wrongMap = new Map()
wrongMap['bla'] = 'blaa'
wrongMap['bla2'] = 'blaaa2'

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

But that way of setting a property does not interact with the Map data structure. It uses the feature of the generic object. The value of 'bla' is not stored in the Map for queries. Othere operations on the data fail:

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

The correct usage for storing data in the Map is through the set(key, value) method.

let 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

Constructor

{{jsxref("Map/Map", "Map()")}}: Creates a new Map object.

Static properties

{{jsxref("Map.@@species", "get Map[@@species]")}}: The constructor function that is used to create derived objects.

Instance properties

{{jsxref("Map.prototype.size")}}: Returns the number of key/value pairs in the Map object.

Instance methods

{{jsxref("Map.prototype.clear()")}}: Removes all key-value pairs from the Map object.
{{jsxref("Map.delete", "Map.prototype.delete(key)")}}: Returns true if an element in the Map object existed and has been removed, or false if the element does not exist. Map.prototype.has(key) will return false afterwards.
{{jsxref("Map.prototype.entries()")}}: Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order.
{{jsxref("Map.forEach", "Map.prototype.forEach(callbackFn[, thisArg])")}}: Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback.
{{jsxref("Map.get", "Map.prototype.get(key)")}}: Returns the value associated to the key, or undefined if there is none.
{{jsxref("Map.has", "Map.prototype.has(key)")}}: Returns a boolean asserting whether a value has been associated to the key in the Map object or not.
{{jsxref("Map.prototype.keys()")}}: Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.
{{jsxref("Map.set", "Map.prototype.set(key, value)")}}: Sets the value for the key in the Map object. Returns the Map object.
{{jsxref("Map.prototype.values()")}}: Returns a new Iterator object that contains the values for each element in the Map object in insertion order.
{{jsxref("Map.@@iterator", "Map.prototype[@@iterator]()")}}: Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order.

Examples

Using the `Map` object

let myMap = new Map()

let keyString = 'a string'
let keyObj    = {}
let keyFunc   = function() {}

// setting the values
myMap.set(keyString, "value associated with 'a string'")
myMap.set(keyObj, 'value associated with keyObj')
myMap.set(keyFunc, 'value associated with keyFunc')

myMap.size              // 3

// getting the values
myMap.get(keyString)    // "value associated with 'a string'"
myMap.get(keyObj)       // "value associated with keyObj"
myMap.get(keyFunc)      // "value associated with keyFunc"

myMap.get('a string')    // "value associated with 'a string'"
                         // because keyString === 'a string'
myMap.get({})            // undefined, because keyObj !== {}
myMap.get(function() {}) // undefined, because keyFunc !== function () {}

Using `NaN` as `Map` keys

{{jsxref("NaN")}} can also be used as a key. Even though every NaN is not equal to itself (NaN !== NaN is true), the following example works because NaNs are indistinguishable from each other:

let myMap = new Map()
myMap.set(NaN, 'not a number')

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

let otherNaN = Number('foo')
myMap.get(otherNaN)
// "not a number"

Iterating `Map` with `for..of`

Maps can be iterated using a for..of loop:

let myMap = new Map()
myMap.set(0, 'zero')
myMap.set(1, 'one')

for (let [key, value] of myMap) {
  console.log(key + ' = ' + value)
}
// 0 = zero
// 1 = one

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

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

for (let [key, value] of myMap.entries()) {
  console.log(key + ' = ' + value)
}
// 0 = zero
// 1 = one

Iterating `Map` with `forEach()`

Maps can be iterated using the {{jsxref("Map.prototype.forEach", "forEach()")}} method:

myMap.forEach(function(value, key) {
  console.log(key + ' = ' + value)
})
// 0 = zero
// 1 = one

Relation with Array objects

let kvArray = [['key1', 'value1'], ['key2', 'value2']]

// Use the regular Map constructor to transform a 2D key-value Array into a map
let myMap = new Map(kvArray)

myMap.get('key1') // returns "value1"

// Use Array.from() to transform a map into a 2D key-value Array
console.log(Array.from(myMap)) // Will show you exactly the same Array as kvArray

// A succinct way to do the same, using the spread syntax
console.log([...myMap])

// Or use the keys() or values() iterators, and convert them to an array
console.log(Array.from(myMap.keys())) // ["key1", "key2"]

Cloning and merging `Map`s

Just like Arrays, Maps can be cloned:

let original = new Map([
  [1, 'one']
])

let clone = new Map(original)

console.log(clone.get(1))       // one
console.log(original === clone) // false (useful for shallow comparison)

Important: Keep in mind that the data itself is not cloned.

Maps can be merged, maintaining key uniqueness:

let first = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
])

let second = new Map([
  [1, 'uno'],
  [2, 'dos']
])

// Merge two maps. The last repeated key wins.
// Spread operator essentially converts a Map to an Array
let merged = new Map([...first, ...second])

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

Maps can be merged with Arrays, too:

let first = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
])

let second = new Map([
  [1, 'uno'],
  [2, 'dos']
])

// Merge maps with an array. The last repeated key wins.
let merged = new Map([...first, ...second, [1, 'eins']])

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

Specifications

Specification
{{SpecName('ESDraft', '#sec-map-objects', 'Map')}}

Description

Key equality

Objects vs. Maps

Setting object properties

Constructor

Static properties

Instance properties

Instance methods

Examples

Using the Map object

Using NaN as Map keys

Iterating Map with for..of

Iterating Map with forEach()