Map

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

Description

A Map object iterates its elements in insertion order — a for...of loop returns an array of [key, value] for each iteration.

Key equality

  • Key equality is based on the sameValueZero algorithm.
  • 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

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), Object has been used as Map historically.

However, there are important differences that make Map preferable in some 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 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 String or a Symbol.
Key Order

The keys in Map are ordered in a simple, straightforward way: A Map object iterates entries, keys, and values in the order of entry insertion.

Although the keys of an ordinary Object are ordered now, this was not always the case, and the order is complex. As a result, it's best not to rely on property order.

The order was first defined for own properties only in ECMAScript 2015; ECMAScript 2020 defines order for inherited properties as well. See the OrdinaryOwnPropertyKeys and EnumerateObjectProperties abstract specification operations. But note that no single mechanism iterates all of an object's properties; the various mechanisms each include different subsets of properties. (for-in includes only enumerable string-keyed properties; Object.keys includes only own, enumerable, string-keyed properties; Object.getOwnPropertyNames includes own, string-keyed properties even if non-enumerable; Object.getOwnPropertySymbols does the same for just Symbol-keyed properties, etc.)

Size

 The number of items in a Map is easily retrieved from its 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.

Object does not implement an iteration protocol, and so objects are not directly iterable using the JavaScript for...of statement (by default).

Note:

  • An object can implement the iteration protocol, or you can get an iterable for an object using Object.keys or Object.entries.
  • The for...in statement allows you to iterate over the enumerable properties of an object.
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.
Serialization and parsing

No native support for serialization or parsing.

(But you can build your own serialization and parsing support for Map by using JSON.stringify() with its replacer argument, and by using JSON.parse() with its reviver argument. See the Stack Overflow question How do you JSON.stringify an ES6 Map?).

Native support for serialization from Object to JSON, using JSON.stringify().

Native support for parsing from JSON to Object, using JSON.parse().

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:

const 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. Other 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. 

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

Constructor

Map()

Creates a new Map object.

Static properties

get Map[@@species]

The constructor function that is used to create derived objects.

Instance properties

Map.prototype.size

Returns the number of key/value pairs in the Map object.

Instance methods

Map.prototype.clear()

Removes all key-value pairs from the Map object.

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.

Map.prototype.get(key)

Returns the value associated to the key, or undefined if there is none.

Map.prototype.has(key)

Returns a boolean asserting whether a value has been associated to the key in the Map object or not.

Map.prototype.set(key, value)

Sets the value for the key in the Map object. Returns the Map object.

Iteration methods

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.

Map.prototype.keys()

Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.

Map.prototype.values()

Returns a new Iterator object that contains the values for each element in the Map object in insertion order.

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.

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.

Examples

Using the Map object

const myMap = new Map()

const keyString = 'a string'
const keyObj    = {}
const 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

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: 

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"

Iterating Map with for..of

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

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

Iterating Map with forEach()

Maps can be iterated using the forEach() method: 

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

Relation with Array objects

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

// Use the regular Map constructor to transform a 2D key-value Array into a map
const 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 Maps

Just like Arrays, Maps can be cloned:

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

const clone = new Map(original)

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

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

Maps can be merged, maintaining key uniqueness:

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

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

// Merge two maps. The last repeated key wins.
// Spread operator essentially converts a Map to an 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

Maps can be merged with Arrays, too:

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

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

// Merge maps with an array. The last repeated key wins.
const 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
ECMAScript Language Specification
# sec-map-objects

Browser compatibility

BCD tables only load in the browser

See also