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.

Syntax

new Map([iterable])

Parameters

iterable

An Array or other iterable object whose elements are key-value pairs. (For example, arrays with two elements, such as [[ 1, 'one' ],[ 2, 'two' ]].) Each key-value pair is added to the new Map.

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 and maps compared

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. Because of this (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:

• The keys of an Object are String and Symbol, whereas they can be any value for a Map (including functions, objects, or any primitive).
• The keys in Map are ordered, while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.

  Note: Since the ECMAScript 2015 spec, objects do preserve creation order for string and Symbol keys; so iterating over an object with only string keys would yield the keys in order of insertion.

• You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
• 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.
• An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful.

  Note: As of ES5, this can be bypassed by using Object.create(null), but this is seldom done.

• A Map may perform better in scenarios involving frequent additions and removals of key-value pairs.

Properties

Map.length

The value of the length property is 0.
To count how many entries are in a Map, use Map.prototype.size.

get Map[@@species]

The constructor function that is used to create derived objects.

Map.prototype

Represents the prototype for the Map constructor. Allows the addition of properties to all Map objects.

Map instances

All Map instances inherit from Map.prototype.

Properties

Map.prototype.constructor

Returns the function that created an instance's prototype. This is the Map function by default.

Map.prototype.size

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

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.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.

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.keys()

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

Map.prototype.set(key, value)

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

Map.prototype.values()

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

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

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 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 Maps

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)

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

Usage notes

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

Therefore, this still sort-of works....

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

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

...But, it does not behave as expected:

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

And there's very little difference from the correct usage, anyway: 

let myMap = new Map()
myMap.set('bla','blaa')
myMap.set('bla2','blaa2')
console.log(myMap)  // Map { 'bla' => 'blaa', 'bla2' => 'blaa2' }

myMap.has('bla')    // true
myMap.delete('bla') // true
console.log(myMap)  // Map { 'bla2' => 'blaa2' }

Specifications

Browser compatibility

See also

• Map and Set bug at Mozilla
• ECMAScript Harmony proposal
• Set
• WeakMap
• WeakSet