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 certain cases:

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. 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.

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

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 [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 [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

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)

Note: 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