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.
A Map
object iterates its elements in insertion order — a for...of
loop returns an array of [key, value]
for each iteration.
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.-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.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
s have been used as Map
s 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 Note: As of ES5, this can be bypassed by using |
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 | Although the keys of an ordinary 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. ( |
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. | 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 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
Map()
Map
object.get Map[@@species]
Map.prototype.size
Map
object.Map.prototype.clear()
Map
object.Map.prototype.delete(key)
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)
key
, or undefined
if there is none.Map.prototype.has(key)
key
in the Map
object or not.Map.prototype.set(key, value)
value
for the key
in the Map
object. Returns the Map
object.Map.prototype[@@iterator]()
[key, value]
for each element in the Map
object in insertion order.Map.prototype.keys()
Map
object in insertion order.Map.prototype.values()
Map
object in insertion order.Map.prototype.entries()
[key, value]
for each element in the Map
object in insertion order.Map.prototype.forEach(callbackFn[, thisArg])
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.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 () {}
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 NaN
s 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"
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
Maps can be iterated using the forEach()
method:
myMap.forEach(function(value, key) { console.log(key + ' = ' + value) }) // 0 = zero // 1 = one
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"]
Just like Array
s, Map
s 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
Desktop | ||||||
---|---|---|---|---|---|---|
Map |
38 | 12 | 13 | 11 | 25 | 8 |
Map() constructor |
38 | 12 | 13 | 11 | 25 | 8 |
clear |
38 | 12 | 19 | 11 | 25 | 8 |
delete |
38 | 12 | 13 | 11 | 25 | 8 |
entries |
38 | 12 | 20 | No | 25 | 8 |
forEach |
38 | 12 | 25 | 11 | 25 | 8 |
get |
38 | 12 | 13 | 11 | 25 | 8 |
has |
38 | 12 | 13 | 11 | 25 | 8 |
Key equality for -0 and 0 | 38 | 12 | 29 | No | 25 | 9 |
keys |
38 | 12 | 20 | No | 25 | 8 |
set |
38 | 12 | 13 | 11
|
25 | 8 |
size |
38 | 12 | 19
|
11 | 25 | 8 |
values |
38 | 12 | 20 | No | 25 | 8 |
@@iterator |
43 | 12 | 36
|
No | 30 | 10 |
@@species |
51 | 13 | 41 | No | 38 | 10 |
@@toStringTag |
44 | 79 | 51 | No | No | 9.1 |
Mobile | ||||||
---|---|---|---|---|---|---|
Map |
38 | 38 | 14 | 25 | 8 | 3.0 |
Map() constructor |
38 | 38 | 14 | 25 | 8 | 3.0 |
clear |
38 | 38 | 19 | 25 | 8 | 3.0 |
delete |
38 | 38 | 14 | 25 | 8 | 3.0 |
entries |
38 | 38 | 20 | 25 | 8 | 3.0 |
forEach |
38 | 38 | 25 | 25 | 8 | 3.0 |
get |
38 | 38 | 14 | 25 | 8 | 3.0 |
has |
38 | 38 | 14 | 25 | 8 | 3.0 |
Key equality for -0 and 0 | 38 | 38 | 29 | 25 | 9 | 3.0 |
keys |
38 | 38 | 20 | 25 | 8 | 3.0 |
set |
38 | 38 | 14 | 25 | 8 | 3.0 |
size |
38 | 38 | 19
|
25 | 8 | 3.0 |
values |
38 | 38 | 20 | 25 | 8 | 3.0 |
@@iterator |
43 | 43 | 36
|
30 | 10 | 4.0 |
@@species |
51 | 51 | 41 | 41 | 10 | 5.0 |
@@toStringTag |
44 | 44 | 51 | No | 9.3 | 4.0 |
Server | |
---|---|
Map |
0.12
|
Map() constructor |
0.12
|
clear |
0.12 |
delete |
0.12
|
entries |
0.12 |
forEach |
0.12 |
get |
0.12
|
has |
0.12
|
Key equality for -0 and 0 | 4.0.0 |
keys |
0.12 |
set |
0.12
|
size |
0.12 |
values |
0.12 |
@@iterator |
0.12 |
@@species |
6.5.0
|
@@toStringTag |
6.0.0 |
© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map