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The Map
object holds key-value pairs. 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 (arrays with two elements, e.g.[[ 1, 'one' ],[ 2, 'two' ]]
). Each key-value pair is added to the newMap
;null
values are treated asundefined
.
Description
A Map
object iterates its elements in insertion order — a for...of
loop returns an array of [key, value]
for each iteration.
It should be noted that a Map
which is a map of an object, especially a dictionary of dictionaries, will only map to the object's insertion order—which is random and not ordered.
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
Objects
are similar to Maps
in that 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), Object
s have been used as Maps
historically; however, there are important differences that make using a Map
preferable in certain cases:
- The keys of an
Object
areStrings
andSymbols
, whereas they can be any value for aMap
, including functions, objects, and any primitive. - You can get the size of a
Map
easily with thesize
property, while the number of properties in anObject
must be determined manually. - A
Map
is an iterable and can thus be directly iterated, whereas iterating over anObject
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. As of ES5 this can be bypassed by usingmap = Object.create(null)
, but this is seldom done. - A
Map
may perform better in scenarios involving frequent addition and removal of key pairs.
Properties
Map.length
- The value of the
length
property is 0. 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 allMap
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 theMap
object existed and has been removed, orfalse
if the element does not exist.Map.prototype.has(key)
will returnfalse
afterwards. Map.prototype.entries()
- Returns a new
Iterator
object that contains an array of[key, value]
for each element in theMap
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
, orundefined
if there is none. Map.prototype.has(key)
- Returns a boolean asserting whether a value has been associated to the
key
in theMap
object or not. Map.prototype.keys()
- Returns a new
Iterator
object that contains the keys for each element in theMap
object in insertion order. Map.prototype.set(key, value)
- Sets the value for the
key
in theMap
object. Returns theMap
object. Map.prototype.values()
- Returns a new
Iterator
object that contains the values for each element in theMap
object in insertion order. Map.prototype[@@iterator]()
- Returns a new
Iterator
object that contains an array of[key, value]
for each element in theMap
object in insertion order.
Examples
Using the Map
object
var myMap = new Map(); var keyString = 'a string', keyObj = {}, 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 NaN
s are indistinguishable from each other:
var myMap = new Map(); myMap.set(NaN, 'not a number'); myMap.get(NaN); // "not a number" var otherNaN = Number('foo'); myMap.get(otherNaN); // "not a number"
Iterating Maps
with for..of
Maps can be iterated using a for..of
loop:
var myMap = new Map(); myMap.set(0, 'zero'); myMap.set(1, 'one'); for (var [key, value] of myMap) { console.log(key + ' = ' + value); } // 0 = zero // 1 = one for (var key of myMap.keys()) { console.log(key); } // 0 // 1 for (var value of myMap.values()) { console.log(value); } // zero // one for (var [key, value] of myMap.entries()) { console.log(key + ' = ' + value); } // 0 = zero // 1 = one
Iterating Maps
with forEach()
Maps can be iterated using the forEach()
method:
myMap.forEach(function(value, key) { console.log(key + ' = ' + value); }); // Will show 2 logs; first with "0 = zero" and second with "1 = one"
Relation with Array
objects
var kvArray = [['key1', 'value1'], ['key2', 'value2']]; // Use the regular Map constructor to transform a 2D key-value Array into a map var myMap = new Map(kvArray); myMap.get('key1'); // returns "value1" // Use the Array.from function to transform a map into a 2D key-value Array console.log(Array.from(myMap)); // Will show you exactly the same Array as kvArray // Or use the keys or values iterators and convert them to an array console.log(Array.from(myMap.keys())); // Will show ["key1", "key2"]
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Map' in that specification. |
Standard | Initial definition. |
ECMAScript Latest Draft (ECMA-262) The definition of 'Map' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 38 | 12 | 13 | 11 | 25 | 8 |
new Map(iterable) | 38 | 12 | 13 | No | 25 | 9 |
new Map(null) | Yes | 12 | 37 | 11 | Yes | 9 |
Map() without new throws | Yes | 12 | 42 | 11 | Yes | 9 |
Key equality for -0 and 0 | 38 | 12 | 29 | No | 25 | 9 |
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 |
keys | 38 | 12 | 20 | No | 25 | 8 |
prototype | 38 | 12 | 13 | 11 | 25 | 8 |
set | 38 | 12 | 13 | 111 | 25 | 8 |
size | 38 | 12 | 192 | 11 | 25 | 8 |
values | 38 | 12 | 20 | No | 25 | 8 |
@@iterator | Yes | Yes | 36 | No | Yes | Yes |
@@species | 51 | 13 | 41 | No | 38 | 10 |
@@toStringTag | 44 | No | No | No | No | No |
Feature | Android webview | Chrome for Android | Edge mobile | Firefox for Android | Opera Android | iOS Safari | Samsung Internet |
---|---|---|---|---|---|---|---|
Basic support | 38 | 38 | 12 | 14 | 25 | 8 | Yes |
new Map(iterable) | 38 | 38 | 12 | 14 | 25 | 9 | Yes |
new Map(null) | Yes | Yes | 12 | 37 | Yes | 9 | Yes |
Map() without new throws | Yes | Yes | 12 | 42 | Yes | 9 | Yes |
Key equality for -0 and 0 | 38 | 38 | 12 | 29 | 25 | 9 | Yes |
clear | 38 | 38 | 12 | 19 | 25 | 8 | Yes |
delete | 38 | 38 | 12 | 14 | 25 | 8 | Yes |
entries | 38 | 38 | 12 | 20 | 25 | 8 | Yes |
forEach | 38 | 38 | 12 | 25 | 25 | 8 | Yes |
get | 38 | 38 | 12 | 14 | 25 | 8 | Yes |
has | 38 | 38 | 12 | 14 | 25 | 8 | Yes |
keys | 38 | 38 | 12 | 20 | 25 | 8 | Yes |
prototype | 38 | 38 | 12 | 14 | 25 | 8 | Yes |
set | 38 | 38 | 12 | 14 | 25 | 8 | Yes |
size | 38 | 38 | 12 | 192 | 25 | 8 | Yes |
values | 38 | 38 | 12 | 20 | 25 | 8 | Yes |
@@iterator | Yes | Yes | Yes | 36 | Yes | Yes | Yes |
@@species | 51 | 51 | 13 | 41 | 38 | 10 | 5.0 |
@@toStringTag | 44 | 44 | No | No | No | No | 4.0 |
1. Returns 'undefined' instead of the 'Map' object.
2. From Firefox 13 to Firefox 18, the size
property was implemented as a Map.prototype.size()
method, this has been changed to a property in later versions conform to the ECMAScript 2015 specification.
3. A placeholder property named @@iterator
is used.
4. Supported as @@iterator
.
5. A placeholder property named iterator
is used.
6. Supported as iterator
.