Map
BaselineWidely available *
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
* Some parts of this feature may have varying levels of support.
TheMap
object holds key-value pairs and remembers the original insertion order of the keys.Any value (both objects andprimitive values) may be used as either a key or a value.
Try it
const map1 = new Map();map1.set("a", 1);map1.set("b", 2);map1.set("c", 3);console.log(map1.get("a"));// Expected output: 1map1.set("a", 97);console.log(map1.get("a"));// Expected output: 97console.log(map1.size);// Expected output: 3map1.delete("b");console.log(map1.size);// Expected output: 2
Description
Map
objects are collections of key-value pairs. A key in theMap
may only occur once; it is unique in theMap
's collection. AMap
object is iterated by key-value pairs — afor...of
loop returns a 2-member array of[key, value]
for each iteration. Iteration happens ininsertion order, which corresponds to the order in which each key-value pair was first inserted into the map by theset()
method (that is, there wasn't a key with the same value already in the map whenset()
was called).
The specification requires maps to be implemented "that, on average, provide access times that are sublinear on the number of elements in the collection". Therefore, it could be represented internally as a hash table (with O(1) lookup), a search tree (with O(log(N)) lookup), or any other data structure, as long as the complexity is better than O(N).
Key equality
Value equality is based on theSameValueZero algorithm. (It used to useSameValue, which treated0
and-0
as different. Checkbrowser compatibility.) This meansNaN
is considered the same asNaN
(even thoughNaN !== NaN
) and all other values are considered equal according to the semantics of the===
operator.
Objects vs. Maps
Object
is similar toMap
—both let you set keys tovalues, retrieve those values, delete keys, and detect whether something isstored at a key. For this reason (and because there were no built-inalternatives),Object
has been used asMap
historically.
However, there are important differences that makeMap
preferable in somecases:
Map | Object | |
---|---|---|
Accidental Keys | AMap does not contain any keys by default. It only contains what is explicitly put into it. | An Note: This can be bypassed by using |
Security | AMap is safe to use with user-provided keys and values. | Setting user-provided key-value pairs on an |
Key Types | AMap 's keys can be any value (including functions, objects, or any primitive). | The keys of anObject must be either aString or aSymbol . |
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. But note that no single mechanism iteratesall of an object's properties; the various mechanisms each include different subsets of properties. ( |
Size | The number of items in aMap is easily retrieved from itssize property. | Determining the number of items in anObject is more roundabout and less efficient. A common way to do it is through thelength of the array returned fromObject.keys() . |
Iteration | AMap is aniterable, so it can be directly iterated. |
Note:
|
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 | Native support for serialization from Native support for parsing from JSON to |
Setting object properties
Setting Object properties works for Map objects as well, and can causeconsiderable 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 datastructure. It uses the feature of the generic object. The value of 'bla' is notstored in the Map for queries. Other operations on the data fail:
wrongMap.has("bla"); // falsewrongMap.delete("bla"); // falseconsole.log(wrongMap); // Map { bla: 'blaa', bla2: 'blaaa2' }
The correct usage for storing data in the Map is through theset(key, value)
method.
const contacts = new Map();contacts.set("Jessie", { phone: "213-555-1234", address: "123 N 1st Ave" });contacts.has("Jessie"); // truecontacts.get("Hilary"); // undefinedcontacts.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"); // falsecontacts.delete("Jessie"); // trueconsole.log(contacts.size); // 1
Map-like browser APIs
BrowserMap
-like objects (or "maplike objects") areWeb API interfaces that behave in many ways like aMap
.
Just likeMap
, entries can be iterated in the same order that they were added to the object.Map
-like objects andMap
also have properties and methods that share the same name and behavior.However unlikeMap
they only allow specific predefined types for the keys and values of each entry.
The allowed types are set in the specification IDL definition.For example,RTCStatsReport
is aMap
-like object that must use strings for keys and objects for values.This is defined in the specification IDL below:
interface RTCStatsReport { readonly maplike<DOMString, object>;};
Map
-like objects are either read-only or read-writable (see thereadonly
keyword in the IDL above).
- Read-only
Map
-like objects have the propertysize
, and the methods:entries()
,forEach()
,get()
,has()
,keys()
,values()
, andSymbol.iterator()
. - Writeable
Map
-like objects additionally have the methods:clear()
,delete()
, andset()
.
The methods and properties have the same behavior as the equivalent entities inMap
, except for the restriction on the types of the keys and values.
The following are examples of read-onlyMap
-like browser objects:
Constructor
Map()
Creates a new
Map
object.
Static properties
Map[Symbol.species]
The constructor function that is used to create derived objects.
Static methods
Map.groupBy()
Groups the elements of a given iterable using the values returned by a provided callback function. The final returned
Map
uses the unique values from the test function as keys, which can be used to get the array of elements in each group.
Instance properties
These properties are defined onMap.prototype
and shared by allMap
instances.
Map.prototype.constructor
The constructor function that created the instance object. For
Map
instances, the initial value is theMap
constructor.Map.prototype.size
Returns the number of key/value pairs in the
Map
object.Map.prototype[Symbol.toStringTag]
The initial value of the
[Symbol.toStringTag]
property is the string"Map"
. This property is used inObject.prototype.toString()
.
Instance methods
Map.prototype.clear()
Removes all key-value pairs from the
Map
object.Map.prototype.delete()
Returns
true
if an element in theMap
object existed and has beenremoved, orfalse
if the element does not exist.map.has(key)
will returnfalse
afterwards.Map.prototype.entries()
Returns a new Iterator object that contains a two-member array of
[key, value]
for each element in theMap
object in insertion order.Map.prototype.forEach()
Calls
callbackFn
once for each key-value pair present in theMap
object, in insertion order. If athisArg
parameter is provided toforEach
, it will be used as thethis
value for each callback.Map.prototype.get()
Returns the value associated to the passed key, or
undefined
if there is none.Map.prototype.has()
Returns a boolean indicating whether a value has been associated with the passed 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()
Sets the value for the passed key in the
Map
object. Returns theMap
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[Symbol.iterator]()
Returns a new Iterator object that contains a two-member array of
[key, value]
for each element in theMap
object in insertion order.
Examples
Using the Map object
const myMap = new Map();const keyString = "a string";const keyObj = {};const keyFunc = () => {};// setting the valuesmyMap.set(keyString, "value associated with 'a string'");myMap.set(keyObj, "value associated with keyObj");myMap.set(keyFunc, "value associated with keyFunc");console.log(myMap.size); // 3// getting the valuesconsole.log(myMap.get(keyString)); // "value associated with 'a string'"console.log(myMap.get(keyObj)); // "value associated with keyObj"console.log(myMap.get(keyFunc)); // "value associated with keyFunc"console.log(myMap.get("a string")); // "value associated with 'a string'", because keyString === 'a string'console.log(myMap.get({})); // undefined, because keyObj !== {}console.log(myMap.get(() => {})); // undefined, because keyFunc !== () => {}
Using NaN as Map keys
NaN
can also be used as a key. Even though everyNaN
isnot equal to itself (NaN !== NaN
is true), the following example works becauseNaN
s 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 afor...of
loop:
const myMap = new Map();myMap.set(0, "zero");myMap.set(1, "one");for (const [key, value] of myMap) { console.log(`${key} = ${value}`);}// 0 = zero// 1 = onefor (const key of myMap.keys()) { console.log(key);}// 0// 1for (const value of myMap.values()) { console.log(value);}// zero// onefor (const [key, value] of myMap.entries()) { console.log(`${key} = ${value}`);}// 0 = zero// 1 = one
Iterating Map with forEach()
Maps can be iterated using theforEach()
method:
myMap.forEach((value, key) => { console.log(`${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 mapconst myMap = new Map(kvArray);console.log(myMap.get("key1")); // "value1"// Use Array.from() to transform a map into a 2D key-value Arrayconsole.log(Array.from(myMap)); // Will show you exactly the same Array as kvArray// A succinct way to do the same, using the spread syntaxconsole.log([...myMap]);// Or use the keys() or values() iterators, and convert them to an arrayconsole.log(Array.from(myMap.keys())); // ["key1", "key2"]
Cloning and merging Maps
Just likeArray
s,Map
s can be cloned:
const original = new Map([[1, "one"]]);const clone = new Map(original);console.log(clone.get(1)); // oneconsole.log(original === clone); // false (useful for shallow comparison)
Note:Keep in mind thatthe data itself is not cloned. In other words, it is only ashallow copy of theMap
.
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 syntax essentially converts a Map to an Arrayconst merged = new Map([...first, ...second]);console.log(merged.get(1)); // unoconsole.log(merged.get(2)); // dosconsole.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, "un"]]);console.log(merged.get(1)); // unconsole.log(merged.get(2)); // dosconsole.log(merged.get(3)); // three
Specifications
Specification |
---|
ECMAScript® 2026 Language Specification # sec-map-objects |