The Map
object holds key-value pairs and remembers the original insertion order of the keys.
Any value (both objects and
) may be used as either a key or a value.
Description
Map
objects are collections of key-value pairs. A key in the Map
may only occur once; it is unique in the Map
's collection. A Map
object is iterated by key-value pairs — a for...of loop returns a 2-member array of [key, value]
for each iteration. Iteration happens in insertion order, which corresponds to the order in which each key-value pair was first inserted into the map by the set()
method (that is, there wasn't a key with the same value already in the map when set()
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 the SameValueZero algorithm. (It used to use SameValue, which treated 0
and -0
as different. Check browser compatibility.) This means 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.
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 some
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: This can be bypassed by using [Object.create(null)](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/Object.create), but this is seldom done. |
Security |
A Map is safe to use with user-provided keys and values.
|
Setting user-provided key-value pairs on an |
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.
But note that no single mechanism
iterates
all of an object's properties; the various mechanisms
each include different subsets of properties.
([for-in](/~bremner/teaching/cs2613/books/mdn/Reference/Statements/for...in)
includes only enumerable string-keyed properties;
[Object.keys](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/Object/keys) includes only own, enumerable,
string-keyed properties;
[Object.getOwnPropertyNames](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/Object/getOwnPropertyNames) includes own,
string-keyed properties even if non-enumerable;
[Object.getOwnPropertySymbols](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/Object/getOwnPropertySymbols) does the same
for just |
Size |
The number of items in a Map is easily retrieved from its
[size](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/Map.prototype.size) property.
|
Determining the number of items in an Object is more roundabout and less efficient. A common way to do it is through the [length](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/Array/length) of the array returned from [Object.keys](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/Object/keys).
|
Iteration |
A Map is an
iterable, 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 Object to JSON, using [JSON.stringify](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/JSON/stringify). Native support for parsing from JSON to Object, using [JSON.parse](/~bremner/teaching/cs2613/books/mdn/Reference/Global_Objects/JSON/parse). |
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:
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 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.
const 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-like browser APIs
Browser Map
-like objects (or "maplike objects") are Web API interfaces that behave in many ways like a Map
.
Just like Map
, entries can be iterated in the same order that they were added to the object.
Map
-like objects and Map
also have properties and methods that share the same name and behavior.
However unlike Map
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,
is a Map
-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 the readonly
keyword in the IDL above).
- Read-only
Map
-like objects have the propertysize
, and the methods:entries()
,forEach()
,get()
,has()
,keys()
,values()
, and@@iterator
. - Writeable
Map
-like objects additionally have the methods:clear()
,delete()
, andset()
.
The methods and properties have the same behavior as the equivalent entities in Map
, except for the restriction on the types of the keys and values.
The following are examples of read-only Map
-like browser objects:
Constructor
- Map()
- : Creates a new
Map
object.
- : Creates a new
Static properties
- Map[@@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.
- : Groups the elements of a given iterable using the values returned by a provided callback function. The final returned
Instance properties
These properties are defined on Map.prototype
and shared by all Map
instances.
- Map.prototype.constructor
- : The constructor function that created the instance object. For
Map
instances, the initial value is the Map constructor.
- : The constructor function that created the instance object. For
- Map.prototype.size
- : Returns the number of key/value pairs in the
Map
object.
- : Returns the number of key/value pairs in the
Map.prototype[@@toStringTag]
- : The initial value of the
@@toStringTag
property is the string"Map"
. This property is used in Object.prototype.toString.
- : The initial value of the
Instance methods
- Map.prototype.clear
- : Removes all key-value pairs from the
Map
object.
- : Removes all key-value pairs from the
- Map.prototype.delete
- : Returns
true
if an element in theMap
object existed and has been removed, orfalse
if the element does not exist.map.has(key)
will returnfalse
afterwards.
- : Returns
- 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.
- : Returns a new Iterator object that contains a two-member array of
- 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.
- : Calls
- Map.prototype.get
- : Returns the value associated to the passed key, or
undefined
if there is none.
- : Returns the value associated to the passed key, or
- Map.prototype.has
- : Returns a boolean indicating whether a value has been associated with the passed key in the
Map
object or not.
- : Returns a boolean indicating whether a value has been associated with the passed key in the
- Map.prototype.keys
- : Returns a new Iterator object that contains the keys for each element in the
Map
object in insertion order.
- : Returns a new Iterator object that contains the keys for each element in the
- Map.prototype.set
- : Sets the value for the passed key in the
Map
object. Returns theMap
object.
- : Sets the value for the passed key in the
- Map.prototype.values
- : Returns a new Iterator object that contains the values for each element in the
Map
object in insertion order.
- : Returns a new Iterator object that contains the values for each element in the
Map.prototype[@@iterator]()
- : Returns a new Iterator object that contains a two-member array of
[key, value]
for each element in theMap
object in insertion order.
- : Returns a new Iterator object that contains a two-member array of
Examples
Using the Map object
const myMap = new Map();
const keyString = "a string";
const keyObj = {};
const 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");
console.log(myMap.size); // 3
// getting the values
console.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(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:
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 a for...of
loop:
const myMap = new Map();
myMap.set(0, "zero");
myMap.set(1, "one");
for (const [key, value] of myMap) {
console.log(`${key} = ${value}`);
}
// 0 = zero
// 1 = one
for (const key of myMap.keys()) {
console.log(key);
}
// 0
// 1
for (const value of myMap.values()) {
console.log(value);
}
// zero
// one
for (const [key, value] of myMap.entries()) {
console.log(`${key} = ${value}`);
}
// 0 = zero
// 1 = one
Iterating Map with forEach()
Maps can be iterated using the forEach() method:
myMap.forEach((value, key) => {
console.log(`${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 map
const myMap = new Map(kvArray);
console.log(myMap.get("key1")); // "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 Array
s, Map
s can be cloned:
const original = new Map(<span class="createlink">1, "one"</span>);
const 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:
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 Array
const 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:
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, "eins"]]);
console.log(merged.get(1)); // eins
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three