const map = new Map(); map.set("a", 1); map.set("b", 2); map.set("c", 3); console.log(map.get("a")); // Expected output: 1 map.set("a", 97); console.log(map.get("a")); // Expected output: 97 console.log(map.size); // Expected output: 3 map.delete("b"); console.log(map.size); // Expected output: 2
Map objects are collections of key-value pairs. A key in the Mapmay only occur once; it is unique in the Map's collection. A Map object is iterated by key-value pairs — a
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
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).
Value equality is based on the
algorithm. (It used to use
, which treated 0 and -0 as different. Check
.) This means
is considered the same as NaN (even though NaN !== NaN) and all other values are considered equal according to the semantics of the === operator. Also, for object keys, equality is based on object identity. They are compared by reference, not by value. See
for examples.
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:
MapObjectAccidental Keys A Map does not contain any keys by default. It only contains what is explicitly put into it. An Object has a prototype, so it contains default keys that could collide with your own keys if you're not careful.
Note: This can be bypassed by using
, 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 Object may allow an attacker to override the object's prototype, which can lead to
or
. Like the accidental keys issue, this can also be mitigated by using a null-prototype object.
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
or a
. Key Order The keys in Map are ordered in a straightforward way: A Map object iterates entries, keys, and values in the order of entry insertion.
Although the keys of an ordinary Object are ordered now, this was not always the case, and the order is complex. As a result, it's best not to rely on property order.
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. (
includes only enumerable string-keyed properties;
includes only own, enumerable, string-keyed properties;
includes own, string-keyed properties even if non-enumerable;
does the same for just Symbol-keyed properties, etc.)
Size
The number of items in a Map is easily retrieved from its
property. Determining the number of items in an Object is more roundabout and less efficient. A common way to do it is through the
of the array returned from
. Iteration A Map is an
, so it can be directly iterated. Object does not implement an
, and so objects are not directly iterable using the JavaScript
statement (by default).
Note:
An object can implement the iteration protocol, or you can get an iterable for an object using
or
.
The
statement allows you to iterate over the enumerable properties of an object.
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 parsingNo native support for serialization or parsing.
(But you can build your own serialization and parsing support for Map by using
with its replacer argument, and by using
with its reviver argument. See the Stack Overflow question
How do you JSON.stringify an ES6 Map?
).
Native support for serialization from
to JSON, using
.
Native support for parsing from JSON to
, using
.
Setting Object properties works for Map objects as well, and can cause considerable confusion.
Therefore, this appears to work in a way:
js
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:
js
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.
js
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
Browser Map-like objects (or "maplike objects") are
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:
webidl
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 property
, and the methods:
,
,
,
,
,
, and
.
Writeable Map-like objects additionally have the methods:
,
, and
.
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:
Creates a new Map object.
The constructor function that is used to create derived objects.
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.
These properties are defined on Map.prototype and shared by all Map instances.
The constructor function that created the instance object. For Map instances, the initial value is the
constructor.
Returns the number of key/value pairs in the Map object.
Map.prototype[Symbol.toStringTag]
The initial value of the
property is the string "Map". This property is used in
.
Removes all key-value pairs from the Map object.
Removes the entry specified by the key from this Map.
Returns a new Iterator object that contains a two-member array of [key, value] for each element in the Map object in insertion order.
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.
Returns the value corresponding to the key in this Map, or undefined if there is none.
Returns the value corresponding to the specified key in this Map. If the key is not present, it inserts a new entry with the key and a given default value, and returns the inserted value.
Map.prototype.getOrInsertComputed()
Returns the value corresponding to the specified key in this Map. If the key is not present, it inserts a new entry with the key and a default value computed from a given callback, and returns the inserted value.
Returns a boolean indicating whether an entry with the specified key exists in this Map or not.
Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.
Adds a new entry with a specified key and value to this Map, or updates an existing entry if the key already exists.
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 the Map object in insertion order.
js
const myMap = new Map(); const keyString = "a string"; const keyObj = {}; const keyFunc = () => {}; // 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(() => {})); // undefined, because keyFunc !== () => {}
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:
js
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"
Maps can be iterated using a for...of loop:
js
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} = ${value}`); } // 0 = zero // 1 = one
Maps can be iterated using the
method:
js
myMap.forEach((value, key) => { console.log(`${key} = ${value}`); }); // 0 = zero // 1 = one
js
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"]
Just like Arrays, Maps can be cloned:
js
const original = new Map([[1, "one"]]); 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. In other words, it is only a
of the Map.
Maps can be merged, maintaining key uniqueness:
js
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:
js
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)); // un console.log(merged.get(2)); // dos console.log(merged.get(3)); // three
Specification