files/en-us/web/javascript/reference/global_objects/map/index.md
The Map object holds key-value pairs and remembers the original insertion order of the keys.
Any value (both objects and {{Glossary("Primitive", "primitive values")}}) may be used as either a key or a value.
{{InteractiveExample("JavaScript Demo: Map", "taller")}}
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 Map may only occur once; it is unique in the Map's collection. A Map object is iterated by key-value pairs — a {{jsxref("Statements/for...of", "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).
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 {{jsxref("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. Also, for object keys, equality is based on object identity. They are compared by reference, not by value. See Using the Map object for examples.
{{jsxref("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:
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
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, {{domxref("RTCStatsReport")}} 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).
Map-like objects have the property {{jsxref("Map/size", "size")}}, and the methods: {{jsxref("Map/entries", "entries()")}}, {{jsxref("Map/forEach", "forEach()")}}, {{jsxref("Map/get", "get()")}}, {{jsxref("Map/has", "has()")}}, {{jsxref("Map/keys", "keys()")}}, {{jsxref("Map/values", "values()")}}, and Symbol.iterator().Map-like objects additionally have the methods: {{jsxref("Map/clear", "clear()")}}, {{jsxref("Map/delete", "delete()")}}, and {{jsxref("Map/set", "set()")}}.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:
Map object.Map[Symbol.species]
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.
Map instances, the initial value is the {{jsxref("Map/Map", "Map")}} constructor.Map object.Map.prototype[Symbol.toStringTag]
[Symbol.toStringTag] property is the string "Map". This property is used in {{jsxref("Object.prototype.toString()")}}.Map object.Map.[key, value] for each element in the Map object in insertion order.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, or undefined if there is none.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. 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.Map or not.Map object in insertion order.Map, or updates an existing entry if the key already exists.Map object in insertion order.Map.prototype[Symbol.iterator]()
[key, value] for each element in the Map object in insertion order.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 !== () => {}
{{jsxref("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
NaNs 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"
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} = ${value}`);
}
// 0 = zero
// 1 = one
Maps can be iterated using the {{jsxref("Map/forEach", "forEach()")}} method:
myMap.forEach((value, key) => {
console.log(`${key} = ${value}`);
});
// 0 = zero
// 1 = one
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:
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 shallow copy of the
Map.
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, "un"]]);
console.log(merged.get(1)); // un
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three
{{Specifications}}
{{Compat}}
Map in core-jsMap