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).

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.

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

Map (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, 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).

• Read-only Map-like objects have the property size, and the methods: entries(), forEach(), get(), has(), keys(), values(), and @@iterator.
• Writeable Map-like objects additionally have the methods: clear(), delete(), and 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:

• AudioParamMap
• RTCStatsReport
• EventCounts
• KeyboardLayoutMap
• MIDIInputMap
• MIDIOutputMap

Map()

Creates a new Map object.

Map[@@species]

The constructor function that is used to create derived objects.

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.

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.

Map.prototype.size

Returns the number of key/value pairs in the Map object.

Map.prototype[@@toStringTag]

The initial value of the @@toStringTag property is the string "Map". This property is used in Object.prototype.toString().

Map.prototype.clear()

Removes all key-value pairs from the Map object.

Map.prototype.delete()

Returns true if an element in the Map object existed and has been removed, or false if the element does not exist. map.has(key) will return false afterwards.

Map.prototype.entries()

Returns a new Iterator object that contains a two-member array of [key, value] for each element in the Map object in insertion order.

Map.prototype.forEach()

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()

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 the Map 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[@@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.

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 forEach() method:

myMap.forEach((value, key) => {
  console.log(`${key} = ${value}`);
});
// 0 = zero
// 1 = one

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)

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

• Polyfill for Map in core-js
• Set
• WeakMap
• WeakSet