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 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 Object.create(null), 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 object injection attacks . 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 String or a Symbol.
Key Order

The keys in Map are ordered in a simple, 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. (for-in includes only enumerable string-keyed properties; Object.keys includes only own, enumerable, string-keyed properties; Object.getOwnPropertyNames includes own, string-keyed properties even if non-enumerable; Object.getOwnPropertySymbols does the same for just Symbol-keyed properties, etc.)

Size

 The number of items in a Map is easily retrieved from its 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 of the array returned from Object.keys().
Iteration A Map is an iterable, so it can be directly iterated.

Object does not implement an iteration protocol, and so objects are not directly iterable using the JavaScript for...of statement (by default).

Note:

  • An object can implement the iteration protocol, or you can get an iterable for an object using Object.keys or Object.entries.
  • The for...in 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 parsing

No native support for serialization or parsing.

(But you can build your own serialization and parsing support for Map by using JSON.stringify() with its replacer argument, and by using JSON.parse() with its reviver argument. See the Stack Overflow question How do you JSON.stringify an ES6 Map?).

Native support for serialization from Object to JSON, using JSON.stringify().

Native support for parsing from JSON to Object, using 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, 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