JavaScript: Map Object

By Xah Lee. Date: 2017-01-22. Last updated: 2020-07-02.

New in JS2015.

Map is the value of the property key "Map" of the Global Object .

console.log( window["Map"] === Map ); // true

Type

Type of Map is a function.

console.log( typeof Map === "function" ); // true

Parent

Parent of Map is Function.prototype. [see Prototype and Inheritance]

console.log( Reflect.getPrototypeOf ( Map ) === Function.prototype ); // true

Purpose

Purpose of Map is:

To create instance of “map” object. Instance of map object is a collection of key/value pairs as a lookup table data structure. This is different from the generic data object. The map object instance is designed specifically as a lookup table data structure.
Used as a namespace to hold general purpose methods for working with “map” instances.
Holds the property key "prototype". The value of Map.prototype is the parent object of all “map” instances.

Facts about map:

Map instances are collections of key/value pairs.
Key can be any type. [see Value Types]
Value can be any type.
Keys are always distinct.
The insertion order of entries are maintained.

[see the Map Object Tutorial]

How Map Determines Uniqueness of Keys

The equality test used for determining whether 2 keys in a map is the same as ===, except treatment of NaN (not a number).

NaN === NaN return false, but for map object, NaN is considered same as any NaN.

// this is false
console.log(NaN === NaN); // false

// for map object, NaN is same as any NaN
const m = new Map();
m.set(NaN, "n1");
m.set(NaN, "n2");

console.log(m) // Map { NaN => 'n2' }