JavaScript: Map Object

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' }

Constructor

Map Constructor

Properties

• prototype