JS: Map Object

New in ES2015.

Map is the value of the property key "Map" of the global object. [see JS: the Global Object]

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

Type

Map is a function.

[see JS: Value Types]

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

Parent

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

// parent of Map
console.log ( Object.getPrototypeOf ( Map ) === Function.prototype ); // true

Purpose

Purpose of Map is:

1. 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 JavaScript object 〔JS: Object Type 〕. The map object instance is designed specifically as a lookup table data structure.
2. Used as a namespace to hold general purpose methods for working with “map” instances.
3. 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.
• The key can be any JavaScript type.
• The value can be any JavaScript type. [see JS: Value Types]
• Keys are always distinct.
• The insertion order of entries are maintained.

[see JS: 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

JS: Map Constructor

Properties

• Map.prototype

Reference

ECMAScript 2015 §Keyed Collection#sec-map-objects