JavaScript: Map Object Tutorial

By Xah Lee. Date: . Last updated: .

New in JS2015.

The map object is designed to be used as a map data structure (similar to Python Dictionary or Ruby Hash Table or Java Map. ).

What is the difference between object and map?

Create Map

new Map()
Create a empty map.
new Map(iterable)
Create a map from values in iterable object iterable.

[see Iterable Object]

const xx = new Map();

// add new item
xx.set(1, "n1");
xx.set(2, "n2");

// Map { 1 => "n1", 2 => "n2" }

There's no literal expression for map. But you can pass in a literal expression of array of array, like this:

new Map([ [key1, val1], [key2, val2] …])

const xx = new Map([[3, "n3"], [4, "n4"]]);

// Map { 3 => "n3", 4 => "n4" }

Note: the map constructor (new Map()) does not accept a object datatype.

[see Map Constructor]

[see Convert Object to/from Map πŸš€]

Parent of Map Objects

The parent of any map object is Map.prototype.

[see Prototype and Inheritance]

[see Map.prototype]

Set/Add Entry

Delete Entry

Check Key Existence

Get Key's Value

Size of Map

Clear Map

Iterate Map

Get All Keys

Get All Values

Nested Map

const nestedMap = new Map([
    new Map([
      ["old", new Map([["John", 59], ["Mary", 68]])],
      ["young", new Map([["Joe", 37], ["Smith", 34]])],
    new Map([
      ["old", new Map([["Jane", 22], ["Connie", 28]])],
      ["young", new Map([["Jenny", 67], ["David", 69]])],

console.log(nestedMap.get("tall").get("old").get("Mary") === 68);

Add Arbitrary Properties to Map

Because map is a object, you can add object properties to it, but you shouldn't do it.

const xx = new Map([[3, "n3"], [4, "n4"], [5, "n5"]]);

// you can add arbitrary properties to a map object
xx.pp = 22;
console.log(xx.pp === 22);

JavaScript Map Object

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