JavaScript: Map Object Tutorial

New in JS2015.

• Map objects are collections of key/value pairs. Each pair is called a entry.
• Key can be any Value Types.
• Value can be any value types.
• Keys are always distinct.
• The insertion order of entries are maintained.

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

console.log(xx);
// 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"]]);

console.log(xx);
// 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 🚀]

Parent of Map Objects

The parent of any map object is Map.prototype.

[see Prototype and Inheritance]

[see Map.prototype]

Set/Add Entry

• Map.prototype.set

Delete Entry

• Map.prototype.delete

Check Key Existence

• Map.prototype.has

Get Key's Value

• Map.prototype.get

Size of Map

• Map.prototype.size

Clear Map

• Map.prototype.clear

Iterate Map

• Iterate Over Map Object

Get All Keys

• Map.prototype.keys

Get All Values

• Map.prototype.values

Nested Map

const nestedMap = new Map([
  [
    "tall",
    new Map([
      ["old", new Map([["John", 59], ["Mary", 68]])],
      ["young", new Map([["Joe", 37], ["Smith", 34]])],
    ]),
  ],
  [
    "short",
    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);