JavaScript Maps — JavaScript interactive lesson

The Map Object

A JavaScript Map is an object that can store collections of key-value pairs, similar to a dictionary in other programming languages.

Maps differ from standard objects in that keys can be of any data type .

Map Characteristics

Key Types Map keys can be any type (strings, numbers, objects, etc).
Insertion Order The Map remembers the original insertion order of the keys.
Size The number of items in a Map is easily retrieved using the size property.
Performance Maps are optimized for frequent additions and removals of key-value pairs.
Iteration Maps are iterable, allowing for direct use of for...of loops or methods like forEach().
Iteration Order The original order is preserved during iteration.

Maps are similar to both Objects (unique key/value collection) and Arrays (ordered values collection).

But if you look close, Maps are most similar to Objects.

How to Create a Map

You create a JavaScript Map by

Create a new Map and add elements with Map.set()
Passing an existing Array to the new Map() constructor

Map.set()

Adding Map Values

You can add elements to a Map with the set() method:

fruits.set("mangos", 100);

Changing Map Values

The set() method can also be used to change existing Map values:

fruits.set("apples", 200);

The get() Method

The get() method gets the value of a key in a Map:

fruits.get("apples"); // Returns 500

Maps are Objects

// Returns object:
typeof fruits;

instanceof Map returns true:

// Returns true:
fruits instanceof Map;

JavaScript Objects vs Maps

Object	Map
Not directly iterable	Directly iterable
Do not have a size property	Have a size property
Keys must be Strings (or Symbols)	Keys can be any datatype
Keys are not well ordered	Keys are ordered by insertion
Have default keys	Do not have default keys

Learn More

JavaScript Map Methods

JavaScript Weak Maps

JavaScript Map Reference

JavaScript Sets

Browser Support

Map is an ES6 feature .

ES6 is fully supported in all modern browsers since June 2017:

Chrome 51	Edge 15	Firefox 54	Safari 10	Opera 38
May 2016	Apr 2017	Jun 2017	Sep 2016	Jun 2016