Map

BrandonDusch

Published Nov 5, 2021Updated Dec 31, 2021

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A Map is an object in JavaScript that stores entries of key-value pairs in their original insertion order.

Values mapped to existing keys can be overwritten later.
Keys/values can either be an object or a variable of any data type.
Maps are directly iterable whereas objects are not.

Syntax

A map can be defined with the new keyword. The example below creates an empty Map object, map:

const map = new Map();

Example

To create a non-empty Map object, an array of arrays is passed into Map(). Each inner array represents a key-value pair:

const hogwartsStudents = new Map([
  ['Gryffindor', 'Harry Potter'],
  ['Slytherin', 'Draco Malfoy'],
  ['Hufflepuff', 'Cedric Diggory'],
]);

console.log(hogwartsStudents);

The output will be:

Map(3) {
  'Gryffindor' => 'Harry Potter',
  'Slytherin' => 'Draco Malfoy',
  'Hufflepuff' => 'Cedric Diggory'
}

Codebyte Example

In the example below, an addressBook maps a person’s name to a phone number. By the end of the program:

Tom has moved and deleted their phone number with the .delete() method.
Paul has changed their phone number using the .set() method.

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Map

.delete(): Removes the entry associated with a given key from a Map object.
.get(): Retrieves a value associated with a given key in a Map object.
.has(): Returns a boolean reflecting whether an entry with a given key exists in a Map object.
.set(): Stores or updates entries of key-value pairs in a Map object.
.size: Returns the number of entries inside of a Map object.