Menu
×
   ❮     
HTML CSS JAVASCRIPT SQL PYTHON JAVA PHP HOW TO W3.CSS C C++ C# BOOTSTRAP REACT MYSQL JQUERY EXCEL XML DJANGO NUMPY PANDAS NODEJS R TYPESCRIPT ANGULAR GIT POSTGRESQL MONGODB ASP AI GO KOTLIN SASS VUE DSA GEN AI SCIPY AWS CYBERSECURITY DATA SCIENCE
     ❯   

JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Let JS Const JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Object Properties JS Object Methods JS Object Display JS Object Constructors JS Events JS Strings JS String Methods JS String Search JS String Templates JS Numbers JS BigInt JS Number Methods JS Number Properties JS Arrays JS Array Methods JS Array Search JS Array Sort JS Array Iteration JS Array Const JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS If Else JS Switch JS Loop For JS Loop For In JS Loop For Of JS Loop While JS Break JS Iterables JS Sets JS Set Methods JS Maps JS Map Methods JS Typeof JS Type Conversion JS Destructuring JS Bitwise JS RegExp JS Precedence JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Arrow Function JS Classes JS Modules JS JSON JS Debugging JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words

JS Versions

JS Versions JS 2009 (ES5) JS 2015 (ES6) JS 2016 JS 2017 JS 2018 JS 2019 JS 2020 JS 2021 JS 2022 JS 2023 JS 2024 JS IE / Edge JS History

JS Objects

Object Definitions Object Prototypes Object Methods Object Properties Object Get / Set Object Protection

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Bind Function Closures

JS Classes

Class Intro Class Inheritance Class Static

JS Async

JS Callbacks JS Asynchronous JS Promises JS Async/Await

JS HTML DOM

DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM Forms DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS Web APIs

Web API Intro Web Forms API Web History API Web Storage API Web Worker API Web Fetch API Web Geolocation API

JS AJAX

AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples

JS JSON

JSON Intro JSON Syntax JSON vs XML JSON Data Types JSON Parse JSON Stringify JSON Objects JSON Arrays JSON Server JSON PHP JSON HTML JSON JSONP

JS vs jQuery

jQuery Selectors jQuery HTML jQuery CSS jQuery DOM

JS Graphics

JS Graphics JS Canvas JS Plotly JS Chart.js JS Google Chart JS D3.js

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Editor JS Exercises JS Quiz JS Website JS Syllabus JS Study Plan JS Interview Prep JS Bootcamp JS Certificate

JS References

JavaScript Objects HTML DOM Objects


JavaScript Map Methods


The new Map() Method

You can create a map by passing an array to the new Map() constructor:

Example

// Create a Map
const fruits = new Map([
  ["apples", 500],
  ["bananas", 300],
  ["oranges", 200]
]);
Try it Yourself »

Map.get()

You get the value of a key in a map with the get() method

Example

fruits.get("apples");
Try it Yourself »

Map.set()

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

Example

// Create a Map
const fruits = new Map();

// Set Map Values
fruits.set("apples", 500);
fruits.set("bananas", 300);
fruits.set("oranges", 200);
Try it Yourself »

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

Example

fruits.set("apples", 500);
Try it Yourself »

Map.size

The size property returns the number of elements in a map:

Example

fruits.size;
Try it Yourself »

Map.delete()

The delete() method removes a map element:

Example

fruits.delete("apples");
Try it Yourself »

Map.clear()

The clear() method removes all the elements from a map:

Example

fruits.clear();
Try it Yourself »

Map.has()

The has() method returns true if a key exists in a map:

Example

fruits.has("apples");
Try it Yourself »

Try This:

fruits.delete("apples");
fruits.has("apples");
Try it Yourself »



Map.forEach()

The forEach() method invokes a callback for each key/value pair in a map:

Example

// List all entries
let text = "";
fruits.forEach (function(value, key) {
  text += key + ' = ' + value;
})
Try it Yourself »

Map.entries()

The entries() method returns an iterator object with the [key,values] in a map:

Example

// List all entries
let text = "";
for (const x of fruits.entries()) {
  text += x;
}
Try it Yourself »

Map.keys()

The keys() method returns an iterator object with the keys in a map:

Example

// List all keys
let text = "";
for (const x of fruits.keys()) {
  text += x;
}
Try it Yourself »

Map.values()

The values() method returns an iterator object with the values in a map:

Example

// List all values
let text = "";
for (const x of fruits.values()) {
  text += x;
}
Try it Yourself »

You can use the values() method to sum the values in a map:

Example

// Sum all values
let total = 0;
for (const x of fruits.values()) {
  total += x;
}
Try it Yourself »

Objects as Keys

Being able to use objects as keys is an important Map feature.

Example

// Create Objects
const apples = {name: 'Apples'};
const bananas = {name: 'Bananas'};
const oranges = {name: 'Oranges'};

// Create a Map
const fruits = new Map();

// Add new Elements to the Map
fruits.set(apples, 500);
fruits.set(bananas, 300);
fruits.set(oranges, 200);
Try it Yourself »

Remember: The key is an object (apples), not a string ("apples"):

Example

fruits.get("apples");  // Returns undefined
Try it Yourself »

JavaScript Map.groupBy()

ES2024 added the Map.groupBy() method to JavaScript.

The Map.groupBy() method groups elements of an object according to string values returned from a callback function.

The Map.groupBy() method does not change the original object.

Example

// Create an Array
const fruits = [
  {name:"apples", quantity:300},
  {name:"bananas", quantity:500},
  {name:"oranges", quantity:200},
  {name:"kiwi", quantity:150}
];

// Callback function to Group Elements
function myCallback({ quantity }) {
  return quantity > 200 ? "ok" : "low";
}

// Group by Quantity
const result = Map.groupBy(fruits, myCallback);
Try it Yourself »

Browser Support

Map.groupby() is an ES2024 feature.

It is supported in new browsers since March 2024:

Chrome 117 Edge 117 Firefox 119 Safari 17.4 Opera 103
Sep 2023 Sep 2023 Oct 2023 Okt 2024 May 2023

Warning

ES2024 features are relatively new.

Older browsers may need an alternative code (Polyfill)


Object.groupBy() vs Map.groupBy()

The difference between Object.groupBy() and Map.groupBy() is:

Object.groupBy() groups elements into a JavaScript object.

Map.groupBy() groups elements into a Map object.


Complete Map Reference

For a complete reference, go to our:

Complete JavaScript Map Reference.

The reference contains descriptions and examples of all Map Properties and Methods.

×

Contact Sales

If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail:
sales@w3schools.com

Report Error

If you want to report an error, or if you want to make a suggestion, send us an e-mail:
help@w3schools.com

W3Schools is optimized for learning and training. Examples might be simplified to improve reading and learning. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content. While using W3Schools, you agree to have read and accepted our terms of use, cookie and privacy policy.

Copyright 1999-2024 by Refsnes Data. All Rights Reserved. W3Schools is Powered by W3.CSS.