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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.

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