JavaScript Maps

A Map holds key-value pairs where the keys can be any datatype.

A Map remembers the original insertion order of the keys.

How to Create a Map

You can create a JavaScript Map by:

  • Passing an Array to new Map()
  • Create a Map and use Map.set()

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]
]);

The set() Method

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);

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

Example

fruits.set("apples", 200);

The get() Method

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

Example

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

Maps are Objects

typeof returns object:

Example

// Returns object:
typeof fruits;

instanceof Map returns true:

Example

// Returns true:
fruits instanceof Map;

JavaScript Objects vs Maps

Differences between JavaScript Objects and 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

 

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]
]);

Map.get()

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

Example

fruits.get("apples");

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);

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

Example

fruits.set("apples", 500);

Map.size

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

Example

fruits.size;

Map.delete()

The delete() method removes a map element:

Example

fruits.delete("apples");

Map.clear()

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

Example

fruits.clear();

Map.has()

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

Example

fruits.has("apples");

Try This:

fruits.delete("apples");
fruits.has("apples");

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;
})

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;
}

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;
}

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;
}

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;
}

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);

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

Example

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

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);