JavaScript Map

The JavaScript ES6 has introduced two new data structures, i.e Map and WeakMap.

Map is similar to objects in JavaScript that allows us to store elements in a key/value pair.

The elements in a Map are inserted in an insertion order. However, unlike an object, a map can contain objects, functions and other data types as key.

Create JavaScript Map

To create a Map, we use the new Map() constructor. For example,

// create a Map
const map1 = new Map(); // an empty map
console.log(map1); // Map {}

Insert Item to Map

After you create a map, you can use the set() method to insert elements to it. For example,

// create a set
let map1 = new Map();

// insert key-value pair
map1.set('info', {name: 'Jack', age: 26});
console.log(map1); // Map {"info" => {name: "Jack", age: 26}}

You can also use objects or functions as keys. For example,

// Map with object key
let map2 = new Map();

let obj = {};
map2.set(obj, {name: 'Jack', age: "26"});

console.log(map2); // Map {{} => {name: "Jack", age: "26"}}

Access Map Elements

You can access Map elements using the get() method. For example,

let map1 = new Map();
map1.set('info', {name: 'Jack', age: "26"});

// access the elements of a Map
console.log(map1.get('info')); // {name: "Jack", age: "26"}

Check Map Elements

You can use the has() method to check if the element is in a Map. For example,

const set1 = new Set([1, 2, 3]);

let map1 = new Map();
map1.set('info', {name: 'Jack', age: "26"});

// check if an element is in Set
console.log(map1.has('info')); // true

Removing Elements

You can use the clear() and the delete() method to remove elements from a Map.

The delete() method returns true if a specified key/value pair exists and has been removed or else returns false. For example,

let map1 = new Map();
map1.set('info', {name: 'Jack', age: "26"});

// removing a particular element
map1.delete('address'); // false
console.log(map1); // Map {"info" => {name: "Jack", age: "26"}} 

map1.delete('info'); // true
console.log(map1); // Map {}

The clear() method removes all key/value pairs from a Map object. For example,

let map1 = new Map();
map1.set('info', {name: 'Jack', age: "26"});

// removing all element
map1.clear();
console.log(map1); // Map {}

JavaScript Map Size

You can get the number of elements in a Map using the size property. For example,

let map1 = new Map();
map1.set('info', {name: 'Jack', age: "26"});

console.log(map1.size); // 1

Iterate Through a Map

You can iterate through the Map elements using the for...of loop or forEach() method. The elements are accessed in the insertion order. For example,

let map1 = new Map();
map1.set('name', 'Jack');
map1.set('age', '27');

// looping through Map
for (let [key, value] of map1) {
    console.log(key + '- ' + value);
}

Output

name- Jack
age- 27

You could also get the same results as the above program using the forEach() method. For example,

// using forEach method()
let map1 = new Map();
map1.set('name', 'Jack');
map1.set('age', '27');

// looping through Map
map1.forEach(function(value, key) {
  console.log(key + '- ' + value)
})

Iterate Over Map Keys

You can iterate over the Map and get the key using the keys() method. For example,

let map1 = new Map();
map1.set('name', 'Jack');
map1.set('age', '27');

// looping through the Map
for (let key of map1.keys()) {
  console.log(key)
}

Output

name
age

Iterate Over Map Values

You can iterate over the Map and get the values using the values() method. For example,

let map1 = new Map();
map1.set('name', 'Jack');
map1.set('age', '27');

// looping through the Map
for (let values of map1.values()) {
    console.log(values);
}

Output

Jack
27

Get Key/Values of Map

You can iterate over the Map and get the key/value of a Map using the entries() method. For example,

let map1 = new Map();
map1.set('name', 'Jack');
map1.set('age', '27');

// looping through the Map
for (let elem of map1.entries()) {
    console.log(`${elem[0]}: ${elem[1]}`);
}

Output

name: Jack
age: 27

JavaScript Map vs Object

JavaScript WeakMap

The WeakMap is similar to a Map. However, WeakMap can only contain objects as keys. For example,

const weakMap = new WeakMap();
console.log(weakMap); // WeakMap {} 

let obj = {};

// adding object (element) to WeakMap
weakMap.set(obj, 'hello');

console.log(weakMap); // WeakMap {{} => "hello"}

When you try to add other data types besides objects, WeakMap throws an error. For example,

const weakMap = new WeakMap();

// adding string as a key to WeakMap
weakMap.set('obj', 'hello');

// throws error
// TypeError: Attempted to set a non-object key in a WeakMap

WeakMap Methods

WeakMaps have methods get(), set(), delete(), and has(). For example,

const weakMap = new WeakMap();
console.log(weakMap); // WeakMap {} 

let obj = {};

// adding object (element) to WeakMap
weakMap.set(obj, 'hello');

console.log(weakMap); // WeakMap {{} => "hello"}

// get the element of a WeakMap
console.log(weakMap.get(obj)); // hello

// check if an element is present in WeakMap
console.log(weakMap.has(obj)); // true

// delete the element of WeakMap
console.log(weakMap.delete(obj)); // true

console.log(weakMap); // WeakMap {} 

WeakMaps Are Not iterable

Unlike Maps, WeakMaps are not iterable. For example,

const weakMap = new WeakMap();
console.log(weakMap); // WeakMap {} 

let obj = {};

// adding object (element) to WeakMap
weakMap.set(obj, 'hello');


// looping through WeakMap
for (let i of weakMap) {

    console.log(i);  // TypeError
}

JavaScript Map and WeakMap were introduced in ES6. Some browsers may not support their use. To learn more, visit JavaScript Map support and JavaScript WeakMap support.