Map() Constructor

The Map() constructor in JavaScript creates a new Map object. A Map holds key-value pairs, where keys can be of any type. The insertion order of elements is preserved, and it provides efficient lookup, addition, and deletion operations.

Syntax

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new Map()
new Map(iterable)

Parameters

ParameterDescription
iterableAn iterable object (such as an array) containing key-value pairs (arrays of length 2). If no iterable is provided, an empty Map is created.

Return Value

The new Map() constructor returns a new Map object containing the key-value pairs from the provided iterable or an empty map if no iterable is given.

Examples

1. Creating an Empty Map

The following example creates an empty Map and then adds key-value pairs using the set() method.

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const myMap = new Map();

myMap.set('name', 'Arjun');
myMap.set('age', 25);

console.log(myMap);

Output

Map(2) { 'name' => 'Arjun', 'age' => 25 }
  1. new Map() creates an empty map.
  2. set() method is used to add key-value pairs.

2. Initializing a Map with an Iterable

You can initialize a Map with an array of key-value pairs.

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const userMap = new Map([
  ['name', 'Bob'],
  ['age', 30],
  ['city', 'New York']
]);

console.log(userMap);

Output

Map(3) { 'name' => 'Bob', 'age' => 30, 'city' => 'New York' }
  1. The new Map(iterable) constructor initializes the Map with pre-defined key-value pairs.
  2. Each pair is stored as an entry in the Map.

3. Using Objects as Keys

The Map object allows keys of any type, including objects.

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const objKey = { id: 1 };
const myMap = new Map();

myMap.set(objKey, 'apple');

console.log(myMap.get(objKey));

Output

apple

Maps allow objects as keys, unlike regular objects where keys are always converted to strings.

4. Checking Map Size

The size property returns the number of key-value pairs in a Map.

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const fruitMap = new Map([
  ['apple', 10],
  ['banana', 5],
  ['cherry', 7]
]);

console.log(fruitMap.size);

Output

3

5. Iterating Over a Map

The forEach() method can iterate over the entries of a Map.

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const myMap = new Map([
  ['name', 'Arjun'],
  ['age', 25]
]);

myMap.forEach((value, key) => {
  console.log(`${key}: ${value}`);
});

Output

name: Arjun
age: 25

Each key-value pair is processed in the order it was inserted.