Creating Maps in JavaScript

In JavaScript, a Map is a collection of key-value pairs where keys can be of any type (objects, functions, primitives). Maps maintain the insertion order of elements and provide useful methods like .set(), .get(), .has(), .delete(), and more.

There are several ways to create a Map:

1. Using the Map Constructor (Empty Map)

You can create an empty Map and then add entries dynamically using the .set() method.

• Keys can be any type: string, number, boolean, object, or even a function.
• Map preserves the insertion order.

Syntax:

const myMap = new Map();

Example:

const myMap = new Map();

myMap.set('name', 'John');
myMap.set('age', 30);
myMap.set(true, 'isActive');

console.log(myMap);
// Output: Map(3) { 'name' => 'John', 'age' => 30, true => 'isActive' }

2. Initialize a Map with Key-Value Pairs

You can create a Map directly with an array of key-value pairs. Each pair is an array [key, value].

• Quick way to create a Map with predefined entries.
• Especially useful when converting objects or arrays into Maps.

Syntax:

const myMap = new Map([
  [key1, value1],
  [key2, value2],
  ...
]);

Example:

const myMap = new Map([
  ['name', 'Alice'],
  ['age', 25],
  [true, 'isActive']
]);

console.log(myMap);
// Output: Map(3) { 'name' => 'Alice', 'age' => 25, true => 'isActive' }

3. Using the .set() Method After Creation

You can start with an empty Map and add entries one by one using .set().

Calling .set() with an existing key overwrites the previous value.

Syntax:

map.set(key, value);

Example:

const myMap = new Map();

myMap.set('country', 'India');
myMap.set('capital', 'New Delhi');
myMap.set(1, 'Number Key');

console.log(myMap);
// Output: Map(3) { 'country' => 'India', 'capital' => 'New Delhi', 1 => 'Number Key' }

myMap.set('country', 'USA');
console.log(myMap.get('country')); // Output: USA

4. Converting Objects or Arrays into Maps

Since Maps support any type of key, you can easily convert an object or array into a Map.

From Object:

const obj = { name: 'John', age: 30 };
const myMap = new Map(Object.entries(obj));

console.log(myMap);
// Output: Map(2) { 'name' => 'John', 'age' => 30 }

From Array of Arrays:

const arr = [['id', 101], ['role', 'admin']];
const myMap = new Map(arr);

console.log(myMap);
// Output: Map(2) { 'id' => 101, 'role' => 'admin' }