Map.prototype.set()

map.set(key, value)

The set() method adds or updates a key-value pair in a Map. It returns the Map itself, enabling method chaining.

Syntax

map.set(key, value)

Parameters

Parameter	Type	Description
key	any	The key to set
value	any	The value to associate with the key

Examples

Basic usage

const userRoles = new Map();

userRoles.set('alice', 'admin');
userRoles.set('bob', 'editor');

console.log(userRoles.get('alice'));
// 'admin'

Updating existing values

const scores = new Map();

scores.set('player1', 100);
scores.set('player1', 150); // Updates the value

console.log(scores.get('player1'));
// 150

Method chaining

const userRoles = new Map();

// set() returns the Map, enabling chaining
userRoles.set('alice', 'admin')
         .set('bob', 'editor')
         .set('charlie', 'viewer');

console.log([...userRoles.entries()]);
// [['alice', 'admin'], ['bob', 'editor'], ['charlie', 'viewer']]

Map with object keys

const visits = new Map();

const page1 = { path: '/home', lang: 'en' };
const page2 = { path: '/home', lang: 'es' };

visits.set(page1, 1500);
visits.set(page2, 800);

console.log(visits.get(page1));
// 1500

Map with various value types

const data = new Map();

data.set('users', ['alice', 'bob']);
data.set('config', { theme: 'dark', lang: 'en' });
data.set('callback', () => console.log('ready'));

console.log(data.get('users'));
// ['alice', 'bob']

console.log(data.get('config').theme);
// 'dark'

data.get('callback')();
// 'ready'

Common Patterns

Caching with Map

function fibonacci(n, cache = new Map()) {
  if (cache.has(n)) return cache.get(n);
  if (n <= 1) return n;
  
  const result = fibonacci(n - 1, cache) + fibonacci(n - 2, cache);
  cache.set(n, result);
  return result;
}

console.log(fibonacci(50));
// 12586269025

Building a Map from an object

const obj = { a: 1, b: 2, c: 3 };
const map = new Map(Object.entries(obj));

map.set('d', 4);
console.log(map.get('b'));
// 2

Conditional updates

const inventory = new Map([
  ['apples', 10],
  ['bananas', 5]
]);

function addStock(fruit, quantity) {
  const current = inventory.get(fruit) || 0;
  inventory.set(fruit, current + quantity);
}

addStock('apples', 5);
addStock('oranges', 20);

console.log([...inventory]);
// [['apples', 15], ['bananas', 5], ['oranges', 20]]

See Also

• map::get
• map::has
• Map