Map.prototype.has()

map.has(key)

Returns: boolean · Updated March 13, 2026 · Map and Set

map has contains

The has() method returns a boolean indicating whether a Map contains a specific key. Unlike object property checking with in operator, has() only checks the Map’s own entries and doesn’t traverse the prototype chain.

Syntax

map.has(key)

Parameters

Parameter	Type	Description
`key`	`any`	The key to check for in the Map

Return Value

Returns true if the key exists in the Map, false otherwise.

Examples

Basic usage

const userRoles = new Map();
userRoles.set('alice', 'admin');
userRoles.set('bob', 'editor');

console.log(userRoles.has('alice'));
// true

console.log(userRoles.has('unknown'));
// false

Using with object keys

const visits = new Map();

const page1 = { path: '/home', lang: 'en' };
visits.set(page1, 1500);

console.log(visits.has(page1));
// true

// Different object reference - not found
console.log(visits.has({ path: '/home', lang: 'en' }));
// false

Conditional value retrieval

const cache = new Map();

function getCached(key) {
  if (!cache.has(key)) {
    return null;
  }
  return cache.get(key);
}

cache.set('users', ['alice', 'bob']);
console.log(getCached('users')); // ['alice', 'bob']
console.log(getCached('missing')); // null

Before delete operation

const queue = new Map([['job1', { status: 'running' }]]);

function cancelJob(jobId) {
  if (!queue.has(jobId)) {
    console.log(`Job ${jobId} not found`);
    return false;
  }
  
  queue.delete(jobId);
  console.log(`Job ${jobId} cancelled`);
  return true;
}

cancelJob('job1'); // Job job1 cancelled
cancelJob('job2'); // Job job2 not found

Common Patterns

Prevent overwriting existing entries

const registry = new Map();

function register(name, handler) {
  if (registry.has(name)) {
    throw new Error(`Handler ${name} already registered`);
  }
  registry.set(name, handler);
}

register('init', () => {});
register('init', () => {}); // Throws Error

Key existence before access

const configs = new Map([
  ['production', { debug: false }],
  ['development', { debug: true }]
]);

function getConfig(env) {
  const exists = configs.has(env);
  console.log(`Config for ${env} exists: ${exists}`);
  return configs.get(env);
}

getConfig('production'); // true
getConfig('staging'); // false - returns undefined