Skip to main content

How to Get the Length (Size) of a Map in JavaScript

The Map object is a powerful data structure for storing key-value pairs, and a fundamental operation is to determine how many entries it contains. Unlike arrays which use the .length property, Map objects have a specific property for this purpose: .size.

This guide will teach you how to use the .size property to get the number of key-value pairs in a Map and explain its key differences from an array's .length property.

The Map.prototype.size property is the standard, built-in way to get the number of elements in a Map. It returns an integer representing the count of key-value pairs.

For example, you have a Map and you want to know how many entries it holds.

// Problem: How to get the number of elements in this Map?
const userRoles = new Map([
  ['user1', 'admin'],
  ['user2', 'editor'],
  ['user3', 'viewer'],
]);

This .size property is all you need as solution:

const userRoles = new Map([
  ['user1', 'admin'],
  ['user2', 'editor'],
  ['user3', 'viewer'],
]);

const mapSize = userRoles.size;

console.log(mapSize); // Output: 3
note

If the Map is empty, .size will correctly return 0. ::

How the .size Property Works​

The .size property is a "getter" that is automatically updated whenever you modify the Map. It provides a live, accurate count of the elements.

const myMap = new Map();
console.log(myMap.size); // Output: 0

// Add elements
myMap.set('a', 1);
myMap.set('b', 2);
console.log(myMap.size); // Output: 2

// Remove an element
myMap.delete('a');
console.log(myMap.size); // Output: 1

// Clear all elements
myMap.clear();
console.log(myMap.size); // Output: 0

A Crucial Distinction: .size vs. Array.prototype.length​

While they serve a similar purpose, there is a key difference between a Map's .size and an Array's .length.

  • map.size is read-only. You cannot change the number of elements in a Map by setting its size property. 
    const myMap = new Map([['a', 1]]);
    myMap.size = 5;           // This has no effect
    console.log(myMap.size);  // Output: 1
  • array.length is writable. You can change an array by setting its length. Setting it to a smaller number will truncate the array. 
    const myArray = [1, 2, 3, 4, 5];
    myArray.length = 2;
    console.log(myArray);     // Output: [1, 2]
note

This read-only nature of .size makes Maps more predictable and less prone to accidental mutation.

Conclusion​

For getting the number of key-value pairs in a Map, the solution is simple and direct.

  • The map.size property is the only correct and recommended best practice.
  • It provides a read-only, live count of the elements in the Map.
  • Avoid manual counting methods (like using forEach and a counter), as the .size property is the most efficient and idiomatic solution.

Table of Contents