How to get the Length of a Map in JavaScript

# Get the Length of a Map in JavaScript

Use the size property to get the length of a Map, e.g. console.log(map.size).

The size property returns the number of elements in the Map object. When accessed on an empty Map, the size property returns 0.

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const map = new Map();

map.set('name', 'bobby');
map.set('age', 30);
map.set('country', 'Chile');

console.log(map.size); // 👉️ 3

We used the Map.size property to get the number of key-value pairs in the Map.

As opposed to the array's length property, the size property is read-only and can't be changed by the user.

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const map = new Map();
map.set('name', 'Tom');

console.log(map.size); // 👉️ 1

map.size = 5;

console.log(map.size); // 👉️ 1

Even though we tried to set the size of the Map, we were unable to.

This is not the case when using the array's length property.

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const arr = ['a', 'b', 'c'];
console.log(arr.length); // 👉️ 3

// 👇️ set the array's length to 1
arr.length = 1;
console.log(arr.length); // 👉️ 1

// 👇️ updating the length removed 2 elements from the array.
console.log(arr); // 👉️ ['a']

The code sample shows that we can update the array's length and contents by using its length property.

As expected, the size of the Map object is updated when:

• you add an element to the Map
• you remove an element from the Map
• you use the Map.clear() method to remove all elements from the Map

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const map = new Map();

console.log(map.size); // 👉️ 0

// ✅ add elements to the map
map.set('name', 'bobby');
map.set('age', 30);

console.log(map.size); // 👉️ 2

// ✅ delete an element from the map
map.delete('age');

console.log(map.size); // 👉️ 1

// ✅ clear the map object
map.clear();

console.log(map.size); // 👉️ 0

We used the Map.set() method to add 2 elements to the Map object.

Accessing the size property on the Map returns 2 at this point.

We used the Map.delete() method to remove an element from the Map.

Accessing the size property returned 1 after removing the element.

Finally, we used the Map.clear() method to remove all elements from the Map, so the size property returned 0.

# Getting the length of a Map with Map.forEach()

You can also use the Map.forEach() method to get the length of a Map.

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const map = new Map();

map.set('name', 'bobby');
map.set('age', 30);

let length = 0;

map.forEach((_value, _key) => {
  length += 1;
});

console.log(length); // 👉️ 2

We declared a length variable and initialized it to 0.

The Map.forEach() method gets called for each key-value pair of the Map.

On each iteration, we increment the length by 1.

Notice that we declared the length variable using let.

This is necessary because variables declared using const cannot be reassigned.

Iterating over a Map to calculate the length is an option, however, it isn't necessary most of the time because the size property is more convenient and gets updated automatically.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Check if an Object is of type Map or Set in JavaScript
• How to convert an Array to a Map in JavaScript
• Convert a Map to an Array of Objects in JavaScript
• How to Convert a Map to JSON in JavaScript
• Convert a Map to an Object in JavaScript
• Convert Map Keys and Values to an Array in JavaScript
• Check if a Map or a Set is Empty in JavaScript
• How to Clear an Object in JavaScript