JavaScript Array Length

Summary: in this tutorial, you’ll learn about the JavaScript Array length property and how to handle it correctly.

What exactly is the JavaScript Array length property

By definition, the length property of an array is an unsigned, 32-bit integer that is always numerically greater than the highest index in the array.

The maximum value of the length is 2³². This means that an array can hold up to 4_294_967_296 (2³²) elements.

The length property behaves differently depending on the array types including dense and sparse.

1) Dense arrays

A dense array is an array where its elements have contiguous indexes starting at zero.

For dense arrays, you can use the length property to get the number of elements in the array. For example:

let colors = ['red', 'green', 'blue'];
console.log(colors.length); // 3Code language: JavaScript (javascript)

In this example, the length property returns three, which is the same as the number of elements in the colors array.

The following adds one more element to the colors array:

let colors = ['red', 'green', 'blue'];
console.log(colors.length); // 3

colors.push('yellow');
console.log(colors.length); // 4Code language: JavaScript (javascript)

Now, the length property of the colors array is 4.

When you empty the colors array, its length is zero:

colors = [];
console.log(colors.length); // 0Code language: JavaScript (javascript)

2) Sparse arrays

A sparse array is an array whose elements don’t have contiguous indexes starting at zero.

For example, the [10,, 20, 30] is a sparse array because the indexes of its elements are 0, 2, and 3.

In a sparse array, the length property doesn’t indicate the actual number of elements. It’s a number that is greater than the highest index. For example:

let numbers = [10, , 20, 30];
console.log(numbers.length); // 4Code language: JavaScript (javascript)

In this example, the number of elements in the numbers array is 3: 10, 20, and 30. The highest index is 3. Therefore, the length property returns 4.

The following adds an element to the numbers array at the index 10:

let numbers = [10, , 20, 30];
console.log(numbers.length); // 4

numbers[10] = 100;
console.log(numbers.length); // 11Code language: JavaScript (javascript)

In this example, the length property returns 11.

Modifying JavaScript Array length property

JavaScript allows you to change the value of the array length property. By changing the value of the length property, you can remove elements from an array or make an array sparse.

1) Emptying an array

If you set the length property of an array to zero, the array will be empty:

const fruits = ['Apple', 'Orange', 'Strawberry'];
fruits.length = 0;

console.log(fruits); // []
Code language: JavaScript (javascript)

Output:

[]Code language: JSON / JSON with Comments (json)

2) Removing elements

If you set the length property of an array to a value lower than the highest index, all the elements whose index is greater than or equal to the new length are removed.

The following example changes the length property of the fruits array to 2, which removes the third element from the array:

const fruits = ['Apple', 'Orange', 'Strawberry'];
fruits.length = 2;

console.log(fruits); Code language: JavaScript (javascript)

Output:

[ 'Apple', 'Orange' ]Code language: JSON / JSON with Comments (json)

3) Making the array sparse

If you set the length property of an array to a value higher than the highest index, the array will be spared. For example:

const fruits = ['Apple', 'Orange', 'Strawberry'];
fruits.length = 5;

console.log(fruits);Code language: JavaScript (javascript)

Output:

[ 'Apple', 'Orange', 'Strawberry', <2 empty items> ]Code language: JSON / JSON with Comments (json)

Summary

• The length property of an array is an unsigned, 32-bit integer that is always numerically greater than the highest index of the array.
• The length property returns the number of elements of a dense array.
• For spare arrays, the length property doesn’t reflect the number of elements.
• Modifying the length property may remove elements from an array or make an array spare.