""Array:

The length data property of an Array instance represents the number of elements in that array. The value is an unsigned, 32-bit integer that is always numerically greater than the highest index in the array.

Value

A nonnegative integer less than 2³².

Description

The value of the length property is a nonnegative integer with a value less than 2³².

const listA = [1, 2, 3];
const listB = new Array(6);

console.log(listA.length);
// 3

console.log(listB.length);
// 6

listB.length = 2 ** 32; // 4294967296
// RangeError: Invalid array length

const listC = new Array(-100); // Negative numbers are not allowed
// RangeError: Invalid array length

The array object observes the length property, and automatically syncs the length value with the array's content. This means:

Setting length to a value smaller than the current length truncates the array — elements beyond the new length are deleted.
Setting any array index (a nonnegative integer smaller than 2³²) beyond the current length extends the array — the length property is increased to reflect the new highest index.
Setting length to an invalid value (e.g. a negative number or a non-integer) throws a RangeError exception.

When length is set to a bigger value than the current length, the array is extended by adding empty slots, not actual undefined values. Empty slots have some special interactions with array methods; see array methods and empty slots.

const arr = [1, 2];
console.log(arr);
// [ 1, 2 ]

arr.length = 5; // set array length to 5 while currently 2.
console.log(arr);
// [ 1, 2, <3 empty items> ]

arr.forEach((element) => console.log(element));
// 1
// 2

Examples

Iterating over an array

In the following example, the array numbers is iterated through by looking at the length property. The value in each element is then doubled.

const numbers = [1, 2, 3, 4, 5];
const length = numbers.length;
for (let i = 0; i < length; i++) {
  numbers[i] *= 2;
}
// numbers is now [2, 4, 6, 8, 10]

Shortening an array

The following example shortens the array numbers to a length of 3 if the current length is greater than 3.

const numbers = [1, 2, 3, 4, 5];

if (numbers.length > 3) {
  numbers.length = 3;
}

console.log(numbers); // [1, 2, 3]
console.log(numbers.length); // 3
console.log(numbers[3]); // undefined; the extra elements are deleted

Create empty array of fixed length

Setting length to a value greater than the current length creates a sparse array.

const numbers = [];
numbers.length = 3;
console.log(numbers); // [empty x 3]

Array with non-writable length

The length property is automatically updated by the array when elements are added beyond the current length. If the length property is made non-writable, the array will not be able to update it. This causes an error in strict mode.

"use strict";

const numbers = [1, 2, 3, 4, 5];
Object.defineProperty(numbers, "length", { writable: false });
numbers[5] = 6; // TypeError: Cannot assign to read only property 'length' of object '[object Array]'
numbers.push(5); // // TypeError: Cannot assign to read only property 'length' of object '[object Array]'