A nonnegative integer less than 2³².

The value of thelength 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);// 3console.log(listB.length);// 6listB.length = 2 ** 32; // 4294967296// RangeError: Invalid array lengthconst listC = new Array(-100); // Negative numbers are not allowed// RangeError: Invalid array length

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

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

Whenlength is set to a bigger value than the current length, the array is extended by addingempty slots, not actualundefined values. Empty slots have some special interactions with array methods; seearray 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

See alsoRelationship betweenlength and numerical properties.

In the following example, the arraynumbers is iterated through by looking at thelength 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]

The following example shortens the arraynumbers 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); // 3console.log(numbers[3]); // undefined; the extra elements are deleted

Settinglength to a value greater than the current length creates asparse array.

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

Thelength property is automatically updated by the array when elements are added beyond the current length. If thelength property is made non-writable, the array will not be able to update it. This causes an error instrict 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]'

• Indexed collections guide
• Array
• TypedArray.prototype.length
• String:length
• RangeError: invalid array length