How do you define the size of an array?

Try it

Value

A non-negative integer less than 232.

Description

The value of the length property is a non-negative 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 non-negative 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

  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
  

  7 exception.

When length is set to a bigger value than the current length, the array is extended by adding empty slots, not actual

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

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 also Relationship between length and numerical properties.

Examples

Iterating over an array

In the following example, the array

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]

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

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]

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.