Array.prototype.length

The length property of an object which is an instance of type Array sets or returns 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.

The value of the length property is an integer with a positive sign and a value less than 2 to the 32nd power (2^32).

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

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

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

listB.length = 4294967296; //2 to the 32nd power = 4294967296
// RangeError: Invalid array length

const listC = new Array(-100) //negative sign
// RangeError: Invalid array length

You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements increases; for example, if you set length to 3 when it is currently 2, the array now contains 3 elements, which causes the third element to be a non-iterable empty slot.

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

As you can see, the length property does not necessarily indicate the number of defined values in the array. See also Relationship between length and numerical properties.

• Writable: If this attribute set to false, the value of the property cannot be changed.
• Configurable: If this attribute set to false, any attempts to delete the property or change its attributes (Writable, Configurable, or Enumerable) will fail.
• Enumerable: If this attribute set to true, the property will be iterated over during for or for...in loops.

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 (var i = 0; i < length; i++) {
  numbers[i] *= 2;
}
// numbers is now [2, 4, 6, 8, 10]

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

• Array
• RangeError: invalid array length