Javascript Array length

Introduction

Javascript array length property sets or gets the number of elements in array.

The value of array length is an unsigned, 32-bit integer. 

const arr = [1, 2];//from   w w w .  ja  v a  2s. com
console.log(arr);// [ 1, 2 ]
arr.length = 5; // set array length to 5 while currently 2.
console.log(arr);
arr.forEach(element => console.log(element));

Return the length of an array: 

var languages = ["CSS", "HTML", "Java", "Javascript"];
console.log(languages.length);

The array numbers is iterated through by looking at the length property.

The value in each element is then doubled. 

var numbers = [1, 2, 3, 4, 5];
var length = numbers.length;
for (var i = 0; i < length; i++) {
  numbers[i] *= 2;/*from  w  ww  .jav a  2  s .  c om*/
}
// numbers is now [2, 4, 6, 8, 10]

Shortening an array. The following example shortens the array numbers to a length of 3. 

var numbers = [1, 2, 3, 4, 5];
if (numbers.length > 3) {
  numbers.length = 3;/*from  w w w.j a  v a  2  s .  c  o m*/
}
console.log(numbers); // [1, 2, 3]
console.log(numbers.length); // 3

Create empty array of fixed length 

var numbers = [];
numbers.length = 3;
console.log(numbers); // [undefined, undefined, undefined]

The number of items in an array is stored in the length property: 

let colors = ["red", "blue", "green"]; // creates an array with three strings 
let names = []; // creates an empty array 

console.log(colors.length); // 3 
console.log(names.length); // 0

Array length is not read-only.

By setting the length property, we can remove items from or add items to the end of the array. 

let colors = ["red", "blue", "green"]; // creates an array with three strings 
console.log(colors); /*from  w ww  .j  a va2s  .co m*/
colors.length = 2;
console.log(colors); 
console.log(colors[2]); // undefined

If the length were set to a number greater than the length, the new items would each get filled with the value of undefined: 

let colors = ["red", "blue", "green"];  // creates an array with three strings 
colors.length = 4; //from  w  w  w  .  j  a v a  2 s  .com
console.log(colors[3]);  // undefined

The length property can add items to the end of an array: 

let colors = ["red", "blue", "green"];  // creates an array with three strings 
console.log(colors);//  www . j a  va 2 s .c o m
colors[colors.length] = "black";        // add a color (position 3) 
console.log(colors);
colors[colors.length] = "brown";        // add another color (position 4) 
console.log(colors);

The last item in an array is always at position length - 1.

The new length is calculated when an item is placed into a position that's outside of the current array size. 

let colors = ["red", "blue", "green"];  // creates an array with three strings 
colors[99] = "black";                   // add a color (position 99) 
console.log(colors.length);                   // 100

The following example creates an array, msgArray, with a length of 0.

Then assigns values to msgArray[0] and msgArray[99], changing the length of the array to 100. 

let msgArray = []
msgArray[0] = 'Hello'
msgArray[99] = 'world'

if (msgArray.length === 100) {
  console.log('The length is 100.')
}

Arrays can contain a maximum of 4,294,967,295 items.




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