JavaScript Array length

let city = ["California", "Barcelona", "Paris", "Kathmandu"];

// find the length of the city array
let len = city.length;
console.log(len);

// Output: 4

length Syntax

The syntax to access the length property is:

arr.length

Here, arr is an array.

Example 1: Finding Number of Elements in an Array

var companyList = ["Apple", "Google", "Facebook", "Amazon"];
console.log(companyList.length); // Output: 4

var randomList = ["JavaScript", 44];
console.log(randomList.length); // Output: 2

var emptyArray = [];
console.log(emptyArray.length); // Output: 0

Output

4
2
0

Here, we can see that length property returns the number of items in each array. It returns the integer just greater than the highest index in an Array.

Example 2: Using Array length in for loop

var languages = ["JavaScript", "Python", "C++", "Java", "Lua"];

// languages.length can be used to find out 
// the number of times to loop over an array
for (i = 0; i < languages.length; i++){
    console.log(languages[i]);
}

Output

JavaScript
Python
C++
Java
Lua

You can also reassign the length property of an Array using the assignment operator =.

Syntax to assign the Array length:

array.length = <Integer>

This can be used to truncate or extend a given array.

Example 3: Changing length property of Array

var languages = ["JavaScript", "Python", "C++", "Java", "Lua"];

// truncate the Array to 3 elements
languages.length = 3

// Output: [ 'JavaScript', 'Python', 'C++' ]
console.log(languages)

// extend the Array to length 6
languages.length = 6

// Output: [ 'JavaScript', 'Python', 'C++', <3 empty items> ]
console.log(languages)

Output

[ 'JavaScript', 'Python', 'C++' ]
[ 'JavaScript', 'Python', 'C++', <3 empty item> ]

Here, we first truncated the length of Array as 3 (assigned value) is less than 4 (original Array length).

If the assigned value is more than the original Array length, empty items are appended to the end of the Array. We can see this happen in the second example.