JavaScript Program to Determine the Length of an Array

Last Updated : 21 Feb, 2024

Finding the length of an array in JavaScript refers to the process of determining the number of elements contained within an array.

Below are the approaches used to Determine the length of an array in JavaScript:

Table of Content

Using length Property to Determine the Length of an Array
Using for of and for in loop to Determine the Length of an Array
Using reduce() function to Determine the Length of an Array
Using Object.keys() method to Determine the Length of an Array

Using length Property to Determine the Length of an Array

Here, we will use the length property of the array to determine the length of the array. It will return the number of elements in an array.

Using length Property to find the Length of an Array Example:

Here, we have used the length() property to determine the length of an array.

Javascript

// Defining an number array 
let arr = [12, 4.7, 9, 10] 
  
// Printing the length using length property 
console.log 
("The length of the array: " + arr.length)

Output

The length of the array: 4

Explanation:

The code initializes an array of numbers. It then uses the length property of the array to determine its size and prints the result, indicating the number of elements in the array.

Using for of and for in loop to Determine the Length of an Array

Here, we will use the for of and for in loop and traverse each element of array and will increase size variable on each iteration by one.

Using for of and for in loop to find the Length of an Array Example:

Here, we have used for of and for in loop to determine the length of an array.

Javascript

// Defining the array 
let arr = [12, 4.7, 9, 10] 
  
// Defining the length variable 
let size = 0; 
  
// Looping using for-in loop 
for (let element in arr) { 
size++; 
} 
  
// Printing the length 
console.log(`Length by using for in loop -> ${size}`) 
  
// Reseting the value of variable 
size = 0; 
  
// Looping using for-or loop 
for (let element of arr) { 
size++; 
} 
// Printing the length 
console.log(`Length by using for of loop -> ${size}`)

Output

Length by using for in loop -> 4
Length by using for of loop -> 4

Explanation:

The above code defines an array arr and initializes a variable size to 0. It then iterates over the elements of the array using both for-in and for-of loops, incrementing size for each element. Finally, it prints the lengths obtained from both loops.

Using reduce() function to Determine the Length of an Array

Here, we will use the reduce function with the array to determine the length.

Using reduce() function to find the Length of an Array Example:

Here, we have used reduce() function to determine the length of an array.

Javascript

// Defining the array 
let arr = [12,4.7,9,10]; 
  
// Using the reduce function to count the elements 
let length=arr.reduce((size)=>size+1,0); 
  
// Printing the Length 
console.log(`length of given array -> ${length}`);

Output

length of given array -> 4

Explanation:

The above code defines an array arr. It then utilizes the reduce() function to count the number of elements in the array by incrementing a counter (size) for each element. Finally, it prints the length of the array.

Using Object.keys() method to Determine the Length of an Array

The Object.keys() method in JavaScript is used to retrieve an array of the enumerable property names of an object. It returns an array containing the keys of the object.

Using Object.keys() method to find the Length of an Array Example :

Here, we have used theObject.keys method to determine the length of an array.

Javascript

const array = [1, 2, 3, 4, 5]; 
const length = Object.keys(array).length; 
console.log(length); // Output: 5 

Output

Explanation:

The above code initializes an array array containing elements. It converts the array indices into keys using Object.keys() method and then calculates the length of the resulting array of keys. Finally, it prints the length of the original array.