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:

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.

1. Using length Property

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

// 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)

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.

2. Using for of and for in loop

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.

// 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}`)

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.

3. Using reduce() function

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

// 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}`);

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.

4. Using Object.keys() method

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.

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

5

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.

5. Using the spread Operator:

The spread operator (…) can determine the length of an array by spreading its elements into a new array and then accessing the length property of the new array. This concise approach simplifies counting array elements in JavaScript.

Example:

const array = [1, 2, 3, 4, 5];
const newArray = [...array];
const length = newArray.length;
console.log(length); 

5

JavaScript Program to Determine the Length of an Array – Use Cases:

Below are the Use case for JavaScript Program to Determine the Length of an Array:

1. JavaScript Array length Property

The array length property in JavaScript is used to set or return the number of elements in an array. 

2. How to Find the Length of an Array in JavaScript ?

Below are the ways to find the length of an array in JavaScript:

• Using the length Property
• Looping Through the Array
• Using forEach Method
• Using Spread Operator
• Conversion to String

3. How to get the size of an array in JavaScript ?

The JavaScript Array Length returns an unsigned integer value that represents the number of elements present in the array. The value is non-negative and always a 32-bit integer.