What is JavaScript Array reduce() Method?

In this article, we will learn JavaScript Array reduce() Method,This free Javascript tutorial for complete beginners will help you learn Javascript from scratch.

JavaScript Array reduce() Method

JavaScript Array push() Method

JavaScript Array reduceRight() Method

The reduce() method in JavaScript executes a reducer function for each array element, returning a single accumulated value. It skips empty array elements and doesn’t modify the original array, making it useful for concise data aggregation.

Syntax:

array.reduce( function(total, currentValue, currentIndex, arr), 
initialValue )

Parameters:

This method accepts five parameters as mentioned above and described below:

function(total, currentValue, index, arr): It is the required parameter and is used to run for each element of the array. It contains four parameters which are listed below:

Parameter Name	Description	Required/Optional
total	Specifies the initial value or previously returned value of the function	Required
currentValue	Specifies the value of the current element	Required
currentIndex	Specifies the array index of the current element	Optional
arr	Specifies the array object the current element belongs to	Optional

initialValue: It is an optional parameter and is used to specify the value to be passed to the function as the initial value.

Return value: The JavaScript array reduce method returns a single value/element after traversing the complete array.

Below are examples of the Array reduce() method.

Example 1: In this example, we will write a reduce function to simply print the difference of the elements of the array.

Javascript

// Input array
let arr = [175, 50, 25];

// Callback function for reduce method
function subofArray(total, num) {
    return total - num;
}

//Fucntion to execute reduce method 
function myBeginner(item) {
    // Display output
    console.log(arr.reduce(subofArray));
}
myBeginner()

Output

Example 2: This example uses reduce() method to return the sum of all array elements.

Javascript

// Input array
let arr = [10, 20, 30, 40, 50, 60];
// Callback function for reduce method
function sumofArray(sum, num) {
    return sum + num;
}
//Fucntion to execute reduce method 
function myBeginner(item) {
    // Display output
    console.log(arr.reduce(sumofArray));
}
myBeginner();

Output

Example 3: This example uses reduce() method to return the round sum of all array elements.

Javascript

// Input array
let arr = [1.5, 20.3, 11.1, 40.7];

// Callback function for reduce method
function sumofArray(sum, num) {
    return sum + Math.round(num);
}

//Fucntion to execute reduce method 
function myBeginner(item) {
    // Display output
    console.log(arr.reduce(sumofArray, 0));
}
myBeginner();