JavaScript Array reduceRight() Method

The Array.reduceRight() method is a built-in JavaScript function that processes each element of an array from right to left (starting from the last element to the first) and reduces it to a single output value.

Syntax

arr.reduceRight(callback(accumulator, currentValue, index, array), initialValue);

Parameters

Parameter	Description
callback	(Required) A function to execute on each element in the array.
accumulator	(Required in callback) The accumulated result from previous callback executions.
currentValue	(Required in callback) The current element being processed.
index	(Optional in callback) The index of the current element being processed.
array	(Optional in callback) The array reduceRight() is being applied on.
initialValue	(Optional) A value to use as the initial accumulator. If not provided, the last element of the array is used.

Return Value

The reduceRight() method returns the final accumulated value after iterating through all elements of the array from right to left.

Example 1: Concatenating Strings in Reverse Order

let words = ['world', ' ', 'hello'];

let result = words.reduceRight((accumulator, currentValue) => accumulator + currentValue, '');
console.log(result); // Output: 'hello world'

Example 2: Summing Array Elements

let arr = [1, 2, 3, 4];
let sum = arr.reduceRight((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(sum); // Output: 10

Example 3: Flattening a Nested Array in Reverse Order

let arr = [[5, 6], [3, 4], [1, 2]];

let flatArray = arr.reduceRight((accumulator, currentValue) => accumulator.concat(currentValue), []);
console.log(flatArray); 
// Output: [5, 6, 3, 4, 1, 2]

Example 4: Using reduceRight() Without an Initial Value

let numbers = [10, 20, 30];

let result = numbers.reduceRight((accumulator, currentValue) => accumulator - currentValue);
console.log(result); // Output: 0

Supported Browsers

Browser	Support
Chrome	3+
Firefox	3+
Safari	4+
Edge	12+
Opera	10.5+
Internet Explorer	9+