JavaScript reduceRight method

last modified April 4, 2025

In this article we show how to reduce arrays from right to left using the reduceRight method in JavaScript.

Array reduction from right

The reduceRight method executes a reducer function on each array element from right to left, resulting in a single output value. It works similarly to reduce, but processes the array in the opposite direction.

This method is useful when the order of operations matters, such as when processing mathematical expressions or nested function calls. The reduceRight method maintains an accumulator that's updated with each iteration.

The reducer function takes four arguments: accumulator, current value, current index, and the source array. The accumulator accumulates the callback's return values. An initial value can be provided as the second argument to reduceRight.

Basic reduceRight example

The following example demonstrates the basic usage of the reduceRight method to concatenate strings from right to left.

const words = ['world', ' ', 'hello'];
const sentence = words.reduceRight((acc, word) => acc + word);

console.log(sentence);

We create an array of words and use reduceRight to concatenate them from right to left. The accumulator starts with the last element ('hello') and builds the string by prepending each subsequent element.

$ node main.js
hello world

Calculating factorial with reduceRight

The reduceRight method can be used for mathematical operations where order matters.

const numbers = [1, 2, 3, 4];
const factorial = numbers.reduceRight((acc, num) => acc * num);

console.log(factorial);

We calculate the factorial of 4 by multiplying numbers from right to left. The operation is equivalent to 4 * 3 * 2 * 1. Using reduceRight ensures the multiplication happens in the correct order.

$ node main.js
24

Flattening nested arrays from right

The reduceRight method can flatten nested arrays starting from the rightmost element.

const nested = [[0, 1], [2, 3], [4, 5]];
const flattened = nested.reduceRight((acc, val) => acc.concat(val), []);

console.log(flattened);

We flatten a nested array starting from the rightmost sub-array. The empty array [] is provided as the initial value. Each sub-array is concatenated to the accumulator from right to left.

$ node main.js
[4, 5, 2, 3, 0, 1]

Composing functions with reduceRight

The reduceRight method is perfect for function composition where functions need to be applied from right to left.

const add5 = x => x + 5;
const multiply3 = x => x * 3;
const subtract2 = x => x - 2;

const operations = [add5, multiply3, subtract2];
const composed = operations.reduceRight((acc, fn) => fn(acc), 4);

console.log(composed);

We compose three functions that operate on a number. The functions are applied from right to left to the initial value 4. The order is: subtract2, then multiply3, then add5.

$ node main.js
17

Reversing an array with reduceRight

The reduceRight method can be used to create a reversed version of an array.

const letters = ['a', 'b', 'c', 'd'];
const reversed = letters.reduceRight((acc, letter) => [...acc, letter], []);

console.log(reversed);

We create a reversed array by accumulating elements from right to left. The spread operator ... is used to create a new array with each iteration. The empty array [] serves as the initial accumulator.

$ node main.js
['d', 'c', 'b', 'a']

Source

Array reduceRight - language reference

In this article we have demonstrated how to use the reduceRight() method to process arrays from right to left in JavaScript.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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