reduceRight()

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ArieGonzAguer
Published Jun 22, 2025
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The reduceRight() method in JavaScript executes a reducer function on each element of an array, from right to left, to produce a single accumulated result. It is commonly used when the order of operations matters—such as evaluating expressions, parsing nested structures, or performing right-associative computations like exponentiation.

Syntax

reduceRight(callbackFn, initialValue)

Parameters:

  • callbackFn: A function to execute on each element in the array. It takes four arguments:
    • accumulator: The accumulated result from the previous callback.
    • currentValue: The current element being processed.
    • currentIndex: The index of the current element.
    • array: The array reduceRight() was called upon.
  • initialValue (Optional): A value to use as the first argument to the first call of callbackFn. If not provided, the last element in the array is used as the initial value, and iteration starts from the second-to-last element.

Return value:

Returns a single value resulting from the reduction of the array, working from right to left.

reduceRight() vs reduce()

  • reduce() processes array elements from left to right, useful for left-associative operations like summing or accumulating values.
  • reduceRight() processes elements from right to left, ideal for right-associative logic like parsing or reversing operations.

In the following example, reduce() combines array elements from left to right, while reduceRight() combines them from right to left, resulting in reversed concatenation:

const stringArray = ['0', '2', '4', '6', '🐈', '→'];


const reduceMethod = stringArray.reduce((prev, cur) => prev + cur);
const reduceRightMethod = stringArray.reduceRight((prev, cur) => prev + cur);


console.log(`reduceMethod : ${reduceMethod}`);
console.log(`reduceRightMethod : ${reduceRightMethod}`);

It generates the following output:

reduceMethod : 0246🐈→
reduceRightMethod : →🐈6420

Example 1: Reverse Sum with reduceRight()

The following code shows the reduceRight() without an initial value, and the order of execution of the callback:

const michiArray = [0, 1, 2, 3, 4];
const reducedMichi = michiArray.reduceRight(
  (accumulator, currentValue) => accumulator + currentValue
);


console.log(reducedMichi);
call # accumulator currentValue index Return value
First call 4 3 3 7
Second call 7 2 2 9
Third call 9 1 1 10
Fourth call 10 0 0 10

The output of this code is:

10

Example 2: Reverse Name Construction with reduceRight()

The following code shows the reduceRight() with an initial value, and the order of execution of the callback:

const favoriteName = ['👑', 'y', 'n', 'a', 'i'];
const princess = favoriteName.reduceRight(
  (accumulator, currentValue) => accumulator + currentValue,
  'L'
);


console.log(princess);
call # accumulator currentValue index Return value
First call L i 4 Li
Second call Li a 3 Lia
Third call Lia n 2 Lian
Fourth call Lian y 1 Liany
Fifth call Liany 👑 0 Liany👑

The output of this code is:

Liany👑

Codebyte Example

Run the following code to understand the working of the reduceRight() method:

Code
Output
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