JavaScript Array reduceRight()
Syntax & Examples

Array.reduceRight() method

The reduceRight() method of the Array class in JavaScript executes a user-supplied "reducer" callback function on each element of the array (from right to left), to reduce it to a single value.

Syntax of Array.reduceRight()

There are 2 variations for the syntax of Array.reduceRight() method. They are:

1.
reduceRight(callbackFn)

Parameters

ParameterOptional/RequiredDescription
callbackFnrequiredA function to execute on each element in the array. It takes four arguments: accumulator, currentValue, currentIndex, and array.

This method executes the callback function on each element of the array (from right to left) without an initial value, to reduce it to a single value.

Returns value of type Any.

2.
reduceRight(callbackFn, initialValue)

Parameters

ParameterOptional/RequiredDescription
callbackFnrequiredA function to execute on each element in the array. It takes four arguments: accumulator, currentValue, currentIndex, and array.
initialValueoptionalA value to use as the first argument to the first call of the callback.

This method executes the callback function on each element of the array (from right to left) with the specified initial value, to reduce it to a single value.

Returns value of type Any.


✐ Examples

1 Using reduceRight() method to concatenate strings in reverse order

In JavaScript, we can use the reduceRight() method to concatenate all strings in an array in reverse order.

For example,

  1. We define an array variable strArr with elements ['Hello', ' ', 'world', '!'].
  2. We define a callback function concat that concatenates the accumulator and the current value.
  3. We use the reduceRight() method with concat to get the concatenated string in reverse order.
  4. The result is stored in the variable message.
  5. We log message to the console using console.log() method to see the concatenated string.

JavaScript Program

const strArr = ['Hello', ' ', 'world', '!'];
const concat = (accumulator, currentValue) => accumulator + currentValue;
const message = strArr.reduceRight(concat);
console.log(message);

Output

!world Hello

2 Using reduceRight() method to sum the values in an array

We can use the reduceRight() method to sum the values of an array from right to left.

For example,

  1. We define an array variable arr with elements [1, 2, 3, 4, 5].
  2. We define a callback function sum that adds the accumulator and the current value.
  3. We use the reduceRight() method with sum to get the total of the array values from right to left.
  4. The result is stored in the variable total.
  5. We log total to the console using console.log() method to see the total sum of the array values.

JavaScript Program

const arr = [1, 2, 3, 4, 5];
const sum = (accumulator, currentValue) => accumulator + currentValue;
const total = arr.reduceRight(sum);
console.log(total);

Output

15

3 Using reduceRight() method with an initial value

We can use the reduceRight() method with an initial value to multiply the values in an array from right to left.

For example,

  1. We define an array variable arr with elements [1, 2, 3, 4].
  2. We define a callback function multiply that multiplies the accumulator and the current value.
  3. We use the reduceRight() method with multiply and an initial value of 2 to get the product of the array values multiplied by the initial value from right to left.
  4. The result is stored in the variable product.
  5. We log product to the console using console.log() method to see the product of the array values multiplied by the initial value.

JavaScript Program

const arr = [1, 2, 3, 4];
const multiply = (accumulator, currentValue) => accumulator * currentValue;
const product = arr.reduceRight(multiply, 2);
console.log(product);

Output

48

Summary

In this JavaScript tutorial, we learned about reduceRight() method of Array: the syntax and few working examples with output and detailed explanation for each example.