JavaScript’s reduceRight() method to iterate over an array from right to left

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reduceRight(), allows you to perform a function on each element of the array from right to left, accumulating a single output value. This article delves into the intricacies of reduceRight() with clear examples and hardcoded values to enhance understanding.

What is reduceRight()?

The reduceRight() method in JavaScript executes a provided callback function once for each element present in the array, from right to left, and accumulates the result into a single value. It takes in a callback function and an optional initial value as arguments.


array.reduceRight(callback(accumulator, currentValue[, index[, array]])[, initialValue])
  • callback: A function to execute on each element in the array.
  • accumulator: The accumulator accumulates the callback’s return values.
  • currentValue: The current element being processed in the array.
  • index (optional): The index of the current element being processed in the array.
  • array (optional): The array reduceRight() was called upon.
  • initialValue (optional): A value to use as the first argument to the first call of the callback function.


Let’s explore some practical examples to understand reduceRight() better:

Example 1: Concatenating Strings

const words = ['hello', 'world', 'this', 'is', 'JavaScript'];

const concatenatedString = words.reduceRight((accumulator, currentValue) => {
  return accumulator + ' ' + currentValue;



JavaScript is this world hello

Example 2: Summing Array Elements

const numbers = [1, 2, 3, 4, 5];

const sum = numbers.reduceRight((accumulator, currentValue) => {
  return accumulator + currentValue;
}, 0);




Example 3: Flattening an Array of Arrays

const arrays = [[1, 2], [3, 4], [5, 6]];

const flattenedArray = arrays.reduceRight((accumulator, currentValue) => {
  return accumulator.concat(currentValue);
}, []);



[5, 6, 3, 4, 1, 2]

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