JavaScript: use of Reduce() or ReduceRight() methods

In this article, we are going to learn about the reduce() and reduceright() functions in JavaScript. array.Reduce() is a standard-built function of the array. It runs a reducer or callback function on each element of the array and returns a single value output. (from left to right).

Why should we use reduce() over For loop in JavaScript

  • Reduce() help us to get rid of for loop to perform any operation on the array. It makes our code less and achieves more.
  • Reducer or callback makes us perform multiple operations on the array at once.
  • Reduce() function support functional programming.
  • It makes our work so easy to automatically set up the iteration variable in the function.

Let us understand with an example:

For Loop to sum the elements of an array

let array = [6, 7, 10,3];

let num=1;
for (let i = 0; i < array.length; i++) {
    num= num*array[i];



array.reduce() function to sum the elements of an array

In this code example, we are calculating the sum of all array elements and printing on the console.

let array = [6, 7, 10,3];
const reducer = (accumulator, currentValue) => accumulator * currentValue;



array.Reduce() step-by-step explanation

array.reduce(callback( accumulator, currentValue, [, index[, array]] )[, initialValue])

Reducer or callback

//This is Callback or reducer
callback( accumulator, currentValue, [, index[, array]] )

Reduce() method calls the reducer (callback( accumulator, currentValue, [, index[, array]] )) a callback function for each element of the array. Which takes four arguments.

Parameters of Reduce() function

  1. Callback function: We also call it reducer, it calls each element of the array.

The callback function has four arguments.

  • accumulator(acc): It is the return value from the previous result innovation or the Initial value if it is supplied.
  • currentValue(optional): The current value of the array element.
  • index(optional): It contains the current element index, If the initial value is passed then it starts from index 0 else index 1.
  • array(optional): array for which we are executing reducer.

2. initalValue(optional): The initial accumulator value that uses when the first time the callback function is called. If initalValue is not given then the first element of the array is used as an accumulator.


If we call reduce() on an empty array without initial value we will get Typerror(is thrown when the operation could not be performed on the object).

Different examples using the Array’s reduce() function

Sum elements of object Array using reduce()

Here, in this example, we are calculating the sum of the array object’s element using reduce() array method.

let initialValue = 0
let sum_arryobject = [{a: 6}, {a: 7}, {a: 8},{a:9}].reduce(function (accu, currValue) {
    return accu + currValue.a
}, initialValue)



sum = 30

Remove duplicate from an array using reduce()

In this code example, we are removing the duplicate array’s elements using reduce() method.

let dup_array = [12, 15, 12, 11, 10, 11, 15]
let uniquearray = dup_array.reduce(function (accum, currentValue) {
  if (accum.indexOf(currentValue) === -1) {
  return accum
}, [])



[12, 15, 11, 10]

Nested Array to Object Using java script reduce()

In this code example, we are converting a nested array into an object.

let myarray = [['Name','Jonson'],['Age',20],['Desigi','MGR'],['Comp','UNS']]

let ArrayToObject = (myarray, key) =>
    (obj, element) => ({
      [element[key]]: element
const return_obj = Object.fromEntries(myarray)


  Age: 20,
  Comp: "UNS",
  Desigi: "MGR",
  Name: "Jonson"

Flatten an array using Reduce() in javascript

In this code example, we are converting the array of the array into a 1-dimensional array. We can achieve this easily using reduce() method.

let arraytflatten = [[10, 11], [20, 34], [46, 50],[3,4]].reduce(
  ( accum, currVal ) => accum.concat(currVal),


[10, 11, 20, 34, 46, 50, 3, 4]

Using of reduceRight() in javascript

reduceRight() is a built-in standard function of the array, it uses a callback function on each value of the array( from right to left) and returns a single value as output.

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

Callback function in reducerRight()

array.reduceRight(function(accumulator, currentValue, index, array) {  // ...});

Let’s Understand with an example.

Flatten an array using reducerRight()

var arraytoflat = [[7, 1], [6, 1], [10, 5]].reduceRight(function(accum,currval ) {
    return accum.concat(currval);
}, []);


[10, 5, 6, 1, 7, 1]

Here we can see reducerRight() concate from right to left.[10,5] is the last array in arraytoflat.