JavaScript Array reduce()

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Examples

Subtract all numbers in an array:

const numbers = [175, 50, 25];

document.getElementById("demo").innerHTML = numbers.reduce(myFunc);

function myFunc(total, num) {
return total - num;
}

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Round all the numbers and display the sum:

const numbers = [15.5, 2.3, 1.1, 4.7];
document.getElementById("demo").innerHTML = numbers.reduce(getSum, 0);

function getSum(total, num) {
return total + Math.round(num);
}

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Description

The reduce() method executes a reducer function for array element.

The reduce() method returns a single value: the function's accumulated result.

The reduce() method does not execute the function for empty array elements.

The reduce() method does not change the original array.

Note

At the first callback, there is no return value from the previous callback.

Normally, array element 0 is used as initial value, and the iteration starts from array element 1.

If an initial value is supplied, this is used, and the iteration starts from array element 0.

Syntax

array. reduce( function(total, currentValue, currentIndex, arr), initialValue)

Parameters

Parameter

Description

function()

Required.
A function to be run for each element in the array.

Reducer function parameters:

total	Required. The initialValue, or the previously returned value of the function.
currentValue	Required. The value of the current element.
currentIndex	Optional. The index of the current element.
arr	Optional. The array the current element belongs to.