JavaScript Array reduce() Method

The Array.reduce() method is a powerful JavaScript function that processes each element of an array to reduce it to a single output value. It iterates through the array while accumulating a result based on the logic provided in a callback function.

Syntax

arr.reduce(callback(accumulator, currentValue, index, array), initialValue);

Parameters

Return Value

The reduce() method returns the final accumulated value after iterating through all elements of the array.

Example 1: Summing Array Elements

let arr = [10, 20, 30, 40, 50];
let sum = arr.reduce((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(sum); // Output: 150

Example 2: Multiplying Array Elements

let arr = [1, 2, 3, 4];
let product = arr.reduce((accumulator, currentValue) => accumulator * currentValue, 1);
console.log(product); // Output: 24

Example 3: Flattening an Array

let arr = [[1, 2], [3, 4], [5, 6]];
let flatArray = arr.reduce((accumulator, currentValue) => accumulator.concat(currentValue), []);
console.log(flatArray); 
// Output: [1, 2, 3, 4, 5, 6]

Example 4: Counting Occurrences

let arr = ['apple', 'banana', 'apple', 'cherry', 'banana', 'banana'];

let count = arr.reduce((accumulator, fruit) => {
    accumulator[fruit] = (accumulator[fruit] || 0) + 1;
    return accumulator;
}, {});

console.log(count); 
// Output: { apple: 2, banana: 3, cherry: 1 }

Example 5: Finding the Maximum Value

let arr = [10, 25, 5, 30, 15];

let max = arr.reduce((accumulator, currentValue) => 
    currentValue > accumulator ? currentValue : accumulator
);
console.log(max); // Output: 30

Supported Browsers