Blog 
Jul 19, 2021-3 MIN READ
Which is Array Reduce method in Javascript
Learn how the Array reduce method works in JavaScript with practical examples for summing values, flattening arrays, and more.
By Baljeet Singh
Reduce method in javascript is used to reduce the array to a single value. It takes a callback function which takes four parameters (accumulator, currentValue, index, array), but mostly we use the first two.
Now let's take a look at some of the examples,
Calculate sum of all the elements in an array
const arr = [2, 3, 5, 4];
// here accumulator is names sum
// item is the current value
// 0 is the initial value
const arraySum = arr.reduce((sum, item) => sum + item, 0);
console.log('Output is: ', arraySum);
// Output is: 14
Calculate total number of fruits in the array
const fruits = [
{ name: 'Apple', quantity: 3 },
{ name: 'Banana', quantity: 4 },
{ name: 'Mango', quantity: 2 },
];
const totalFruits = fruits.reduce((total, fruit) => total + fruit.quantity, 0);
console.log('Total Fruits are:', totalFruits);
// Total Fruits are: 9
Group Students by ClassName
const students = [
{ name: 'Baljeet', className: 10 },
{ name: 'Fatehjit', className: 2 },
{ name: 'Ekjeet', className: 10 },
{ name: 'Manvir', className: 5 },
];
const output = students.reduce((groupedStudents, student) => {
const className = student.className;
if (groupedStudents[className] == null) {
groupedStudents[className] = [];
}
groupedStudents[className].push(student);
return groupedStudents;
}, {});
console.log('Grouped Students: ', output);
/* output
{ 2: [ { name: 'Fatehjit', className: 2 } ],
5: [ { name: 'Manvir', className: 5 } ],
10: [ { name: 'Baljeet', className: 10 },
{ name: 'Ekjeet', className: 10 } ] } */