The reduce() method runs a function on each array element to produce (reduce it to) a single value.
The reduce() method works from left-to-right in the array. See also reduceRight().
The reduce() method does not reduce the original array.
This example finds the sum of all numbers in an array:
Example
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);
function myFunction(total, value, index, array) {
return total + value;
}
let sum = numbers.reduce(myFunction);
function myFunction(total, value, index, array) {
return total + value;
}
Note that the function takes 4 arguments:
- The total (the initial value / previously returned value)
- The item value
- The item index
- The array itself
The example above does not use the index and array parameters. It can be rewritten to:
Example
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);
function myFunction(total, value) {
return total + value;
}
let sum = numbers.reduce(myFunction);
function myFunction(total, value) {
return total + value;
}
The reduce() method can accept an initial value:
Example
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction, 100);
function myFunction(total, value) {
return total + value;
}
let sum = numbers.reduce(myFunction, 100);
function myFunction(total, value) {
return total + value;
}
Array.reduce() is supported in all browsers except Internet Explorer 8 or earlier. Practice Excercise Practice now