JavaScript Array reduce()
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;
}
Try it Yourself »
document.getElementById("demo").innerHTML = numbers.reduce(myFunc);
function myFunc(total, num) {
return total - num;
}
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);
}
Try it Yourself »
document.getElementById("demo").innerHTML = numbers.reduce(getSum, 0);
function getSum(total, num) {
return total + Math.round(num);
}
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.
See Also:
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.
A function to be run for each element in the array.
Reducer function parameters:
initialValue
Optional.
A value to be passed to the function as the initial value.
total
Required.
The initialValue, or the previously returned value of the function.
The initialValue, or the previously returned value of the function.
currentValue
Required.
The value of the current element.
The value of the current element.
currentIndex
Optional.
The index of the current element.
The index of the current element.
arr
Optional.
The array the current element belongs to.
The array the current element belongs to.
A value to be passed to the function as the initial value.
Return Value
The accumulated result from the last call of the callback function.
Array Tutorials:
Browser Support
reduce() is an ECMAScript5 (ES5 2009) feature.
JavaScript 2009 is supported in all browsers since July 2013:
| Chrome 23 |
IE/Edge 11 |
Firefox 21 |
Safari 6 |
Opera 15 |
| Sep 2012 | Sep 2012 | Apr 2013 | Jul 2012 | Jul 2013 |