What is a real world application of `.reduce()`?

What would be a real world example for using this method. I understand what its doing but it I guess i’m trying to picture what I’d use it for.

For a lot of these excessive I like to think about how i can apply to a real world application


Explanation of javascript’s reduce() with a real world use case

As we see, this method is not just for numeric computation.

This will seem a silly way to compute factorial…

[1,2,3,4,5,6,7].reduce((a, b) => a * b)    // 5040

but it is short and sweet. And this would seem a strange way to join strings…

array = ['A', 'quick', 'brown', 'fox', 'jumps', 'over', 'the', 'lazy', 'dog'];
array.reduce((a, b) => a + ' ' + b)
// "A quick brown fox jumps over the lazy dog"

Neither is very real world when compared with the example given in the article. Perhaps in your reading you will find some other examples. Please do share!


Accumulator acts as a running total.
I think it would be helpful to calculate the cumulative sums of sales metric for each day.
For exmaple , sales_jan = [200,300,400,500, 100].
Day 1: $200
Day 2: $500
Day 3: $900
and so on…


I think it’s really a good and concise method to perform numeric computation and as well as concatenation on all the elements of an array, looking at the examples @mtf gave.

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Can someone explain to me why I am putting the parenthesis outside of the code block? I think I missed something. See below.

const newNumbers = newSum.reduce((accumulator, currentValue) => {
return accumulator + currentValue

The parens are the delimiters of the argument for the reduce method.


The callback may be a literal function…

iterable.reduce(function (a, b) {
    return a + b;


iterable.reduce(f(a, b) => {
    return a + b;


iterable.reduce((a, b) => a + b)

or, we can write the callback as a standalone function that can be accessed by the rest of the program (optimal)…

a_add_b = (a, b) => a + b;


It should be noted that iterable above is an array.

Let’s play with a number in which we wish to add the digits…

a = 123456789

s = a.toString().split('').reduce((m,n) => +m + +n)

console.log(s)    //  45

Great code, though how does this syntax work?

Have never seen this before. Why are there + signs before m and n?

The + is one of the unary operators which in some cases can be counted upon to coerce an object to a Number. It comes with a disclaimer in as much as it is not a silver bullet.

Our data in the array is String and we know the characters are number representations that can be coerced to Number with certainty. It’s a controlled situation.

+m + +n


'1' + '2'  // would result in '12'

and turns it into,

1 + 2      // which gives 3

Taking this a bit further…

 > +[]
<- 0
 > +{}
<- NaN
 > +[1]
<- 1
 > +[5]
<- 5
 > +[2, 3, 4]
<- NaN
 > +{1}
 X Uncaught SyntaxError: Unexpected token '}'
 > +{a: 1}
<- NaN
 > +"two"
<- NaN
 > +"42"
<- 42
 > +NaN
<- NaN
 > +true
<- 1
 > +false
<- 0
 > +null
<- 0
 > +undefined
<- NaN