Basics Of JavaScript Reduce() Method

The reduce() method is one of Javascript's array method. As a JavaScript developer, the reduce() method comes in handy when working with arrays. In this article, I would try my best to explain the basics behind the reduce() method and how it works. L...

barnabas.hashnode.dev4 min read

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Coincidentally, I googled about the reduce method just last night, so waking up to this well explained article already made my day. Thanks for breaking down this topic.

I have a question though. In the 2nd example you gave were we had

const arr = [10, 20, 30, 40, 50]
const arrSum = arr.reduce((acc, curr) => {
    return acc + curr
})

You said the output was going to be 150

I find this quite confusing since you also stated that if an initial value is not stated just as above, the reducer starts from index 1 which in this case is 20.

So shouldn't the result be 140 instead of 150?

Barnabas Asha

Frontend developer

Aug 11, 2020

I'm glad you liked the article, In the case where an initial value is not specified the reducer starts from index 1 and the first member of the array (index 0) is taken as the accumulator for the first execution.

arr = [10, 20, 30, 40, 50]
callback = function (acc, cur) {
return acc + cur
}

arr.reduce(callback)

/* first instance: 10 is taken as the initial val since one isn't specified and used as the accumulator 
So the callback returns */

return 10 + 20

/* Then 30 becomes the accumulator for the next execution. And it goes on */

I hope I was able to shed more light and not confuse you. Feel free to reach me.

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Basics Of JavaScript Reduce() Method

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