Javascript reduce with a nested array

Question

I'm trying to sum a nested array with the reduce method. My dat array looks like this:

var data = [
    [1389740400000, 576],
    [1389741300000, 608],
    [1389742200000, 624],
    [1389743100000, 672],
    [1389744000000, 691]
];

I got this:

// es5
data.reduce(function(prev, next) { return prev + next[1]; })

// es6 syntax
data.reduce((prev, next) => prev + next[1])

However I only do need the second value from each (nested) array. Any hints or tipps for me? I'm trying to sum all values within the array.

// Edit: Thanks for the answers. The problem was, that I missed the initialValue at the end.

// es6 solution
data.reduce((prev, next) => prev + next[1], 0)

I simple way to sum all values.

wiesson
– wiesson

2014-08-06 11:08:55 +00:00
Commented Aug 6, 2014 at 11:08 — wiesson
– wiesson, Commented Aug 6, 2014 at 11:08

Mritunjay · Accepted Answer · 2014-08-06 11:15:27Z

18

Do it as following

var result = data.reduce(function (prev,next) {
    return prev + next[1];
},0);

console.log(result);//prints 3171

Here I am sending 0 as prev initially. So it will go like this

First Time  prev->0 next->[1389740400000, 576]
Second Time prev->576 next->[1389740400000, 608]

Do a console.log(prev,next) to understand much better.

If you'll see in docs you will get it.

edited Aug 6, 2014 at 11:15

answered Aug 6, 2014 at 11:09

Mritunjay

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1 Comment

wiesson Over a year ago

Thanks for this. I tried different approaches, however I missed the last element. Maybe I should read the docs more carefully. :)

Nina Scholz · Accepted Answer · 2017-09-12 07:40:57Z

3

A generic approach for all array, even if they are irregular styled.

Use: array.reduce(sum, 0)

function sum(r, a) {
    return Array.isArray(a) ? a.reduce(sum, r) : r + a;
}

console.log([
    [1389740400000, 576],
    [1389741300000, 608],
    [1389742200000, 624],
    [1389743100000, 672],
    [1389744000000, 691]
].reduce(sum, 0));

console.log([
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12, [13, 14, 15, 16]]
].reduce(sum, 0));

edited Sep 12, 2017 at 7:40

answered Aug 23, 2015 at 16:14

Nina Scholz

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Comments

coder.in.me · Accepted Answer · 2014-08-06 11:16:14Z

0

What you have written would work if data is an array of integers. In your case, data is an array of arrays. Hence the return statement should operate on elements of the array:

return [previousValue[0] + currentValue[0], previousValue[1] + currentValue[1]];

answered Aug 6, 2014 at 11:16

coder.in.me

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