Aggregate multiple values in of javascript

Question

I want to reduce an array of objects like this:

var data = [{
    item: "Item 1",
    pages: 30,
    chapters: 3
    },
    {
    item: "Item 1",
    pages: 50,
    chapters: 3
    },
    {
    item: "Item 2",
    pages: 10,
    chapters: 3
    }
    ]

But aggregating two "fields", so in this case it should end like this:

  [
    {item: "Item 1", pages: 80, chapters: 6},
    {item: "Item 2", pages: 10, chapters: 3}
  ]

I've tried with a reduce but I can't aggregate more than one field:

data.reduce((acc,cur) => {
    acc[cur.item][pages] = acc[cur.item][pages] + cur.item.pages || cur.item.pages
    acc[cur.item][chapters] = acc[cur.item][chapters] + cur.item.chapters || cur.item.chapters
    return acc
},{})

But this throws errors since it doesn't find the pages item in the accumulator.

Is it possible to reduce aggregating more than one field of data?

the problem is acc[cur.item] doesn't exist, not that pages doesn't — Jaromanda X
– Jaromanda X, Commented Jun 10, 2024 at 0:00
oh, yeah, and pages doesn't exist, not does cur.item.pages — Jaromanda X
– Jaromanda X, Commented Jun 10, 2024 at 0:03
But I can aggregate in acc[cur.item]. This is working fine, but only sums the pages for every Item: data.reduce((acc,cur) => { acc[cur.item] = acc[cur.item] + cur.pages || cur.pages return acc },{}) — Ikzer
– Ikzer, Commented Jun 10, 2024 at 0:11
your code throws an error, so "this is working fine" is a statement that is clearly NOT about this code — Jaromanda X
– Jaromanda X, Commented Jun 10, 2024 at 0:14
I mean, what do you think acc[cur.item][pages] is? it's not a property called .pages ... it's a property called 30, 50 or 10 in the case of the data you've presented — Jaromanda X
– Jaromanda X, Commented Jun 10, 2024 at 0:16

user1514042 · Accepted Answer · 2024-06-10 00:11:59Z

2

var data = [{
    item: "Item 1",
    pages: 30,
    chapters: 3
},
{
    item: "Item 1",
    pages: 50,
    chapters: 3
},
{
    item: "Item 2",
    pages: 10,
    chapters: 3
}];

const processedData = data.reduce((items, { item, pages, chapters }) => {
    const ensureItemData = () =>
        items.find(itemData => itemData.item === item)
        ?? (() => {
            const itemData = { item, pages: 0, chapters: 0 };
            items.push(itemData);
            return itemData;
        })();

    const itemData = ensureItemData();
    itemData.pages += pages;
    itemData.chapters += chapters;

    return items;
}, []);

answered Jun 10, 2024 at 0:11

user1514042

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Comments

Sash Sinha · Accepted Answer · 2024-06-10 00:21:53Z

0

You can achieve this by using reduce and properly initializing the accumulator:

const data = [
  { item: "Item 1", pages: 30, chapters: 3 },
  { item: "Item 1", pages: 50, chapters: 3 },
  { item: "Item 2", pages: 10, chapters: 3 }
];

let result = data.reduce((acc, cur) => {
  if (!acc[cur.item]) {
    acc[cur.item] = { item: cur.item, pages: 0, chapters: 0 };
  }
  acc[cur.item].pages += cur.pages;
  acc[cur.item].chapters += cur.chapters;
  return acc;
}, {});

result = Object.values(result);
console.log(result);

answered Jun 10, 2024 at 0:21

Sash Sinha

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