Reduce array of objects without using .push()

Question

I have this array of objects:

[
    {
        user: 'User_1',
        date: 1603926000000,
        count: 3,
    },
    {
        user: 'User_2',
        date: 1603926000000,
        count: 10,
    },
    {
        user: 'User_2',
        date: 1604876400000,
        count: 1,
    },
]

I reduce it with this function:

const reducedDataByDate = dataByDate.reduce((acc, d) => {
        const foundUser = acc.find((a) => a.user === d.user)
        const value = { date: formatDate(d.date), count: d.count }
        if (!foundUser) {
            acc.push({ user: d.user, data: [value] })
        } else {
            foundUser.data.push(value)
        }
        return acc
    }, [])

with this outcome:

[
    {
        user: 'User_1',
        data: [
            {
                date: '2020-10-29',
                count: 10,
            },
            {
                date: '2020-11-09',
                count: 1,
            },
        ],
    },
    {
        user: 'User_2',
        data: [
            {
                date: '2020-10-29',
                count: 3,
            },
        ],
    },
]

Ideally, I would like get rid of pushing values to original acc and foundUser array but have little idea how to go about that. Any input is much appreciated!

Thanks for your comment. I am looking for a way to not mutate the original array. I guess a function collectDataForUser that for a specific user collects all data then add the result of that to acc/ foundUser. — suuuriam
– suuuriam, Commented Nov 11, 2020 at 14:37

Nina Scholz · Accepted Answer · 2020-11-11 14:46:49Z

1

You could collect the data with a Map and get the wanted format from it.

const 
    data = [{ user: 'User_1', date: 1603926000000, count: 3 }, { user: 'User_2', date: 1603926000000, count: 10 }, { user: 'User_2', date: 1604876400000, count: 1 }],
    result = Array.from(
        data.reduce(
            (m, { user, ...o }) => m.set(user, [...(m.get(user) || []), o]),
            new Map
        ),
        ([user, data]) => ({ user, data })
    );

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

answered Nov 11, 2020 at 14:46

Nina Scholz

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2 Comments

suuuriam Over a year ago

Thanks, that works indeed! However, I don't really understand what's going on here ([user, data]) => ({ user, data }). Could you clear that up?

Nina Scholz Over a year ago

Array.from takes an iterable, the map, which is the same as Map#entries which returns an array of arrays with key/value pairs and this array is mapped by using the key/userand value/data as short hand properties for a new object.

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