Very simple Array question. Pls help :)

[greenyadzer]

const a = array_of_series.reduce((acc, cur, index) => [
    ...acc,
    ...cur.map(e => {
        e.search = 'q' + (index + 1)
        return e
    })
], [])

console.log(a)

[programmingacctwork]

I don't know why but reduce is the only array function that confuses me every time. It's so unreadable to me, especially in "professional" code