reverseFlatten

reverseFlatten.js

// I wrote this file in order to reverse the result from https://gist.github.com/aherve/3c958b02c8262e64e0873f05377129f1

const expand = (array, eachGroupSize) => {
    return array.reduce((acc, item, index) => {
        index % eachGroupSize === 0 ? acc.push([]) : null
        acc[acc.length - 1].push(item)
        return acc
    }, [])
}

const expandWithFill = (array, eachGroupSize, fillitem) => {
    return array.reduce((acc, item, index) => {
        index % eachGroupSize === 0 ? acc.push(new Array(eachGroupSize).fill(fillitem)) : null
        acc[acc.length - 1][index % eachGroupSize] = item
        return acc
    }, [])
}

// test
let a = [1, 2, 3, 4, 5]

console.log(JSON.stringify(expand(a, 4)))
// => [[1, 2, 3, 4], [5]]

console.log(JSON.stringify(expandWithFill(a, 4, null)))
// => [[1, 2, 3, 4], [5, null, null, null]]