unflatten.js

let a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
 
const unflatten = (arrayToUnflatten, unflattenLength) => {
    return Promise.all(new Array(Math.ceil(arrayToUnflatten.length / unflattenLength)).fill('').map((_, index) => new Promise(resolve => {
        resolve(arrayToUnflatten.slice(index * unflattenLength, index * unflattenLength + unflattenLength))
    })))
}

unflatten(a, 2).then(result => console.log(result))

birkhoff at Birkhoff-MBPR in ~/Desktop
$ node Untitled-1.js
[ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ], [ 10 ] ]