return a flatten map resolved by async function

async-flat-map.js

async function asyncFlatMap (arr, asyncFn) {
    return Promise.all(flatten(await asyncMap(arr, asyncFn)))
}

function asyncMap (arr, asyncFn) {
    return Promise.all(arr.map(asyncFn))
}

function flatMap (arr, fn) {
    return flatten(arr.map(fn))
}

function flatten (arr) {
    return [].concat(...arr)
}

module.exports = {
    asyncFlatMap,
    asyncMap,
    flatMap,
    flatten
}