Check the original: https://gist.github.com/JosePedroDias/76bbb68ade995751ffc6a2fbc2597ae9

mapLimit.js

const split = (arr = [], n, ...arrs) => arr.length ? split(arr.slice(n), n, ...arrs, arr.slice(0, n)) : arrs
const splitIn = (arr = [], n) => split(arr, Math.ceil(arr.length / n))

const flat = (arr) => arr.reduce((a, b) => a.concat(b), [])

const mapParallel = (prom) => (arr) => Promise.all(arr.map(prom))
const mapSeries = (arr, prom, prevProm = Promise.resolve()) =>
  Promise.all(arr.map(val => (prevProm = prevProm.then(() => prom(val)))))

const allLimit = (arr, limit) => ({ then: (prom) => mapSeries(split(arr, limit), mapParallel(prom)).then(flat) })
const mapLimit = (arr, prom, limit) => mapSeries(split(arr, limit), mapParallel(prom)).then(flat)

const _mapLimit = (arr, prom, limit) => allLimit(arr, limit).then(prom)

usage-example.js

const sleep = (ms = 100) =>
  (n) =>
    new Promise(resolve => {
      console.log('FIRED ' + n);

      setTimeout(() => {
        console.log(n)

        resolve(n)
      }, ms)
    })

var arr = [1, 2, 3, 4, 5, 6, 7]

mapLimit(arr, sleep(500), 3).then(r => console.log(r))
allLimit(arr, 3).then(sleep(500)).then(r => console.log(r))