chunking promise.all and capturing all the promises

chunkingpromises.js

const chunkAsync = (arr, callback, chunkSize = 1) => {
  const results = []
  const chunks = chunkArray(arr, chunkSize)
  const work = chunks.reduce((previousPromise, chunk) => {
    return previousPromise.finally(() => {
      results.push(...chunk.map(callback))
      return Promise.all(results)
    })
  }, Promise.resolve())
  return work.finally(() => results)
}

const chunkArray = (arr, chunkSize) => {
  const result = []
  for (let i = 0; i < arr.length; i += chunkSize) {
    result.push(arr.slice(i, i + chunkSize))
  }
  return result
}

// Example

const stuff = [1, 2, 3, 4, 5]

const doAsyncWork = (i) => {
  return new Promise((resolve) => {
    console.log(`starting ${i}...`)
    setTimeout(() => {
      console.log(`...finished ${i}`)
      resolve(i)
    }, 1000)
  })
}

chunkAsync(stuff, doAsyncWork, 2).then((result) => console.log(result))
// starting 1...
// starting 2...
// ...finished 1
// ...finished 2
// starting 3...
// starting 4...
// ...finished 3
// ...finished 4
// starting 5...
// ...finished 5
// [ Promise { 1 },
//   Promise { 2 },
//   Promise { 3 },
//   Promise { 4 },
//   Promise { 5 } ]