concurrent-n-promises.ts

async function processItems (
  items: unknown[],
  operation: () => Promise,
  concurrency = 25
) {
  const errors = []
  let id = 0
  const exec = async () => {
    if (id === items.length) return
    const item = items[id++]
    await operation(item).catch(error => errors.push({ item, error }))
    return exec()
  }
  const workers = Array.from({ length: concurrency }, exec)
  await Promise.all(workers)
  return errors
}