async_map.js

function map(collection, iteratee, callback) {
  let running = collection.length
  const results = []
  collection.forEach((value, index) =>
    iteratee(value, (err, data) => {
      if (err) {
        callback(err)
        return
      }
      results[index] = data
      running--
      if (running === 0) {
        callback(null, results)
      }
    })
  )
}