Two different ways to process asynchronous requests serially without the use of convenient library like async (https://caolan.github.io/async/v3/docs.html#eachSeries).

async-serial-reduction.js

(async function () {

  // Setup
  const values = [1, 2, 3]
  const makeTimestamp = () => Math.round(Date.now() / 1000)
  const request = (value, time = 1000) => {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve(value)
      }, time)
    })
  }

  // async/await serial reduction
  const result = await values.reduce(async (prev, value) => {
    const collection = await prev

    console.log(`START ${value}:`, makeTimestamp())
    collection.push(await request(value))
    console.log(`END ${value}:`, makeTimestamp())

    return collection
  }, (async () => [])())
  console.log(`result:`, result)

  // promise-based serial reduction
  const promise = values.reduce((prev, value) => {

    return prev.then(collection => {
      console.log(`START ${value}:`, makeTimestamp())

      return request(value).then(result => {
        collection.push(result)
        console.log(`END ${value}:`, makeTimestamp())

        return collection
      })
    })

  }, Promise.resolve([]))
  promise.then(result => {
    console.log(`result:`, result)
  })

})()