serialCon-v2.js

const serialCon = (timeArray, maxConcurrency) => {
  let activePromiseCount = 0
  const times = [...timeArray]

  const processNextPromise = () => {
    const time = times.shift()

    if (!time) return

    const newPromise = new Promise((resolve) =>
      setTimeout(() => {
        console.log(time)
        resolve()
      }, time)
    )
    activePromiseCount += 1

    // Finished processed one promise
    // Now we can process another one!
    newPromise.then(() => {
      activePromiseCount -= 1

      processNextPromise()
    })

    // Maximum concurrency not reached yet, let's process another one
    if (activePromiseCount < maxConcurrency) {
      processNextPromise()
    }
  }

  // Start the recursive calls off!
  processNextPromise()
}

const timeoutArray = [1000, 100, 200, 300, 400, 500]

// serialCon(timeoutArray, 1)
serialCon(timeoutArray, 2)