parallel.js

'use strict'

function parallel(tasks, max, done) {
  let running = 0
  let completed = 0
  next()

  function taskCompleted() {
    completed++
    console.log(`finished task #${completed}`)
    if (completed === tasks.length) return done()
    running--
    next()
  }

  function next() {
    while (running < max && completed + max - 1 < tasks.length) spawnNext()
  }

  function spawnNext() {
    tasks[completed](taskCompleted)
    running++
    console.log(`added task, now running: ${running}`)
  }
}

function asyncThing(callback) {
  setTimeout(() => {
    callback()
  }, Math.random() * 2500)
}

parallel([asyncThing, asyncThing, asyncThing, asyncThing], 3, () => {
  console.log('finished everything!')
})

queue.js

'use strict'

function queue(tasks, callback) {
  function process(index) {
    if (index === tasks.length) {
      console.log('queue has finished processing all tasks')
      if (callback) return callback()
      return
    }
    tasks[index](index, err => {
      if (err) console.log('error occured', err)
      process(index + 1)
    })
  }
  process(0)
}

function asyncThing(index, callback) {
  setTimeout(() => {
    console.log(`did async thing #${index}`)
    callback(null)
  }, 500)
}

const tasks = [asyncThing, asyncThing, asyncThing, asyncThing, asyncThing]

queue(tasks)