defer.js

var waitall = {
  defer: [],
  final: null
}

// will make sure to call `waitall.final` when all `waitall.defer` has cleared
function defer (fn) {
  waitall.defer.push(fn)
  return function (...args) {
    var result = fn(...args)
    waitall.defer.shift() // self
    if (waitall.final && waitall.defer.length === 0) {
      setTimeout(waitall.final)
      waitall.final = null // for re-used lambda, this resets the state for next invoke
    }
    return result
  }
}

// ensure `fn` is called when `waitall.defer` is empty
function final (fn) {
  waitall.final = fn
  if (waitall.final && waitall.defer.length === 0) {
    setTimeout(waitall.final)
    waitall.final = null // for re-used lambda, this resets the state for next invoke
  }
}

module.exports = { final, defer }

/**

  const { final, defer } = require('./defer.js')

  function dosomething (n) {
    console.log(n, 'start')
    setTimeout(defer(function () {
      console.log(n, 'end')
    }), n * 1000)
  }

  dosomething(1)
  dosomething(2)
  dosomething(3)
  dosomething(4)
  final(function () { console.log('exit') })

**/