call multiple async methods in parallel and receive the result in a callback

parallel.js

function parallel() {
  var args = Array.apply(null, arguments)
    , callback = args.pop()
    , returns = []
    , len = 0
  args.forEach(function (el, i) {
    el(function () {
      var a = Array.apply(null, arguments)
        , e = a.shift()
      if (e) return callback(e)
      returns[i] = a
      if (args.length == ++len) returns.unshift(null) && callback.apply(null, returns)
    })
  })
}



parallel(
  function (fn) {
    getTimeline(function (e, timeline) {
      fn(e, timeline)
    })
  }
, function (fn) {
    getUser(function (e, user) {
      fn(e, user)
    })
  }
, function (e, timeline, user) {
    if (e) return console.log(e)
    console.log(timeline, user)
  }
)