lib/async.js
function acb(args) {
  args = _.toArray(args)
  return _.find(_.reverse(args), function (arg) { return _.isFunction(arg) }) || function () { }
}
function acb(args) {
  args = _.toArray(args)
  return _.find(_.reverse(args), function (arg) { return _.isFunction(arg) }) || function () { }
}
reverse scans args for [Function] || null -> used for grabbing callback from dynamic args returns first arg that is typeof function from right to left