async stuff

stuff.js

function joinCalls(doneAfter, callback) {
  var counter = 0
  return function wrapped() {
    if (++counter === doneAfter) callback()
  }
}

function joinCallsSingleValue(doneAfter, joinValuesFn, callback) {
  var counter = 0
  var result = null
  return function wrapped() {
    result = joinValuesFn.bind(null, result).apply(null, arguments)
    if (++counter === doneAfter) callback(result)
  }
}

var runner = {
  tasks: {},
  register: function register(key, fn) {
    if (this.tasks[key] === undefined) this.tasks[key] = []
    this.tasks[key].push(fn)
  },
  run: function run(key, callback) {
    var fns = this.tasks[key]
    callback = joinCalls(fns.length, callback)
    for (var i = 0; i < fns.length; i++) {
      fns[i](callback)
    }
  },
  runSingleValue: function runSingleValue(key, joinValuesFn, callback) {
    var fns = this.tasks[key]
    callback = joinCallsSingleValue(fns.length, joinValuesFn, callback)
    for (var i = 0; i < fns.length; i++) {
      fns[i](callback)
    }
  }
}

runner.register('test', function (callback) {
  setImmediate(function () {
    callback('hello ', 1)
  })
})
runner.register('test', function (callback) {
  setImmediate(function () {
    callback('world', 2)
  })
})

runner.run('test', function () {
  console.log('i should be called once')
})

runner.runSingleValue('test', function joinValues(results, str, i) {
  if (results === null) results = ''
  return results += str +  i
}, console.log)