index.js

function limiter (count) {
  var outstanding = 0
  var jobs = []

  function remove () {
    outstanding--

    if (outstanding < count) {
      dequeue()
    }
  }

  function dequeue () {
    var job = jobs.shift()
    semaphore.queue = jobs.length

    if (job) {
      run(job.fn).then(job.resolve).catch(job.reject)
    }
  }

  function queue (fn) {
    return new Promise(function (resolve, reject) {
      jobs.push({fn: fn, resolve: resolve, reject: reject})
      semaphore.queue = jobs.length
    })
  }

  function run (fn) {
    outstanding++
    try {
      return Promise.resolve(fn()).then(function (result) {
        remove()
        return result
      }, function (error) {
        remove()
        throw error
      })
    } catch (err) {
      remove()
      return Promise.reject(err)
    }
  }

  var semaphore = function (fn) {
    if (outstanding >= count) {
      return queue(fn)
    } else {
      return run(fn)
    }
  }

  return semaphore
}

function map (items, mapper) {
  var failed = false

  var limit = this

  return Promise.all(items.map(function () {
    var args = arguments
    return limit(function () {
      if (!failed) {
        return mapper.apply(undefined, args).catch(function (e) {
          failed = true
          throw e
        })
      }
    })
  }))
}

function addExtras (fn) {
  fn.queue = 0
  fn.map = map
  return fn
}

module.exports = function (count) {
  if (count) {
    return addExtras(limiter(count))
  } else {
    return addExtras(function (fn) {
      return fn()
    })
  }
}