get a new/clean port with node.js

gistfile1.js

var portrange = 45032

function getPort (cb) {
  var port = portrange
  portrange += 1

  var server = net.createServer()
  server.listen(port, function (err) {
    server.once('close', function () {
      cb(port)
    })
    server.close()
  })
  server.on('error', function (err) {
    getPort(cb)
  })
}

@ronen & @jonathantneal, reraise:

import { createServer } from 'net'

function getPort (port = 80, maxPort = 65535) {
  if ((maxPort = parseInt(maxPort)) < (port = parseInt(port))) {
    return Promise.reject((() => {
      const e = new Error('EPORTSPEC')
      e.code = e.message
      return e
    })())
  }
  const server = createServer()
  return new Promise((resolve, reject) => server
    .once('error', error => error.code !== 'EADDRINUSE' || ++port > maxPort ? reject(error) : server.listen(port))
    .once('listening', () => server.close(() => resolve(port)))
    .listen(port)
  )
}

NB: edited to remove port range checks since server.listen has that check; only need to check that maxPort > port.