get-port.js

// stolen from https://gist.github.com/mikeal/1840641#gistcomment-3126524

import { createServer } from 'net'

export default function getPort (port = 80, maxPort = 65535) {
  if ((maxPort = parseInt(maxPort)) < (port = parseInt(port)) || isNaN(port) || isNaN(maxPort) || port < 0 || port > 65535 || maxPort < 0 || maxPort > 65535) {
    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)
  )
}