check if a port is being used with nodejs

isPortTaken.js

var isPortTaken = function(port, fn) {
  var net = require('net')
  var tester = net.createServer()
  .once('error', function (err) {
    if (err.code != 'EADDRINUSE') return fn(err)
    fn(null, true)
  })
  .once('listening', function() {
    tester.once('close', function() { fn(null, false) })
    .close()
  })
  .listen(port)
}

Test without producing an error
on mac and most unix/linux

const { spawnSync } = require('child_process')

function checkPort(port) {
  const output = spawnSync(
    `lsof -i tcp:${port} | awk '{print $2}' |grep --invert PID`,
    { shell: true }
  )
  if (output.error) {
    console.error(output.error)
    return
  }
  const pid = Buffer.from(output.stdout.buffer).toString().split('\n')[0]
  console.log({ pid })
  return pid
}

const pid = checkPort(443)) 
if (pid) {
  console.log(`server is running, process id  ${pid}`)
}

const pid = checkPort(443)) 

@tysonrm – nice technique! FYI, there's an extra ) when making the checkPort() example.