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)
}

a promise implement

import { createServer } from "net";

const isPortTaken = (port: number, type: 'IPv4' | 'IPv6' = 'IPv4') => {
  let hasError = 0;
  return new Promise((res) => {
    const server = createServer()
      .once('error', err => { if (err) { res(false) } })
      .once('listening', () => {
        server
          .once('close', () => {
            hasError++;
            if (hasError > 1) {
              res(false)
            } else {
              res(true)
            }
          })
          .close()
      })
      .listen(port, type === 'IPv4' ? '0.0.0.0' : '::')
  })

};

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.