NodeJS script to check if a machine & port is reachable (e.g. telnet debugging replacement on minimal alpine image)

port-reachable.js

// USAGE
// node port-reachable.js google.com 80
// NOTE: if you want to get the file onto a machine without a text editor then try
// `echo "<copy & paste code here>" > port-reachable.js`

const dns = require('dns')
const net = require('net')

/**
 * Find IP for dns
 * e.g. dnsLookup('www.google.com', console.log)
 */
const dnsLookup = (domain, cb) => {
  dns.lookup(domain, (err, ip, _) => {
    if (err) throw err
    cb(ip)
  })
}

/**
 * Establish connection to port
 * e.g. connect(8080)('192.168.1.1')
 */
const connect = (port) => (ip) => {
  const client = new net.Socket();
  console.log('TRYING TO CONNECT TO: ' + ip + ':' + port)

  client.connect(port, ip, () => {
    console.log('CONNECTED!')
    client.destroy()
  })
  client.on('error', console.error)
}

// e.g. connDomain('www.google.com', 443)
const connDomain = (domain, port) => dnsLookup(domain, connect(port))