node-http-socket-lookup.js

const http = require('http')
const net = require('net')
const { performance } = require('perf_hooks')

const HOST = 'example.com'
const ADDRESS = '93.184.216.34'
const FAMILY = 4

let lookupCountCreateConnection = 0
let lookupCountSocketEvent = 0

const log = (...args) => console.log('[' + String(performance.now().toFixed(3)).padStart(8) + ']', ...args)

const createConnection = (options, cb) => {
  log('createConnection()')
  const socket = net.createConnection(options, () => {
    log('socket connected')
    if (cb) {
      cb()
    }
  })
  socket.on('lookup', () => {
    log('createConnection() -> lookup event')
    ++lookupCountCreateConnection
  })
  return socket
}

const lookup = (hostname, options, cb) => {
  if (typeof options === 'function') {
    cb = options
    options = {}
  }
  process.nextTick(cb, null, ADDRESS, FAMILY)
}

const req = http.request({
  host: HOST,
  createConnection,
  lookup
})

req.on('response', res => {
  log('request -> response event')
  res.resume()
  res.on('end', () => {
    log('request -> response event -> end event')
    log({ lookupCountCreateConnection, lookupCountSocketEvent })
  })
  res.on('error', err => {
    log('response error:', err)
  })
})

req.on('socket', socket => {
  log('request -> socket event')
  socket.on('lookup', () => {
    log('request -> socket event -> lookup event')
    ++lookupCountSocketEvent
  })
})

req.on('error', err => {
  log('request error:', err)
})

req.end()

Output:

[  75.599] createConnection()
[  89.827] createConnection() -> lookup event
[  94.179] request -> socket event
[ 193.222] socket connected
[ 298.619] request -> response event
[ 301.219] request -> response event -> end event
[ 301.530] { lookupCountCreateConnection: 1, lookupCountSocketEvent: 0 }