Express.js getting address and port while still executing

Question

I've read many posts on Nodejs and Expressjs for that matter but I still don't understand how this works:

This is the basic Hello World application with Express.js (taken from http://expressjs.com/starter/hello-world.html).

var express = require('express')
var app = express()

app.get('/', function (req, res) {
  res.send('Hello World!')
})

var server = app.listen(3000, function () {

var host = server.address().address
var port = server.address().port

console.log('Example app listening at http://%s:%s', host, port)

})

How are we able to get host and port using server when we are still in the process of getting what we'll eventually bind to the var server?

Answer 1

Because it's asynchronous. The callback is only being run later, after server is defined and initialized.

var server = app.listen(3000, function () {

    var host = server.address().address
    var port = server.address().port

    console.log('Example app listening at http://%s:%s', host, port)

})

Proper indenting sometimes helps see this.