Locate HTTP Port Number from http.Server Instance

This is a bare minimal HTTP server:

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require('http').createServer().listen(port);

If the port number is evaluated to be falsy, such as 0, null, undefined or an empty string, a random port will be assigned.

Begin accepting connections on the specified port and hostname. If the hostname is omitted, the server will accept connections on any IPv6 address (::) when IPv6 is available, or any IPv4 address (0.0.0.0) otherwise. A port value of zero will assign a random port. - https://iojs.org/api/http.html#http_server_listen_port_hostname_backlog_callback

We can use command line tool such as netstat to find out the port number:

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$ netstat -ltpn | grep node
tcp6     0    0 :::44055             :::*            LISTEN      5624/node

However, it will be very convenient without relying on external tools.

Luckily, we can find out the port number by using the address method:

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require('http').createServer().listen().address()

The result will be similar to:

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{ address: '::', family: 'IPv6', port: 44055 }

Furthermore, it works with all popular libraries that create an http.Server instance:

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> require('http').createServer().listen().address()
> require('express')().listen().address()
> require('koa')().listen().address()

They are all having the same result.

Finally, here is a Koa implementation that shows the port number the server is running at:

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var app = require('koa')();
app.listen(function () {
  console.log('Server is running at port ' + this.address().port);
});