In Node.js, get the external IP address of the host machine.

nodeExternalIp.js

function getExternalIp() {
	var ifconfig = require('os').networkInterfaces();
	var device, i, I, protocol;

	for (device in ifconfig) {
		// ignore network loopback interface
		if (device.indexOf('lo') !== -1 || !ifconfig.hasOwnProperty(device)) {
			continue;
		}
		for (i=0, I=ifconfig[device].length; i<I; i++) {
			protocol = ifconfig[device][i];

			// filter for external IPv4 addresses
			if (protocol.family === 'IPv4' && protocol.internal === false) {
				console.log('found', protocol.address);
				return protocol.address;
			}
		}
	}

	return null;
}

This snippet won't work all the time. Take a look:

> os.networkInterfaces()
{ lo0: 
   [ { address: '::1',
       family: 'IPv6',
       internal: true },
     { address: '127.0.0.1',
       family: 'IPv4',
       internal: true },
     { address: 'fe80::1',
       family: 'IPv6',
       internal: true } ],
  en0: 
   [ { address: 'fe80::82e6:50ff:fe05:a19e',
       family: 'IPv6',
       internal: false },
     { address: '192.168.1.3',
       family: 'IPv4',
       internal: false } ],
  vboxnet0: 
   [ { address: '192.168.33.1',
       family: 'IPv4',
       internal: false } ] }

So the output will be "found 192.168.1.3"

Some providers switched to IPv6.
I solved it by using a tunnel (https://github.com/localtunnel/)