Make a best guess at the IP address for the local machine, preferring the first non-loopback non-internal IPv4 address.

getIP.js

function getIP() {
  var interfaces = require('os').networkInterfaces();
  var names = Object.keys(interfaces);
  var addresses = [];
  var idx = 0;

  for (var i = 0; i < names.length; i++) {
    var name = names[i];
    var nic = interfaces[name];

    for (var j = 0; j < nic.length; j++) {
      var address = nic[j];

      address.lo = name.indexOf('lo') === 0;
      address.ipv4 = address.family.toLowerCase() === 'ipv4';
      address.idx = idx++;
      addresses.push(address);
    }
  }

  addresses.sort(function(a, b) {
    if (a.lo && !b.lo) return 1;
    else if (!a.lo && b.lo) return -1;

    if (a.internal && !b.internal) return 1;
    else if (!a.internal && b.internal) return -1;

    if (a.ipv4 && !b.ipv4) return -1;
    else if (!a.ipv4 && b.ipv4) return 1;

    return a.idx - b.idx;
  });

  return addresses.length ? addresses[0].address : '127.0.0.1';
}