index.js

const os = require('os');

function _normalizeFamily(family) {
  return family ? family.toLowerCase() : 'ipv4';
}

function isLoopback(addr) {
  return /^(::f{4}:)?127\.([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})/
    .test(addr) ||
    /^fe80::1$/.test(addr) ||
    /^::1$/.test(addr) ||
    /^::$/.test(addr);
};

function isPrivate(addr) {
  return /^(::f{4}:)?10\.([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})$/i
    .test(addr) ||
    /^(::f{4}:)?192\.168\.([0-9]{1,3})\.([0-9]{1,3})$/i.test(addr) ||
    /^(::f{4}:)?172\.(1[6-9]|2\d|30|31)\.([0-9]{1,3})\.([0-9]{1,3})$/i
      .test(addr) ||
    /^(::f{4}:)?127\.([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})$/i.test(addr) ||
    /^(::f{4}:)?169\.254\.([0-9]{1,3})\.([0-9]{1,3})$/i.test(addr) ||
    /^f[cd][0-9a-f]{2}:/i.test(addr) ||
    /^fe80:/i.test(addr) ||
    /^::1$/.test(addr) ||
    /^::$/.test(addr);
};

function isPublic(addr) {
  return !isPrivate(addr);
};

//
// Default to `ipv4`
//
function loopback(family) {
  family = _normalizeFamily(family);

  if (family !== 'ipv4' && family !== 'ipv6') {
    throw new Error('family must be ipv4 or ipv6');
  }

  return family === 'ipv4' ? '127.0.0.1' : 'fe80::1';
};


//
// ### function address (name, family)
// #### @name {string|'public'|'private'} **Optional** Name or security
//      of the network interface.
// #### @family {ipv4|ipv6} **Optional** IP family of the address (defaults
//      to ipv4).
//
// Returns the address for the network interface on the current system with
// the specified `name`:
//   * String: First `family` address of the interface.
//             If not found see `undefined`.
//   * 'public': the first public ip address of family.
//   * 'private': the first private ip address of family.
//   * undefined: First address with `ipv4` or loopback address `127.0.0.1`.
//
module.exports = function localIpUrl(name, family) {
  var interfaces = os.networkInterfaces();
  var all;
  //
  // Default to `ipv4`
  //
  family = _normalizeFamily(family);

  //
  // If a specific network interface has been named,
  // return the address.
  //
  if (name && name !== 'private' && name !== 'public') {
    var res = interfaces[name].filter(function (details) {
      var itemFamily = details.family.toLowerCase();
      return itemFamily === family;
    });
    if (res.length === 0)
      return undefined;
    return res[0].address;
  }

  var all = Object.keys(interfaces).map(function (nic) {
    //
    // Note: name will only be `public` or `private`
    // when this is called.
    //
    var addresses = interfaces[nic].filter(function (details) {
      details.family = details.family.toLowerCase();
      if (details.family !== family || isLoopback(details.address)) {
        return false;
      } else if (!name) {
        return true;
      }

      return name === 'public' ? isPrivate(details.address) : isPublic(details.address);
    });

    return addresses.length ? addresses[0].address : undefined;
  }).filter(Boolean);

  return !all.length ? loopback(family) : all[0];
}