Get domain name without subdomains using JavaScript? -- Simple solution?

extract-root-domain-from-hostname.js

/**
 * Was "Get domain name without subdomains using JavaScript?" on StackOverflow
 * https://stackoverflow.com/questions/9752963/get-domain-name-without-subdomains-using-javascript
 */

const tests = {
  'www.sidanmor.com':             'sidanmor.com',
  'exemple.com':                  'exemple.com',
  'argos.co.uk':                  'argos.co.uk',
  'www.civilwar.museum':          'civilwar.museum',
  'www.sub.civilwar.museum':      'civilwar.museum',
  'www.xxx.sub.civilwar.museum':  'civilwar.museum',
  'www.exemple.com':              'exemple.com',
  'main.testsite.com':            'testsite.com',
  'www.ex-emple.com.ar':          'ex-emple.com.ar',
  'main.test-site.co.uk':         'test-site.co.uk',
  'en.tour.mysite.nl':            'mysite.nl',
  'www.one.lv':                   'one.lv',
  'www.onfdsadfsafde.lv':         'onfdsadfsafde.lv',
  'aaa.onfdsadfsafde.aa':         'onfdsadfsafde.aa',
  // angry comments for...
  'www.d7143.test.me':            'test.me',
  'main.test-site.fr':            'test-site.fr'
};

console.log(Object.keys(tests).reduce((a, k) => {
  let result = rootDomain(k);
  a[k] = `${result} ---> ${result === tests[k]}`
  return a;
}, {}))

function rootDomain(hostname) {
  let parts = hostname.split(".");
  if (parts.length <= 2)
    return hostname;
  
  parts = parts.slice(-3);
  if (['co','com'].indexOf(parts[1]) > -1)
    return parts.join('.');
  
  return parts.slice(-2).join('.');
}