Find the root/apex domain from a web page's hostname by setting a cookie repeatedly starting from the top-level domain.

find-root-domain-with-cookie.js

const ROOT_DOMAIN_NOT_FOUND = '';
const FAKE_COOKIE_STRING = 'fakeCookieName=fakeCookieValue';

const HOSTNAME = window.location.hostname;
const REVERSED_HOSTNAME_PARTS = HOSTNAME.split('.').reverse();

let tryIndex = 1;
let rootDomain = ROOT_DOMAIN_NOT_FOUND;

while (rootDomain === ROOT_DOMAIN_NOT_FOUND) {
  try {
    let tryRootDomain = REVERSED_HOSTNAME_PARTS.slice(0, tryIndex).reverse().join('.');
    let tryCookie = `${FAKE_COOKIE_STRING};domain=${tryRootDomain}`;

    document.cookie = tryCookie;
    let didNotFindRootDomain = document.cookie.split(';').map((c) => c.trim()).indexOf(FAKE_COOKIE_STRING) === -1;
    if (didNotFindRootDomain) {
      throw tryRootDomain;
    }
    document.cookie = `${tryCookie};max-age=0`; // delete the fake cookie

    rootDomain = tryRootDomain;
  } catch (e) {
    tryIndex += 1;

    if (tryIndex === REVERSED_HOSTNAME_PARTS.length) {
      // short-circuit the loop and return the hostname
      rootDomain = HOSTNAME;
    }
  }
}

return rootDomain;