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

/* globals document, event, window, _satellite */

/**
 * Get the root domain of the current page.
 * The main purpose is so that cookies can be saved to it rather than the current hostname.
 * (The current hostname may be a subdomain of the root domain.)
 *
 * Use the cookie method to find the root domain.
 * I.e. a cookie can be saved only when its "domain" parameter is valid, e.g. "enterprise.gov.sg" is valid, but "sg" is not.
 * Reference: https://gist.github.com/yuhui/e51ece7a95bf819d5422e71c35cb1dd2
 */

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 tryRootDomain;
let tryCookie;
let foundRootDomain = false;
let rootDomain = ROOT_DOMAIN_NOT_FOUND;

while (rootDomain === ROOT_DOMAIN_NOT_FOUND) {
  try {
    /*eslint no-unused-vars: [ "error", { "caughtErrors": "none" } ] */

    tryRootDomain = REVERSED_HOSTNAME_PARTS.slice(0, tryIndex).reverse().join('.');
    tryCookie = `${FAKE_COOKIE_STRING};domain=${tryRootDomain}`;
    document.cookie = tryCookie;

    foundRootDomain = document.cookie.split(';').map((c) => c.trim()).includes(FAKE_COOKIE_STRING);
    if (!foundRootDomain) {
      throw tryRootDomain;
    }

    document.cookie = `${tryCookie};domain=${tryRootDomain};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;