node_modules/tldts-core/src/domain.ts

import { IOptions } from './options';

/**
 * Check if `vhost` is a valid suffix of `hostname` (top-domain)
 *
 * It means that `vhost` needs to be a suffix of `hostname` and we then need to
 * make sure that: either they are equal, or the character preceding `vhost` in
 * `hostname` is a '.' (it should not be a partial label).
 *
 * * hostname = 'not.evil.com' and vhost = 'vil.com'      => not ok
 * * hostname = 'not.evil.com' and vhost = 'evil.com'     => ok
 * * hostname = 'not.evil.com' and vhost = 'not.evil.com' => ok
 */
function shareSameDomainSuffix(hostname: string, vhost: string): boolean {
  if (hostname.endsWith(vhost)) {
    return (
      hostname.length === vhost.length ||
      hostname[hostname.length - vhost.length - 1] === '.'
    );
  }

  return false;
}

/**
 * Given a hostname and its public suffix, extract the general domain.
 */
function extractDomainWithSuffix(
  hostname: string,
  publicSuffix: string,
): string {
  // Locate the index of the last '.' in the part of the `hostname` preceding
  // the public suffix.
  //
  // examples:
  //   1. not.evil.co.uk  => evil.co.uk
  //         ^    ^
  //         |    | start of public suffix
  //         | index of the last dot
  //
  //   2. example.co.uk   => example.co.uk
  //     ^       ^
  //     |       | start of public suffix
  //     |
  //     | (-1) no dot found before the public suffix
  const publicSuffixIndex = hostname.length - publicSuffix.length - 2;
  const lastDotBeforeSuffixIndex = hostname.lastIndexOf('.', publicSuffixIndex);

  // No '.' found, then `hostname` is the general domain (no sub-domain)
  if (lastDotBeforeSuffixIndex === -1) {
    return hostname;
  }

  // Extract the part between the last '.'
  return hostname.slice(lastDotBeforeSuffixIndex + 1);
}

/**
 * Detects the domain based on rules and upon and a host string
 */
export default function getDomain(
  suffix: string,
  hostname: string,
  options: IOptions,
): string | null {
  // Check if `hostname` ends with a member of `validHosts`.
  if (options.validHosts !== null) {
    const validHosts = options.validHosts;
    for (const vhost of validHosts) {
      if (/*@__INLINE__*/ shareSameDomainSuffix(hostname, vhost)) {
        return vhost;
      }
    }
  }

  let numberOfLeadingDots = 0;
  if (hostname.startsWith('.')) {
    while (
      numberOfLeadingDots < hostname.length &&
      hostname[numberOfLeadingDots] === '.'
    ) {
      numberOfLeadingDots += 1;
    }
  }

  // If `hostname` is a valid public suffix, then there is no domain to return.
  // Since we already know that `getPublicSuffix` returns a suffix of `hostname`
  // there is no need to perform a string comparison and we only compare the
  // size.
  if (suffix.length === hostname.length - numberOfLeadingDots) {
    return null;
  }

  // To extract the general domain, we start by identifying the public suffix
  // (if any), then consider the domain to be the public suffix with one added
  // level of depth. (e.g.: if hostname is `not.evil.co.uk` and public suffix:
  // `co.uk`, then we take one more level: `evil`, giving the final result:
  // `evil.co.uk`).
  return /*@__INLINE__*/ extractDomainWithSuffix(hostname, suffix);
}