isFQDN.js

// https://github.com/validatorjs/validator.js/blob/master/src/lib/isFQDN.js

const default_fqdn_options = {
  require_tld: true,
  allow_underscores: false,
  allow_trailing_dot: false,
  allow_numeric_tld: false,
};

function isFQDN_(str, options) {
  assertString_(str);
  options = merge_(options, default_fqdn_options);

  /* Remove the optional trailing dot before checking validity */
  if (options.allow_trailing_dot && str[str.length - 1] === ".") {
    str = str.substring(0, str.length - 1);
  }
  const parts = str.split(".");
  const tld = parts[parts.length - 1];

  if (options.require_tld) {
    // disallow fqdns without tld
    if (parts.length < 2) {
      return false;
    }

    if (!/^([a-z\u00a1-\uffff]{2,}|xn[a-z0-9-]{2,})$/i.test(tld)) {
      return false;
    }

    // disallow spaces && special characers
    if (
      /[\s\u2002-\u200B\u202F\u205F\u3000\uFEFF\uDB40\uDC20\u00A9\uFFFD]/.test(
        tld
      )
    ) {
      return false;
    }
  }

  // reject numeric TLDs
  if (!options.allow_numeric_tld && /^\d+$/.test(tld)) {
    return false;
  }

  return parts.every((part) => {
    if (part.length > 63) {
      return false;
    }

    if (!/^[a-z_\u00a1-\uffff0-9-]+$/i.test(part)) {
      return false;
    }

    // disallow full-width chars
    if (/[\uff01-\uff5e]/.test(part)) {
      return false;
    }

    // disallow parts starting or ending with hyphen
    if (/^-|-$/.test(part)) {
      return false;
    }

    if (!options.allow_underscores && /_/.test(part)) {
      return false;
    }

    return true;
  });
}