How to use the bns.util.fqdn function in bns

fromString(str) {
    assert(typeof str === 'string');
    assert(str.length <= 255);

    str = str.toLowerCase();

    const parts = str.split('@');

    if (parts.length > 1) {
      const name = util.fqdn(parts[0]);
      assert(isName(name));

      const ips = parts[1].split(',');
      assert(ips.length <= 2);

      this.type = types.GLUE;
      this.target = name;

      for (const ip of ips) {
        const [type, addr] = parseIP(ip);

        switch (type) {
          case types.ONION:
            throw new Error('Bad glue address.');
          case types.INET4:
            if (addr === '0.0.0.0')

assert(str[index - 1] === '?' || str[index - 1] === '&');

    str = str.substring(index + 7);

    const parts = str.split(/[:&]/);
    assert(parts.length >= 2);

    const [nid, nin] = parts;

    assert(nid.length <= 255);
    assert(nin.length <= 255);

    assert(isSingle(nid));

    this.nid = util.fqdn(nid);
    this.nin = nin;

    return this;
  }

fromString(str) {
    assert(typeof str === 'string');
    assert(str.length <= 512);

    const parts = str.split(':');
    assert(parts.length === 2);

    const [currency, address] = parts;

    assert(currency.length <= 255);
    assert(address.length <= 255);
    assert(isSingle(currency));

    this.currency = util.fqdn(currency);
    this.address = address;

    return this;
  }

fromJSON(json) {
    assert(json && typeof json === 'object');

    if (json.service != null) {
      assert(isSingle(json.service));
      this.service = util.fqdn(json.service);
    }

    if (json.protocol != null) {
      assert(isSingle(json.protocol));
      this.protocol = util.fqdn(json.protocol);
    }

    if (json.priority != null) {
      assert((json.priority & 0xff) === json.priority);
      this.priority = json.priority;
    }

    if (json.weight != null) {
      assert((json.weight & 0xff) === json.weight);
      this.weight = json.weight;
    }

    if (json.target != null)
      this.target.fromJSON(json.target);

    if (json.port != null) {

function isSingle(label) {
  label = util.fqdn(label);

  if (!isName(label))
    return false;

  if (util.countLabels(label) !== 1)
    return false;

  return true;
}

return this;
    }

    if (onion.isLegacyString(str)) {
      this.type = types.ONION;
      this.target = onion.normalizeLegacy(str);
      return this;
    }

    if (onion.isNGString(str)) {
      this.type = types.ONIONNG;
      this.target = onion.normalizeNG(str, sha3.digest);
      return this;
    }

    const name = util.fqdn(str);
    assert(isName(name));

    this.type = types.NAME;
    this.target = name;

    return this;
  }