How to use the rfc4648.base64url.parse function in rfc4648

async decrypt(ciphertext, key, aad = '')
    {
        if (!sodium) sodium = await SodiumPlus.auto();
        let header = ciphertext.slice(0, 5);
        if (!await Util.hashEquals(MAGIC_HEADER, header)) {
            throw new CryptoOperationException('Invalid ciphertext header.');
        }
        let decoded = await Util.toBuffer(base64url.parse(ciphertext.slice(5)));
        let hkdfSalt = decoded.slice(0, SALT_SIZE);
        let ctrNonce = decoded.slice(
            SALT_SIZE,
            SALT_SIZE + NONCE_SIZE
        );
        let mac = decoded.slice(
            SALT_SIZE + NONCE_SIZE,
            SALT_SIZE + NONCE_SIZE + MAC_SIZE
        );
        let cipher = decoded.slice(SALT_SIZE + NONCE_SIZE + MAC_SIZE);

        let macKey = await Util.HKDF(key, hkdfSalt, 'HMAC-SHA-384');
        let recalc;
        if (aad.length > 0) {
            recalc = await Util.hmac(
                'sha384',

{
        if (!sodium) sodium = await SodiumPlus.auto();
        let encKey = Buffer.alloc(32, 0);
        if (Buffer.isBuffer(key)) {
            key.copy(encKey, 0);
        } else if (SymmetricKey.isSymmetricKey(key)) {
            key.getRawKey().copy(encKey, 0);
        } else {
            throw new TypeError('Argument 1 must be a SymmetricKey');
        }

        let header = ciphertext.slice(0, 5);
        if (!await Util.hashEquals(MAGIC_HEADER, header)) {
            throw new CryptoOperationException('Invalid ciphertext header.');
        }
        let decoded = await Util.toBuffer(base64url.parse(ciphertext.slice(5)));
        let nonce = decoded.slice(0, NONCE_SIZE);
        let encrypted = decoded.slice(NONCE_SIZE);

        if (aad.length >= 0) {
            if (!Buffer.isBuffer(aad)) {
                aad = await Util.toBuffer(aad);
            }
            aad = Buffer.concat([nonce, aad]);
        } else {
            aad = nonce;
        }

        let decrypted;
        try {
            decrypted = await sodium.crypto_aead_xchacha20poly1305_ietf_decrypt(
                encrypted,