How to use the xpring-common-js.XRPAmount function in xpring-common-js

public static defaultFeeResponse(): Fee {
    const amount = new XRPAmount();
    amount.setDrops("10");

    const fee = new Fee();
    fee.setAmount(amount);

    return fee;
  }

public static defaultAccountInfoResponse(): AccountInfo {
    const balance = new XRPAmount();
    balance.setDrops("4000");

    const accountInfo = new AccountInfo();
    accountInfo.setBalance(balance);

    return accountInfo;
  }