index.js
function getBalance(callback, currency){
    if (currency === undefined) {
        var url = 'balance';
    } else {
        var url =  'balance/' + currency.toUpperCase();
    }
    sendRequest( url, 'GET', {}, true, callback );
}
function getBalance(callback, currency){
    if (currency === undefined) {
        var url = 'balance';
    } else {
        var url =  'balance/' + currency.toUpperCase();
    }
    sendRequest( url, 'GET', {}, true, callback );
}
Get balance

@param callback
@param currency