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 );
}
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
Get balance