Float to Currency

I have the client script shown below, and I need to convert the variable tad into a currency field because the destination field (custbody35) it's a Currency field, far as I know the code should be working because of the function nlapiFormatCurrency but I get nothing.

please help, this is weird because everything seems to be right.

function Cantidad_a_DepositarSC()
{
var lineas = nlapiGetLineItemCount('item');
var tad = 0.0;
for (var i = 1; i <= lineas; i++)
     {
   var cant_disp = nlapiGetFieldValue('item', 'quantityavailable', i);
   var cant = nlapiGetFieldValue('item', 'quantity', i);
   var tasa = nlapiGetFieldValue('item', 'rate', i);
   var imp_str = nlapiGetFieldValue('item', 'taxrate1', i);