Base Currency from Accounting Book Record
Depending on the transaction type, you can get the base currency from the transaction record. For example, a Cash Sale record contains information about the accounting book and the associated base currency for the transaction:

Use the following example code to find the base currency for the current accounting book using the Record object:
function customizeGlImpact(transactionRecord, standardLines, customLines, book)
{
...
var bookId = book.getId();
var baseCurrency;
for (var line = 1; line <= transactionRecord.getLineItemCount('accountingbookdetail'); line++)
{
if (bookId == transactionRecord.getLineItemValue('accountingbookdetail','bookid',line))
{
baseCurrency = transactionRecord.getLineItemValue('accountingbookdetail','currency',line);
break;
}
}
...
}