@Sam-I-Am your tip resulted into this code
var currencyrateSearch = nlapiSearchRecord("currencyrate", null,
[
["basecurrency", "anyof", "1"],
"AND",
["transactioncurrency", "anyof", "2"],
"AND",
["effectivedate", "on", toDate]
],
[
new nlobjSearchColumn("exchangerate", null, null)
]
);