JavaScript can handle both simple and complex calculations.

The loan and repayment JavaScript calculators use the "nominal" interest rate which the lender divides by the number of times it is to be applied in the year to get the rate for each interest calculation. So a nominal rate of 12% calculated monthly means 1% per month.
Calculating interest at 1% per month actually gives a higher "effective" interest rate for a year than the 12% we started with because the interest is compounded. This JavaScript Interest Rate Converter will allow you to find out the effective rate if you know the nominal rate and vice versa.