calculateCompoundInterest.js

/**
 * Calculates a compound interest (procent składany) for a given period of time
 * with given annual interest rate, number of capitalizations and saving period.
 * 
 * @param amount
 *         initial investment
 * 
 * @param interestRate
 *         nominal annual interest rate (in decimal, so if interest rate is 5%,
 *         we give 0.05)
 * 
 * @param capitalizations
 *         number of capitalizations per year
 *
 * @param period
 *         saving period (in years, as decimal if shorter period needed)
 *
 * @return calculated compound interest
 */
function compoundInterest( amount, interestRate, capitalizations, period ) {
    return amount * Math.pow( ( 1 + ( interestRate / capitalizations ) ),
        capitalizations * period );
}

alert( compoundInterest( 10000.0, 0.04, 365, 0.4 ) );