Calculate the monthly payments for paying off a loan

calculateMonthlyPayments.js

/**
 *  Calculate the monthly payments for paying off a loan
 *  Source: http://www.1728.org/loanform.htm
 *
 *  @param   integer  apr         APR of the loan
 *  @param   integer  term        Length of repaying the loan in months
 *  @param   integer  loanAmount  Amount that is being borrowed
 *  @return  integer              The monthly payments amount
 */
function calculateMonthlyPayments(apr, term, loanAmount) {
    var monthlyPayments = 0,
        rate            = apr / 1200;

    // Return false if one of the variables isn't a number
    if(isNaN(apr) || isNaN(term) || isNaN(loanAmount)) {
        return false;
    }

    // Calculate the payment differently if the apr is 0
    if(apr == 0) {
        monthlyPayments = loanAmount / term;
    } else {
        monthlyPayments = (rate + (rate / (Math.pow((rate + 1), term) - 1))) * loanAmount;
    }

    // If we get a negative amount, return 0
    if(monthlyPayments <= 0) {
        return '0';
    }

    // Return the calculated monthly payment
    return Math.round(monthlyPayments);
}