let calculateMonthlyPayment = function (principal, years, rate) {
    let monthlyRate = 0;
    if (rate) {
        monthlyRate = rate / 100 / 12;
    }
    let monthlyPayment = principal * monthlyRate / (1 - (Math.pow(1 / (1 + monthlyRate), years * 12)));
    return {principal, years, rate, monthlyPayment, monthlyRate};
    // Creating Objects from Variables ## ES6
    // shorted for the following ES5 syntax
    // return { principal: principal,
    //      years: years,
    //      rate: rate,
    //      monthlyPayment: monthlyPayment,
    //      monthlyRate: monthlyRate };
};

document.getElementById('calcBtn').addEventListener('click', function () {
    let principal = document.getElementById("principal").value;
    let years = document.getElementById("years").value;
    let rate = document.getElementById("rate").value;
    let {monthlyPayment, monthlyRate} = calculateMonthlyPayment(principal, years, rate);
    document.getElementById("monthlyPayment").innerHTML = monthlyPayment.toFixed(2);
    document.getElementById("monthlyRate").innerHTML = (monthlyRate*100).toFixed(2);
});