Excel PMT function in JS

Question

I found i little snipet on internet, about PMT calculate.

function PMT(i, n, p) {
 return i * p * Math.pow((1 + i), n) / (1 - Math.pow((1 + i), n));
}
function CalculatePMTFromForm(idLoanAmount, idAnnualInterestRate, idMonths, idResult) {
 var i = jQuery('#' + idAnnualInterestRate).val() / 1200;
 var n = jQuery('#' + idMonths).val();
 var p = jQuery('#' + idLoanAmount).val();
 var pmt = PMT(i, n, -p);
jQuery('#' + idResult).val(pmt.toFixed(2));
}
function performCalc() {
 CalculatePMTFromForm('LoanAmount', 'InterestRate', 'Months', 'Payment');
}
jQuery(document).ready(function() { performCalc(); jQuery('.calc').keyup(performCalc); });

When the page is load, in the result input box I see "NaN" , and when i tpye some irrelevant number then "-Infinity" msg appear. I search to "NaN" in files and i found in jquery.js, but after I modify, nothing change. And I can't find Infinity

How can I change this messages?

Edit

Calling code:-

function performCalc() {
  CalculatePMTFromForm('LoanAmount', 'InterestRate', 'Months', 'Payment');
}

jQuery(document).ready(function() {
  performCalc(); jQuery('.calc').keyup(performCalc);
});

This is worked for me:

if(pmt>0 && pmt<Number.MAX_VALUE) {jQuery('#' + idResult).val(pmt.toFixed(2));}

Answer 1

This question's been dead for over a year, but I recently needed to do the same thing. Here's what I came up with:

function pmt(rate_per_period, number_of_payments, present_value, future_value, type){
    if(rate_per_period != 0.0){
        // Interest rate exists
        var q = Math.pow(1 + rate_per_period, number_of_payments);
        return -(rate_per_period * (future_value + (q * present_value))) / ((-1 + q) * (1 + rate_per_period * (type)));

    } else if(number_of_payments != 0.0){
        // No interest rate, but number of payments exists
        return -(future_value + present_value) / number_of_payments;
    }

    return 0;
}

type needs to be 1 or 0, same as Excel's. The rate_per_period needs to be a decimal (eg: 0.25, not 25%).

An example:

/* Example: */
var interest    = 0.07,     // Annual interest
    years       = 5,        // Lifetime of loan (in years)
    present     = 10000,    // Present value of loan
    future      = 20000,    // Future value of loan
    beginning   = 1;        // Calculated at start of each period

var payment = -pmt(interest / 12,   // Annual interest into months
                   years * 12,      // Total months for life of loan
                   present,
                   future,
                   beginning);

And the payment for the example period (month) is ~$474.60.

Note the negation of the result, as the amount is a dedection - ie: costs you $474 - the result is a negative value. Were the result to be a credit, the result would be a positive. Generally you'll want to keep it as a negative/positive, but if you were displaying it in a format like Total Debt: $XYZ, you'd want to convert it to a positive.