PMT function in Javascript

Question

I want to use the Excel PMT function in Javascript. The parameter would be:

Pmt( interest_rate, number_payments, PV, FV, Type )

interest_rate : the interest rate for the loan.
number_payments : the number of payments for the loan.
PV : the present value or principal of the loan.
FV : It is the future value or the loan amount outstanding after all payments have been made. 

Type is : It indicates when the payments are due. Type can be one of the following values:
 0, 1

You can refer to: http://www.techonthenet.com/excel/formulas/pmt.php

this is the code I use, I am stuck on the last parameter. Which is "type is" 0 or 1. How does it affect the calculations, please?

function PMT (ir, np, pv, fv ) {
 /*
 ir - interest rate per month
 np - number of periods (months)
 pv - present value
 fv - future value (residual value)
 */
 pmt = ( ir * ( pv * Math.pow ( (ir+1), np ) + fv ) ) / ( ( ir + 1 ) * ( Math.pow ( (ir+1), np) -1 ) );
 return pmt;
}

I need it in plain Javascript and not in jQuery, please.

Answer 1

Try the following settings to better understand the effect of the Type parameter: Present Value = 100, Annual Interest = 12%, Number of Months = 1.

When Type=0 (the default), the PMT() function will yield 101

When Type=1, the PMT() function will yield 100

Because it is assumed that the payment would be made at the end of the month when Type=0, the interest is calculated for 1 month. Because the payment is made at the beginning of the month, Type=1 computes interest for 0 months.