PMT function in Javascript
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

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;
}