Calculate mortage payments pr month

calculate-mortage.js

/**
 * Function for calculating loan ammount.
 * Sum is without fees and additional costs.
 */
function calculate_mortage( totalammount, ownmoney, interest, years ) {
	/**
	 * Formula M = P * ( J / (1 - (1 + J)^-N))
	 * M = Payment monthly
	 * P = Loan ammount
	 * J = Effective intrest in decimal (x% / 100) / 12 months
	 * N = Num Payments
	 * 
	 * Effective interest formula (1 + J)^12 – 1
	 */
	var m, p, j, n;
	p = totalammount - ownmoney;
	j = (interest / 100) / 12;
	n = years * 12;
	m = p * ( j / ( 1 - Math.pow(1+j, -n) ) );
	
	// return with rounded monthly payment
	return Math.round( m );
}