pmt.js

function pmt(rate_per_period, number_of_payments, present_value, future_value, type){
    future_value = typeof future_value !== 'undefined' ? future_value : 0;
    type = typeof type !== 'undefined' ? type : 0;

	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;
}

console.log(pmt(0.08,6,-1.08,0,1)) //0.21631538622900973