index.js

const defaultPrice = 500000;
const defaultDownPayment = 100000;
const defaultInterestRate = 0.05;
const defaultMonths = 360;
const defaultTaxRate = 0.0126;
const defaultInsuranceRate = 0.0014;
const defaultMortgageInsuranceRate = 0.011;
const defaultMortgageInsuranceEnabled = true;
const defaultMortgageInsuranceThreshold = 0.2;
const defaultAdditionalPrincipalPayment = 0;

class MortgageCalculator {

    constructor() {
        this.totalPrice = defaultPrice;
        this.downPayment = defaultDownPayment;
        this.interestRate = defaultInterestRate;
        this.months = defaultMonths;
        this.taxRate = defaultTaxRate;
        this.insuranceRate = defaultInsuranceRate;
        this.mortgageInsuranceRate = defaultMortgageInsuranceRate;
        this.mortgageInsuranceEnabled = defaultMortgageInsuranceEnabled;
        this.mortgageInsuranceThreshold = defaultMortgageInsuranceThreshold;
        this.additionalPrincipalPayment = defaultAdditionalPrincipalPayment;
    }

    calculatePayment() {
        let loanAmount = this.totalPrice - this.downPayment;
        let paymentSchedule = MortgageCalculator.calculatePaymentSchedule(loanAmount, this.interestRate, this.months, this.additionalPrincipalPayment);
        let piPayment = paymentSchedule.length ? paymentSchedule[0].totalPayment : 0;
        let downPaymentPercentage = this.downPayment / this.totalPrice;
        let mortgageInsurance = 0;
        if (this.mortgageInsuranceEnabled && downPaymentPercentage < this.mortgageInsuranceThreshold) {
            mortgageInsurance = (loanAmount * this.mortgageInsuranceRate) / 12;
        }

        let propertyTax = (this.totalPrice * this.taxRate) / 12;
        let homeOwnerInsurance = MortgageCalculator.roundPenny((this.totalPrice * this.insuranceRate) / 12);
        return {
            loanAmount: loanAmount,
            principalAndInterest: piPayment,
            tax: propertyTax,
            insurance: homeOwnerInsurance,
            total: piPayment + propertyTax + homeOwnerInsurance + mortgageInsurance,
            termMonths: this.months,
            paymentSchedule: paymentSchedule,
            mortgageInsurance: mortgageInsurance
        };
    }

    static calculatePaymentSchedule(loanAmount, annualRate, termMonths, additionalPrincipalPayments = 0) {
        const monthlyRate = annualRate / 12;
        const monthlyPayment = MortgageCalculator.calculateMonthlyPIPayment(loanAmount, annualRate, termMonths);
        let principal = MortgageCalculator.roundPenny(loanAmount);
        let payments = [];
        let totalInterest = 0;
        let totalPayments = 0;
        let i = 0;
        while (principal > 0 && i < termMonths) {
            let interestPayment = MortgageCalculator.roundPenny(principal * monthlyRate);
            let principalPayment = MortgageCalculator.roundPenny(monthlyPayment - interestPayment + additionalPrincipalPayments);
            if (principal > principalPayment) {
                principal = MortgageCalculator.roundPenny(principal - principalPayment);
            }
            else {
                principalPayment = principal;
                principal = 0;
            }
            let totalPayment = interestPayment + principalPayment;
            totalInterest += interestPayment;
            totalPayments += totalPayment;
            payments[i] = {
                count: i+1,
                interestPayment: interestPayment,
                totalInterest: totalInterest,
                principalPayment: principalPayment,
                totalPayment: totalPayment,
                totalPayments: totalPayments,
                balance: principal
            };
            i++;
        }

        return payments;
    }

    static calculateMonthlyPIPayment(loanAmount, annualRate, termMonths) {
        let monthlyRate = annualRate / 12;
        let payment = (monthlyRate * loanAmount * Math.pow(1 + monthlyRate, termMonths)) / (Math.pow(1 + monthlyRate, termMonths) - 1);
        return this.nextPenny(payment);
    }

    static roundPenny(value) {
        return Math.round(value * 100)/100;
    }

    static nextPenny(value) {
        return Math.ceil(value * 100)/100;
    }
}

const _calc = new MortgageCalculator();

module.exports = {
    createMortgageCalculator: function(){
        return new MortgageCalculator();
    },
    calculatePayment: function(totalPrice = defaultPrice,
                               downPayment = defaultDownPayment,
                               interestRate = defaultInterestRate,
                               months = defaultMonths,
                               taxRate = defaultTaxRate,
                               insuranceRate = defaultInsuranceRate,
                               mortgageInsuranceRate = defaultMortgageInsuranceRate,
                               mortgageInsuranceEnabled = defaultMortgageInsuranceEnabled,
                               mortgageInsuranceThreshold = defaultMortgageInsuranceThreshold,
                               additionalPrincipalPayment = defaultAdditionalPrincipalPayment) {
        _calc.totalPrice = totalPrice;
        _calc.downPayment = downPayment;
        _calc.interestRate = interestRate;
        _calc.months = months;
        _calc.taxRate = taxRate;
        _calc.insuranceRate = insuranceRate;
        _calc.mortgageInsuranceRate = mortgageInsuranceRate;
        _calc.mortgageInsuranceEnabled = mortgageInsuranceEnabled;
        _calc.mortgageInsuranceThreshold = mortgageInsuranceThreshold;
        _calc.additionalPrincipalPayment = additionalPrincipalPayment;
        return _calc.calculatePayment();
    },
    nextPenny: MortgageCalculator.nextPenny
};