hypotheek.js

'use strict'

const AMOUNT = 300000
const YEARS = 25
const RATE = 2.19

const MONTHS = yearsToMonths(YEARS)
const MONTHLY = monthly(AMOUNT, YEARS, RATE)

// generate an array with shorted name of each month
const MONTHNAMES = new Array(12).fill().map((_, i) => {
  return new Date(0, i).toLocaleString('en', { month: 'short' })
})

function monthly (amount, years, rate) {
  const mPrate = prate(rate)
  const months = yearsToMonths(years)
  const mPow = Math.pow(1 + mPrate, months)

  return amount * (mPrate * mPow) / (mPow - 1)
}

function monthlyBreakdown (amount, years, rate) {
  let leftToPay = amount
  let totalInterest = 0
  const monthlyPay = monthly(amount, years, rate)

  return new Array(yearsToMonths(years)).fill().map((_, month) => {
    const interest = getInterest(leftToPay, rate)
    const aflos = monthlyPay - interest

    leftToPay -= aflos
    totalInterest += interest

    return { month, monthlyPay, aflos, interest, leftToPay }
  })
}

const breakdown = monthlyBreakdown(AMOUNT, YEARS, RATE)
const totalCap = sum(breakdown, 'aflos')
const totalInterest = sum(breakdown, 'interest')
const totalSum = totalCap + totalInterest

const monthlyBreakdownLines = breakdown.map((perMonth, i) => {
  const { month, monthlyPay, aflos, interest, leftToPay } = perMonth
  return `${MONTHNAMES[i]}  ${monthlyPay.toFixed(2)}  ${aflos.toFixed(2)}  ${interest.toFixed(2)} ${leftToPay.toFixed(2)}`
})

const yearlyBreakdownLines = chunk(breakdown, 12).map((months, i) => {
  const yearlyPay = sum(months, 'monthlyPay').toFixed(2)
  const yearlyAflos = sum(months, 'aflos').toFixed(2)
  const yearlyInterest = sum(months, 'interest').toFixed(2)
  const leftToPay = months.slice(12)
  return `Year   ${i + 1} ${yearlyPay}  ${yearlyAflos}  ${yearlyInterest} ${leftToPay}`
})

console.log(`
Quick & dirty mortgage simulator
Written by nib, (c) GPL
Modified by Gilles De Mey

Amount loaned ..... : € ${AMOUNT}
Duration .......... : ${YEARS} years
Duration .......... : ${yearsToMonths(YEARS)} months
Fixed rate ........ : ${RATE} %
Periodic rate...... : ${(prate(RATE) * 100).toFixed(3)} %
Monthly pay ....... : € ${MONTHLY.toFixed(6)}

Year   1
 ${monthlyBreakdownLines.slice(0, 12).join('\n ')}
${yearlyBreakdownLines.slice(1).join('\n')}

Total capital ..... : € ${totalCap.toFixed(2)}
Total interest .... : € ${totalInterest.toFixed(2)}
Sum ................: € ${totalSum.toFixed(2)}
`)

function yearsToMonths (years) {
  return years * 12
}

function prate (rate) {
  return Math.pow((rate / 100) + 1, 1 / 12) - 1
}

function getInterest (amount, rate) {
  return prate(rate) * amount
}

// split array in smaller chunks
function chunk (list, size) {
  return new Array(Math.ceil(list.length / size)).fill().map(_ => list.splice(0, size))
}

// sum of a property in an array
function sum (array, prop) {
  return array.reduce((sum, obj) => sum += obj[prop], 0)
}