Annuity and linear payments

money-value-of-time.js

const principal = 100000;
const interestRate = 0.12 / 12;
const instalments = 24;

function annuityFactor(interestRate, instalments) {
  return (interestRate + ( interestRate / ( Math.pow(1 + interestRate, instalments) - 1 ) ) );
}

function annuityPayment(principal, interestRate, instalments) {
  return principal * annuityFactor(interestRate, instalments);
}

function annuityTotalCost(principal, interestRate, instalments, annuityPayment) {
  return annuityPayment * instalments;
}

function annuityOverpayment(principal, interestRate, instalments, annuityPayment) {
  return annuityTotalCost(principal, interestRate, instalments, annuityPayment) - principal;
}

function percentageInAnnuityPayment(annuityPayment, balance, interestRate, period) {
    if( period < 1) {
      return 0;
    } else if(period === 1) {
      return balance * interestRate;
    } else if (period > 1) {
        const currentPaymentBody = annuityPayment - (balance * interestRate);
        const remainBalance = balance - currentPaymentBody;
        return percentageInAnnuityPayment(annuityPayment, remainBalance, interestRate, period - 1);
    }
}

function bodyInAnnuityPayment(annuityPayment, balance, interestRate, period) {
  return annuityPayment - percentageInAnnuityPayment(annuityPayment, balance, interestRate, period);
}

function percentageInLinearPayment(balance, interestRate, body, period) {
  return (balance - ((period-1) * body)) * interestRate;
}

function bodyInLinearPayment(balance, instalments) {
  return balance / instalments;
}

function linearPayment(principal, interestRate, body, period) {
  const percentage = percentageInLinearPayment(principal, interestRate, body, period);
  
  return percentage + body;
}

function linearTotalCost(principal, interestRate, instalments, body) {
  function calculateLinearPayment(_, index) {
    return linearPayment(principal, interestRate, body, index + 1);
  }
  
  function sumPayments(acc, item) {
    return acc + item;
  }
  
  return (Array.apply(null, Array(instalments)))
    .map(calculateLinearPayment)
    .reduce(sumPayments);
}

function linearOverpayment(principal, interestRate, instalments, body) {
  return linearTotalCost(principal, interestRate, instalments, body) - principal;
}

function annualPercentageRateOfCharge(totalCost, principal, instalments) {
  return (( (totalCost / principal) - 1 ) / (instalments / 12));
}