leave-money-invested-and-pay-installments.js

/**
 * @param {number} num
 * @returns {string}
 */
function formatMoney(num) {
  return `R$ ${num
    .toFixed(2)
    .replace('.', ',')
    .replace(/(\d)(?=(\d\d\d)+(?!\d))/g, '$1.')}`;
}

function formatInteresPercentage(num) {
  return (num * 100).toFixed(2) + '%';
}

function deductIR(investimentReturn, totalDaysInvested) {
  if (totalDaysInvested < 0) {
    return investimentReturn;
  }

  const [_, __, interestPercentage] = [
    [0, 180, 0.225],
    [181, 360, 0.2],
    [361, 720, 0.175],
    [721, Infinity, 0.15],
  ].find(([min, max]) => min <= totalDaysInvested && totalDaysInvested <= max);

  return investimentReturn * (1 - interestPercentage);
}

function leaveMoneyInvestedAndPayInstallments(
  totalAmount,
  totalInstallments,
  annualInterestPercentage,
) {
  const installmentAmount = totalAmount / totalInstallments;
  let currentTotalAmount = totalAmount;

  console.log(
    `Investing ${formatMoney(totalAmount)} for ${
      totalInstallments - 1
    } months at ${formatInteresPercentage(annualInterestPercentage)}/year`,
  );

  for (
    let currentInstallmentNumber = 1;
    currentInstallmentNumber <= totalInstallments;
    currentInstallmentNumber++
  ) {
    const monthlyInterestPercentage = Math.pow(annualInterestPercentage + 1, 1 / 12) - 1;
    const currentInvestimentReturn = currentTotalAmount * monthlyInterestPercentage;
    const actualCurrentInvestimentReturn = deductIR(
      currentInvestimentReturn,
      currentInstallmentNumber * 30,
    );
    let text = `\n\n${currentInstallmentNumber}/${totalInstallments}\n${formatMoney(
      currentTotalAmount,
    )} - ${formatMoney(installmentAmount)} + ${formatMoney(
      actualCurrentInvestimentReturn,
    )} (${formatInteresPercentage(monthlyInterestPercentage)}/month)`;
    currentTotalAmount -= installmentAmount;
    currentTotalAmount += actualCurrentInvestimentReturn;
    text += ` = ${formatMoney(currentTotalAmount)}`;
    console.log(text);
  }

  return currentTotalAmount;
}

/**
 * console.log(formatMoney(leaveMoneyInvestedAndPayInstallments(10000.0, 10, 0.105)));
 *
 * 1/10
 * R$ 10.000,00 - R$ 1.000,00 + R$ 64,75 (0.84%/month) = R$ 9.064,75
 *
 *
 * 2/10
 * R$ 9.064,75 - R$ 1.000,00 + R$ 58,70 (0.84%/month) = R$ 8.123,45
 *
 *
 * 3/10
 * R$ 8.123,45 - R$ 1.000,00 + R$ 52,60 (0.84%/month) = R$ 7.176,05
 *
 *
 * 4/10
 * R$ 7.176,05 - R$ 1.000,00 + R$ 46,47 (0.84%/month) = R$ 6.222,52
 *
 *
 * 5/10
 * R$ 6.222,52 - R$ 1.000,00 + R$ 40,29 (0.84%/month) = R$ 5.262,81
 *
 *
 * 6/10
 * R$ 5.262,81 - R$ 1.000,00 + R$ 34,08 (0.84%/month) = R$ 4.296,89
 *
 *
 * 7/10
 * R$ 4.296,89 - R$ 1.000,00 + R$ 28,72 (0.84%/month) = R$ 3.325,61
 *
 *
 * 8/10
 * R$ 3.325,61 - R$ 1.000,00 + R$ 22,23 (0.84%/month) = R$ 2.347,84
 *
 *
 * 9/10
 * R$ 2.347,84 - R$ 1.000,00 + R$ 15,69 (0.84%/month) = R$ 1.363,53
 *
 *
 * 10/10
 * R$ 1.363,53 - R$ 1.000,00 + R$ 9,11 (0.84%/month) = R$ 372,64
 * R$ 372,64
 */