How to Build a Long Term Calculator

function calculateCompoundInterest(principal, annualRate, years, compoundsPerYear = 12, monthlyContribution = 0) {
  const r = annualRate / 100;
  const n = compoundsPerYear;
  const t = years;

  // Future value of lump sum
  const fvPrincipal = principal * Math.pow(1 + r / n, n * t);

  // Future value of contribution series
  const fvContributions = monthlyContribution > 0
    ? monthlyContribution * ((Math.pow(1 + r / n, n * t) - 1) / (r / n))
    : 0;

  const futureValue = fvPrincipal + fvContributions;
  const totalContributed = principal + (monthlyContribution * n * t);
  const totalInterest = futureValue - totalContributed;

  return {
    futureValue: Math.round(futureValue * 100) / 100,
    totalContributed: Math.round(totalContributed * 100) / 100,
    totalInterest: Math.round(totalInterest * 100) / 100,
    effectiveReturn: ((futureValue / totalContributed - 1) * 100).toFixed(2)
  };
  }

  function generateYearlyBreakdown(principal, annualRate, years, compoundsPerYear, monthlyContribution) {
  const breakdown = [];
  for (let year = 1; year <= years; year++) {
    const result = calculateCompoundInterest(principal, annualRate, year, compoundsPerYear, monthlyContribution);
    breakdown.push({ year, ...result });
  }
  return breakdown;
  }