I built a personal finance brand from scratch.

function buildYearlyData(initial, monthly, rate, years) {
  const r = rate / 100 / 12;
  const data = [];
  for (let y = 1; y <= years; y++) {
    const months = y * 12;
    const futureInitial = initial * Math.pow(1 + r, months);
    const futureMonthly = r > 0
      ? monthly * ((Math.pow(1 + r, months) - 1) / r)
      : monthly * months;
    const total    = futureInitial + futureMonthly;
    const invested = initial + monthly * months;
    const gains    = Math.max(0, total - invested);
    data.push({ year: y, invested: Math.round(invested), gains: Math.round(gains), total: Math.round(total) });
  }
  return data;
}