Code for calculating interest on a recurring deposit

recurring_deposit.js

// For now only works for number of months that are multiples of 3
// Compunding quarterly (every 3 months)

function rd(monthly_contribution, interest_rate, duration_years) {
  const duration_months = Math.floor(duration_years * 12);
  const months_compounding = 3;

  let total = 0;
  const quarterly_contribution =
    monthly_contribution * (1 + (1 * interest_rate) / 1200) +
    monthly_contribution * (1 + (2 * interest_rate) / 1200) +
    monthly_contribution * (1 + (3 * interest_rate) / 1200);
  
  for (let i = 1; i <= duration_months / 3; i++) {
    total =
      total +
      quarterly_contribution *
        (1 + interest_rate / 400) ** Math.floor(duration_months / 3 - i);
  
  return {
    total,
    interest_earned:
      total - monthly_contribution * Math.floor(duration_years * 12),
  };
}

console.log(JSON.stringify(rd(1000, 4.9, 1)));
console.log(JSON.stringify(rd(1000, 5, 2)));
console.log(JSON.stringify(rd(1000, 5.35, 5)));
console.log(JSON.stringify(rd(1000, 5.5, 10)));