How to Build a Pay Off Calculator

function simulatePayoff(debts, extraPayment, method = 'avalanche') {
  // Deep copy and sort by strategy
  let sorted = debts.map(d => ({ ...d, remaining: d.balance }));
  if (method === 'avalanche') {
    sorted.sort((a, b) => b.apr - a.apr);
  } else {
    sorted.sort((a, b) => a.balance - b.balance);
  }

  let month = 0;
  let totalInterest = 0;
  const timeline = [];

  while (sorted.some(d => d.remaining > 0)) {
    month++;
    let extra = extraPayment;

    for (const debt of sorted) {
      if (debt.remaining <= 0) continue;

      const monthlyRate = debt.apr / 100 / 12;
      const interest = debt.remaining * monthlyRate;
      totalInterest += interest;

      let payment = debt.minPayment + (sorted.indexOf(debt) === sorted.findIndex(d => d.remaining > 0) ? extra : 0);
      payment = Math.min(payment, debt.remaining + interest);

      debt.remaining = debt.remaining + interest - payment;
      if (debt.remaining < 0.01) debt.remaining = 0;
    }

    timeline.push({
      month,
      balances: sorted.map(d => ({ name: d.name, remaining: Math.round(d.remaining * 100) / 100 })),
      totalRemaining: sorted.reduce((sum, d) => sum + d.remaining, 0)
    });

    if (month > 600) break; // safety limit: 50 years
  }

  return {
    method,
    months: month,
    years: (month / 12).toFixed(1),
    totalInterest: Math.round(totalInterest * 100) / 100,
    totalPaid: Math.round((debts.reduce((s, d) => s + d.balance, 0) + totalInterest) * 100) / 100,
    timeline
  };
  }

  function compareStrategies(debts, extraPayment) {
  const avalanche = simulatePayoff(debts, extraPayment, 'avalanche');
  const snowball = simulatePayoff(debts, extraPayment, 'snowball');

  return {
    avalanche,
    snowball,
    interestSaved: Math.round((snowball.totalInterest - avalanche.totalInterest) * 100) / 100,
    monthsDifference: snowball.months - avalanche.months
  };
  }