How to Build a Loan Calculator

function calculateLoanPayment(principal, annualRate, months) {
  const monthlyRate = annualRate / 100 / 12;

  if (monthlyRate === 0) return principal / months;

  return principal *
    (monthlyRate * Math.pow(1 + monthlyRate, months)) /
    (Math.pow(1 + monthlyRate, months) - 1);
}

function calculateTotalInterest(principal, monthlyPayment, months) {
  return (monthlyPayment * months) - principal;
}

function calculateAPR(principal, fees, annualRate, months) {
  const effectivePrincipal = principal + fees;
  const payment = calculateLoanPayment(effectivePrincipal, annualRate, months);
  return annualRate * (effectivePrincipal / principal);
}