Mortgage Calculator — JavaScript source

/**
 * mortgage-calculator - amortized monthly payment.
 *
 * Display snippet: ports the core monthlyPayment() formula from the
 * TypeScript lib (src/lib/mortgage.ts). The full amortization schedule
 * (integer-cent tracking, extra payments, payoff dates) lives in the
 * TypeScript/Go sources.
 *
 * Standard amortization formula: P*r*(1+r)^n / ((1+r)^n - 1) with
 * r = annualRatePct/100/12 and n = years*12. A zero rate divides the
 * principal evenly. Returns the payment rounded to cents, or null on
 * invalid input.
 */

function monthlyPayment(principal, ratePct, years) {
  if (!Number.isFinite(principal) || !Number.isFinite(ratePct) || !Number.isFinite(years)) {
    return null;
  }
  if (principal <= 0 || years <= 0 || !Number.isInteger(years)) return null;

  const r = ratePct / 100 / 12;
  const n = years * 12;
  const payment =
    r === 0
      ? principal / n
      : (principal * r * Math.pow(1 + r, n)) / (Math.pow(1 + r, n) - 1);
  return Math.round((payment + Number.EPSILON) * 100) / 100;
}

// Example:
//   monthlyPayment(300000, 6, 30) -> 1798.65