/**
 * Format a number as a localized currency string.
 * @param {number} amount - The value to format
 * @param {string} currency - ISO 4217 currency code
 */
function formatCurrency(amount, currency = 'USD') {
  return new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency,
    minimumFractionDigits: 2,
  }).format(amount);
}

// Usage
formatCurrency(4299);  // → "$4,299.00"