JavaScript: Round to 2 Decimal Places

/**
 * Round to any number of decimal places
 * @param {number} num - Number to round
 * @param {number} decimals - Number of decimal places (default: 2)
 * @returns {number} Rounded number
 */
function roundToDecimals(num, decimals = 2) {
  const factor = Math.pow(10, decimals);
  return Math.round(num * factor) / factor;
}

// Examples
console.log(roundToDecimals(12.345));        // 12.35
console.log(roundToDecimals(12.345, 1));     // 12.3
console.log(roundToDecimals(12.345, 3));     // 12.345
console.log(roundToDecimals(12.345, 0));     // 12

/**
 * Round and format number as string with 2 decimals
 * @param {number} num - Number to format
 * @returns {string} Formatted string
 */
function formatTo2Decimals(num) {
  return (Math.round(num * 100) / 100).toFixed(2);
}

// Examples
console.log(formatTo2Decimals(12.3));        // "12.30"
console.log(formatTo2Decimals(12.345));      // "12.35"
console.log(formatTo2Decimals(0.1 + 0.2));   // "0.30"

/**
 * Round currency value safely
 * @param {number} amount - Amount to round
 * @returns {number} Rounded amount (cents precision)
 */
function roundCurrency(amount) {
  // Handle potential floating point issues
  return Math.round((amount + Number.EPSILON) * 100) / 100;
}

// Examples
console.log(roundCurrency(12.345));          // 12.35
console.log(roundCurrency(0.1 + 0.2));       // 0.3
console.log(roundCurrency(2.555));           // 2.56

// Product pricing component
function formatPrice(price) {
  const formatter = new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency: 'USD'
  });
  return formatter.format(price);
}

// Cart total calculation
function calculateTotal(items) {
  const subtotal = items.reduce((sum, item) => {
    return sum + (item.price * item.quantity);
  }, 0);

  const tax = subtotal * 0.08;
  const total = subtotal + tax;

  // Round to 2 decimals for accurate total
  return Math.round(total * 100) / 100;
}

// Example usage
const cart = [
  { price: 12.99, quantity: 2 },
  { price: 7.50, quantity: 1 }
];

const total = calculateTotal(cart);
console.log(formatPrice(total));  // "$35.93"

// Calculate and display percentage
function formatPercentage(value, total) {
  const percentage = (value / total) * 100;
  const rounded = Math.round(percentage * 100) / 100;
  return rounded.toFixed(2) + '%';
}

// Examples
console.log(formatPercentage(45, 200));    // "22.50%"
console.log(formatPercentage(1, 3));       // "33.33%"

// Progress bar
function getProgress(completed, total) {
  const percent = (completed / total) * 100;
  return Math.round(percent * 100) / 100;
}

console.log(getProgress(7, 10));           // 70

// Interest calculation
function calculateInterest(principal, rate, years) {
  const interest = principal * rate * years;
  return Math.round(interest * 100) / 100;
}

// Loan payment calculator
function calculateMonthlyPayment(principal, annualRate, years) {
  const monthlyRate = annualRate / 12;
  const numPayments = years * 12;

  const payment = principal *
    (monthlyRate * Math.pow(1 + monthlyRate, numPayments)) /
    (Math.pow(1 + monthlyRate, numPayments) - 1);

  return Math.round(payment * 100) / 100;
}

// Example
const payment = calculateMonthlyPayment(200000, 0.045, 30);
console.log(`Monthly payment: $${payment.toFixed(2)}`);
// "Monthly payment: $1013.37"

// TypeScript utility
const formatCurrency = (amount: number): string => {
  return new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency: 'USD'
  }).format(amount);
};

// React component
interface PriceProps {
  amount: number;
}

const Price: React.FC<PriceProps> = ({ amount }) => {
  const rounded = Math.round(amount * 100) / 100;
  return <span className="price">{formatCurrency(rounded)}</span>;
};

// Usage
<Price amount={12.345} />  // Displays: $12.35