# Small Functions

Functions should be small and do one thing well.

## The Rule

- Functions should be small (ideally < 20 lines)
- They should do one thing
- They should do it well
- They should do it only

## Bad Example

```javascript
function processOrder(order) {
  // Validate
  if (!order.items || order.items.length === 0) {
    throw new Error("No items");
  }
  
  // Calculate total
  let total = 0;
  for (const item of order.items) {
    total += item.price * item.quantity;
  }
  
  // Apply discount
  if (order.coupon) {
    total *= 0.9;
  }
  
  // Save to database
  db.orders.insert(order);
  
  // Send email
  sendEmail(order.customer.email, "Order confirmed");
  
  return total;
}
```

## Good Example

```javascript
function processOrder(order) {
  validateOrder(order);
  const total = calculateOrderTotal(order);
  saveOrder(order);
  notifyCustomer(order);
  return total;
}

function validateOrder(order) {
  if (!order.items?.length) {
    throw new Error("No items");
  }
}

function calculateOrderTotal(order) {
  const subtotal = calculateSubtotal(order.items);
  return applyDiscount(subtotal, order.coupon);
}

function calculateSubtotal(items) {
  return items.reduce((sum, item) => 
    sum + item.price * item.quantity, 0
  );
}
```

## Benefits

- Easier to understand
- Easier to test
- Easier to reuse
- Easier to modify
- Self-documenting code