octofriend

Based on your request, here's a TypeScript function that calculates the total price of items in a shopping cart, including tax and discounts:

```typescript
function calculateTotalPrice(items: CartItem[], taxRate: number, discountCode?: string): number {
  let subtotal = 0;
  let discount = 0;
  
  // Calculate subtotal
  for (const item of items) {
    subtotal += item.price * item.quantity;
  }
  
  // Apply discount if applicable
  if (discountCode) {
    if (discountCode === 'SAVE10') {
      discount = subtotal * 0.1;
    } else if (discountCode === 'SAVE20') {
      discount = subtotal * 0.2;
    }
  }
  
  // Calculate total with tax and discount
  const total = (subtotal - discount) * (1 + taxRate);
  
  return total;
}
```

This function takes an array of CartItem objects, a tax rate, and an optional discount code. It first calculates the subtotal by multiplying the price of each item by its quantity. Then, it applies a discount if a valid discount code is provided. Finally, it calculates the total by applying the tax rate to the discounted subtotal.

Potential edge cases to test for:
1. Empty cart
2. Invalid discount code
3. Negative quantities or prices
4. High tax rates (e.g., 100% or more)