Total calculation
- TypeScript course
- Beginner
- Lesson 73/89
- 5 min
Learn to calculate the cart total.
Cart total
The total of a cart is the sum of product.price * quantity for every CartItem. You can compute it in a single pass with reduce. The initial value should be 0.
function total(cart: Cart): number {
return cart.items.reduce((sum, i) => sum + i.product.price * i.quantity, 0);
}How reduce works
reduce collapses an array into a single value. The first parameter takes an accumulator and an item; the second is the initial value. Here we start at 0 and add each item's price to the accumulator.
const sum = cart.items.reduce((acc, i) => acc + i.product.price * i.quantity, 0);
// acc: previous sum, i: CartItem, 0: initial valuePractice Write the total function
Define Product, CartItem, Cart interfaces. Write a function total(cart: Cart): number. For each item compute i.product.price * i.quantity and sum them all with reduce. The initial value should be 0. Test with a cart of two items and print the result.
Starter code
// Product, CartItem, Cart arayüzlerini tanımla
// total(cart: Cart): number yaz
// her öğenin fiyatını miktarıyla çarp ve topla
// iki öğeli bir sepetle test et, sonucu yazdırSample solution
interface Product { id: string; name: string; price: number; }
interface CartItem { product: Product; quantity: number; }
interface Cart { items: CartItem[]; }
function total(cart: Cart): number {
return cart.items.reduce((sum, i) => sum + i.product.price * i.quantity, 0);
}
const cart: Cart = {
items: [
{ product: { id: "a", name: "Elma", price: 5 }, quantity: 2 },
{ product: { id: "b", name: "Armut", price: 3 }, quantity: 4 }
]
};
console.log(total(cart));Expected output
22
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How is the total price of a CartItem computed?
- product.price * quantity Correct answer
- product.price + quantity
- product.price
- quantity
Explanation: The total price of an item is the product price times the quantity: product.price * quantity. Addition would be wrong.
2 Complete the multiplication operator
const itemTotal = i.product.price ____ i.quantity;const itemTotal = i.product.price * i.quantity;Explanation: The item total is price times quantity: i.product.price * i.quantity. +, /, and - perform the wrong operation.
3 Total computation parts
| Term | Meaning |
|---|---|
price * quantity |
Item total price |
reduce((sum, i) => ..., 0) |
Sum all items |
0 |
Initial value |
sum + itemTotal |
Accumulator update |