TypeScript
Total calculation
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 value
Exercise 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.
Show example 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));
Test yourself
-
How is the total price of a CartItem computed?
- product.price * quantity
- product.price + quantity
- product.price
- quantity
Show answer
product.price * quantity — The total price of an item is the product price times the quantity: product.price * quantity. Addition would be wrong.
-
Complete the multiplication operator
const itemTotal = i.product.price ____ i.quantity;
Show answer
const itemTotal = i.product.price * i.quantity;
-
Total computation parts
- price * quantity
- Item total price
- reduce((sum, i) => ..., 0)
- Sum all items
- 0
- Initial value
- sum + itemTotal
- Accumulator update
Next lesson: Apply discount
