We can use classes and interfaces together by defining the contract in an interface and then implementing the class as per the interface. We specify that a class is implementing a particular interface using the implements keyword.

As an example, we can define an interface for the order detail and then a class that implements this interface:

interface IOrderDetail {
  product: Product;
  quantity: number;
  getTotal(discount: number): number;
}

class OrderDetail implements IOrderDetail {
  product: Product;
  quantity: number;

  getTotal(discount: number): number {
    const priceWithoutDiscount = this.product.unitPrice *  
     this.quantity;
    const discountAmount = priceWithoutDiscount * discount;
    return priceWithoutDiscount - discountAmount;
  }
}

In the preceding example, we've prefixed the interface with I so that readers of the code can quickly see when we are referencing interfaces.

Why would we use this approach? It seems like more code than we need to write. So, what's the benefit? This approach allows us to have multiple implementations of an interface, which can be useful in certain situations.