We might want to make a property optional because not every situation where the interface is implemented requires it. Let's take the following steps in our OrderDetail interface:

1. Let's create an optional property for the date it was added. We specify an optional value by putting a ? at the end of the property name but before the type annotation:

interface OrderDetail {
  product: Product;
  quantity: number;
  dateAdded?: Date,
  getTotal(discount: number): number;
}

We'll see that our implementation of this interface, tableOrder, isn't broken. We can choose to add dateAdded to tableOrder but it isn't required.

2. We might also want to make a method parameter optional. We do this in a similar way by putting a ? after the parameter name. In our example, let's make discount optional in the OrderDetail interface:

interface OrderDetail {
  product: Product;
  quantity: number;
  dateAdded?: Date,
  getTotal(discount?: number): number;
}

3. We can change the method implementation signature as well:

getTotal(discount?: number): number {
  const priceWithoutDiscount = this.product.unitPrice * this.quantity;
  const discountAmount = priceWithoutDiscount * (discount || 0);
  return priceWithoutDiscount - discountAmount;
}

We've also dealt with the case when a discount isn't passed into the method by using (discount || 0) in the discountAmount variable assignment.

4. With our optional parameter in place, we can call getTotal without passing a value for the discount parameter:

tableOrder.getTotal()

The preceding line doesn't upset the TypeScript compiler.