T for testable

Even if you don't follow the test-driven development process, testable user stories will be useful. This is because if you don't know how to test something, it usually means that you don't understand it well enough. This also enforces the smallness, as smaller user stories are easier to test.

You can think about testability as what would make you accept that user story as done? How do you know it's working as intended? For example, we know that the shopping cart view works if it shows the contents of the shopping cart, including the unit prices, total amount, weight, and shipping costs. Moreover, it enables us to remove and change the quantity of the items, it leads to the checkout through a highly visible call to action button, while it also allows us to get back to our previous view. As you can see, this can lead to rather complex criteria. Thinking about that will be helpful when we create the Wireflows in Chapter 4, Wireflows - Plan Your Product. For now, testability helps to check whether we have a shared understanding or whether we just think we do.