Some code is easier to test than other code. Ideal code to test has a well-defined interface, doesn’t have much state or other “setup,” and doesn’t have much hidden data to inspect.

If you write your code knowing you’ll be writing a test for it later, a funny thing happens: you start designing your code so that it’s easy to test! Fortunately, coding this way also means that you create better code in general. Test-friendly designs often lead naturally to well-organized code, with separate parts to do separate things.