This has become my favorite subject because it turns out that testability, above nearly every other metric, is important to strive for in developing software. And all other code qualities seem to be a reflection of testability.
5 Easy Ways to Write Hard-to-Test Code
When I say testable code, what I mean is code that can be verified programmatically and in a very granular way. When a test fails, we want it to tell us exactly why so that we know what went wrong and can correct the problem immediately. This means that we're verifying very small units of behavior. This not only helps us debug code when there are problems, but it also helps keep us focused on building software that fulfills some desired behavior.