What is the value of test driven development?

Is the value writing tests at the same time as you write the code? Sure, I like that property. It means that at any time you’re one control-Z away from your tests passing; either revert your test change, or fix the code so the test pass. The nice property of this method is once you’ve implemented your feature, by definition, it’s already tested. Push that branch and lean in for the code review.

Another important property of TDD is it forces you to think about writing code that is testable, as a first class citizen. You don’t add testing after the fact, in the same way you don’t add performance or security after the code is “done” — right?

But for me, the most important property of TDD is it forces you to write your tests as a consumer of your own code, making you think about its API, continuously.

Many times people have said to me that they like the idea of TDD in principle, but have found they felt slower when they tried it. I understand completely. TDD does slow you down if you don’t have a design to work from. TDD doesn’t relieve you of the responsibility of designing your code first.

How much design you do is really up to you, but if you find yourself in the situation where you find TDD is slowing you down because you’re fighting the double whammy of changing the code and the tests at the same time, that’s a sure fire sign that you’ve run off the edge of your design map.

Robert Martin says you should not write a line of production code without a failing unit test–the key word is production code. It’s 100% OK to skip writing tests you’re exploring the design space, just remember to budget time to rewrite this code in a TDD fashion. The good news is it won’t take you very long, you’ve already designed the code, and built one to throw away.