My 2 cents, based on personal experience:
- With TDD, I’m a lot more confident about my solutions. If I spot a design improvement, I can quickly jump in and fix it with confidence. When given feedback, I’m able to respond to it quickly. I feel I’m in control.
- It really helps me keep my design and code minimalistic and clean. No bells and whistles. No buy one get one free combo offers. <Perfection in design is achieved not when there is nothing more to add, but rather when there is nothing more to take away>
- Turns out that my code is lot more easier to test. Because its designed to be testable. Lots of people argue that they will write tests after writing the code and it would be more efficient. The biggest problem I find with this approach is that the code ends up being something not readily testable. Then either I’ve to spend time making it testable or I skip testing saying its not possible or not required.
- It helps me build a safety net of executable, living, up-to-date specification for my classes and features. Its a great starting point for new team members to understand my software. Tests are a great reference point for someone who wants to use my code.
- TDD is a great teacher. It helps me listen to my code and learn from it. It forces me to pay close attention to what is happening. Its easy to spot bad things faster. Its all about feedback and visibility.
- TDD forces me to slow down and think. It encourages me to take baby-steps. Sometimes I find people are in such a hurry that they spill mess all over the place. It takes soo much more time and effort to clean up the mess they leave behind.
- My tests tend to communicate my design choices much longer after I’m gone.
- I massively reduce the amount of time I spend in the debugger or trying to manually test (monkey test) from the UI. When something breaks, I no longer need to crawl through the logs to figure out where things are going wrong. I get pin-pointed feedback.
- TDD helps me maintain focus on measurable outcome (producing software that accomplishes a concrete objective). I’m not longer drifting down ratholes.
- TDD also helps me reduce the hand-overs between developers and tests and hence the wastage introduced because of all that overhead and context switching.
- And so on…
Having said that, TDD alone is not sufficient to achieve the above. At times you need to spike/prototype things. One needs to have (or at least start focusing on building) a good knowledge of Design Principles and Patterns. Its easy to get lost, having a pair can really help.
Again I don’t want to sound dogmatic. I don’t think TDD is the only way to build great software. There are lots of great developers out there, building amazing software without TDD. However I think TDD is a much easier approach to achieve the same.