Testing an application is one of the main concerns for programmers. It does not matter what paradigms, languages, or frameworks you are using to develop your program, you must always be careful with your tests.
A good test suite ensures better quality for your code and for your application. It can also be used by other programmers to help them read and understand your code, and it helps you decouple and improve the architecture for your program.
This book is not about tests, so I will not try to evangelize them but I advise you to always test your code, and it's vital. I will give you some advice and show how functional reactive programming can help you to decouple your code, increasing its testability.
Before jumping ...