DEPENDENCY INJECTION

What's your opinion on DI? is it just enterprise overengineering shit or it actually makes your code better and testable?

In a nutshell, types™ are simple functions, so if function A calls function B, then A is is by definition a subtype of B. When function calls several other functions, we have multi-inheritance.

So if you want to change the way function behaves, just wrap it inside of another function.