I'm absolutely terrified of refactoring in big python, ruby, or javascript codebases, because there's less chance that I'll be able to find all the side effects

I'm absolutely terrified of refactoring in big python, ruby, or javascript codebases, because there's less chance that I'll be able to find all the side effects of changing something. Tests help, but maintaining high-enough test coverage over possible code paths can be harder and less useful than working with a type system and writing fewer tests.