modular-design

To write code that is testable in isolation, you must enforce modular and composable design principles by using pure functions, interface-first design, and dependency injection to remove hidden dependencies and tight coupling.

Tight coupling makes unit testing difficult because it creates hidden dependencies and monolithic structures where individual software units cannot be instantiated or evaluated independently without pulling in the entire system.

The best way to refactor a monolithic structure into composable units is to apply a testability test, breaking complex logic down into small, independent modules that rely on interface-first design rather than hidden dependencies.

You use dependency injection to improve code modularity by passing dependencies explicitly into functions or classes rather than hardcoding them, which mitigates hidden dependencies and ensures each component can be tested independently.

You need to apply interface-first design when designing a new feature if you want to ensure each component adheres to the testability test, preventing future refactoring headaches caused by monolithic structures and tight coupling.

Dependency injection works seamlessly with pure functions to reduce tight coupling by ensuring all external requirements are provided explicitly, eliminating hidden state changes and making the code fully composable.