The author advocates for designing compounding functions, which are small, pure, and composable units of code, to create robust and maintainable software. Purity is foundational; a pure function consistently returns the same output for the same input without side effects, making it trustworthy and easily testable. Functions should be small and single-purpose, breaking down complex tasks into manageable units, which improves clarity and reusability. Composition becomes natural when chaining these small, pure functions, often using pipelines to create a clear flow of operations.Designing functions with data flow in mind ensures they accept precisely what they need, promoting greater composability. Currying allows for partial application, creating specialized functions from generic ones, thereby enhancing reusability. A significant benefit of compounding functions is simplified testing, as pure and small functions require minimal setup and no mocking. While aiming for composability, it's crucial to avoid over-abstraction, ensuring functions provide clear conceptual value rather than merely wrapping other calls.Refactoring toward compounding involves incrementally converting impure functions to pure ones, extracting logical blocks, and recomposing them with pipelines, guided by tests. This discipline offers substantial benefits across development, including improved readability, maintainability, and debugging. When functions are designed as reliable building blocks, new features feel like assembly rather than starting from scratch. Starting small, by refactoring one complex function, demonstrates the surprising and growing positive impact of good design.