Which of the following is a core clean code principle?

• A. Small cohesive functions.
• B. Long, multi-purpose functions.
• C. Deep nesting and side effects.
• D. Global state usage.

Answer: (A)

Small, cohesive functions are a hallmark of clean code because they keep responsibilities clear and narrow. When a function does one thing well, it’s easier to read, reason about, and name, which makes the overall program flow understandable. This clarity also makes testing simpler—you can verify the exact behavior of a tiny piece of logic without worrying about a tangle of side effects or unrelated tasks. By keeping functions short and focused, you reduce duplication, improve reusability, and make maintenance changes safer since the impact is isolated to a small unit.

Long, multi-purpose functions blur responsibilities and increase cognitive load; readers have to track many tasks at once, which invites mistakes. Deep nesting and side effects complicate control flow and make bugs harder to trace. Relying on global state introduces hidden dependencies and coupling that can break tests and lead to unpredictable behavior. Keeping constraints tight around small, well-named functions avoids these pitfalls and aligns with clean code practices.