Refactoring Legacy Code: The Jedi Move of Extracting Pure Functions

Short summary

Legacy code often tangles side effects with core logic, making it brittle and hard to test. Extract pure functions—small, deterministic pieces that depend only on their inputs—to isolate calculations from database calls, file I/O, and state mutations. This refactoring improves testability, code reasoning, and composability while quarantining side effects at the system edges.

• •Isolate pure functions from side effects for dramatically better testability
• •Refactor legacy monsters by extracting calculation logic into small, deterministic helpers
• •Pure functions composed together eliminate coupling and make requirement changes safer

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