How Replacing Global State with Explicit Dependencies Rescues Legacy Codebases

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A software developer recounts inheriting a sprawling legacy codebase built with mixed PHP, JavaScript, and Bash, where pervasive global singletons made even minor changes unpredictable and dangerous. Critical objects like a Config singleton and a static Logger were accessed directly throughout the code, causing hidden side effects that were nearly impossible to trace or test in isolation. The turning point came after reading Michael Feathers' 'Working Effectively with Legacy Code', which introduced the concept of seams — points where behavior can be changed or tested without altering existing code. The developer adopted the practice of passing dependencies explicitly as parameters rather than relying on globals, which improved code transparency, testability, and safety. This architectural shift allowed unit tests to use mock objects without bootstrapping the full application, dramatically reducing debugging time and making the codebase easier to maintain.

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