Legacy Code Refactoring
Safely transform tangled legacy code by adding tests first and refactoring in small steps.
The Strangler Fig Pattern
Rather than rewriting legacy code all at once, grow new clean code around it like a strangler fig grows around a tree — until the old code can be safely removed. Incremental replacement keeps the system running throughout the transformation.
Tests Are Your Safety Net
Before touching legacy code, write characterization tests. These tests don't describe what the code should do — they describe what it currently does, preserving behavior during refactoring. If a test breaks after your change, you accidentally changed behavior.
Code Challenge
Extract testable units from the monolithic function.
💡Key takeaway
Refactor in small, verifiable steps. Each step should leave the code better and all tests green.
🔧 Some exercises may still have errors. If something seems wrong, use the Feedback button (bottom-right of the page) to report it — it helps us fix it fast.
Hint: The Golden Rule of Legacy Refactoring: never refactor without a test net beneath you.
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