The Smallest Safe Change: Refactor a Messy Function One Step at a Time
The biggest refactor failures come from big steps. A 200-line function turns into a rewrite. The rewrite ships with a regression. The fix is to make the smallest safe change. Lock behavior first, then move one logical block at a time. This post shows that workflow on a real messy function. You'll see how characterization tests and mutation testing verify every step. You'll also learn where a free…
The shortest safe route to refactoring a convoluted function involves making incremental changes. A 200-line pricing function in a JavaScript storefront mixes line totals, coupon calculations, and rounding, making it a prime candidate for a gradual refactor. The key to success lies in characterizing the existing behavior before making any modifications.
Characterization tests capture the function's current output for various inputs, including cases with no coupon, a percentage-based coupon, a fixed coupon value, and a negative total. These tests verify the function's behavior without prescribing what it should do, relying instead on observed results. Once the characterization tests are in place, they can be used to refactor the function without fear of breaking existing functionality.
To ensure the tests' reliability, mutation testing is employed. Mutation testing introduces small changes to the source code, such as swapping - for + or deleting lines, and checks whether the tests still pass. If the tests fail, it indicates that they may not have adequately covered the function's behavior, revealing gaps that need to be addressed by adding more tests.
Running mutation testing locally helps identify weak tests with high mutation scores. To alleviate the computational burden during mutation testing, a remote runner can be utilized. Some platforms offer free model access and server options for running mutation tests remotely, without requiring additional dependencies. After establishing a solid test suite and confirming its effectiveness through mutation testing, the actual refactoring can begin.
The focus should be on making small, incremental changes, extracting one logical block at a time, such as moving the coupon logic into a separate function. By adhering to this step-by-step approach, the function can be gradually refined, minimizing the risk of introducing regressions and ensuring that the core behavior remains intact throughout the process.
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