IntermediateSITUATIONAL
You’re tasked with refactoring a legacy part of your existing full-stack application that mixes business logic in the UI, API, and database layer. How would you approach decomposing and reorganizing this code while minimizing risk to ongoing feature work?
Full Stack Developer
General

Sample Answer

I’d start by making the change safe before making it pretty. First step is mapping what actually exists: sequence diagrams of the main flows, identify shared procedures, and add missing tests around critical paths. On my last team, we brought a legacy module from ~20% to 70% test coverage before any major refactor, which gave us a safety net. Then I’d define a target architecture: clear domain layer in the API, thin UI, and simple data layer. I like the strangler-fig approach: introduce a new service or module, route a small, well-understood slice of functionality through it, and slowly expand. During this, I’d add feature flags so new and old paths can coexist. We did this over 6–8 weeks while still shipping features, and defect rates in that area dropped by about 60% while onboarding time for new devs shrank from weeks to a few days.

Keywords

Map current behavior and add tests around critical flows before refactoringDefine a clear target architecture with separation of concerns across layersUse a strangler-fig pattern and feature flags to migrate behavior incrementallyRefactor in small, low-risk slices while allowing ongoing feature development
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