IntermediateBEHAVIORAL
Describe a situation where you inherited a legacy codebase or module that was hard to understand or extend. What specific steps did you take to improve its readability, test coverage, and overall design without breaking existing functionality?
Software Engineer
General

Sample Answer

At my previous company, I inherited a 6‑year‑old billing module—about 12k lines in a single Rails model with almost no tests and a bug rate of 8–10 support tickets per week. Before touching behavior, I added characterization tests around the public methods using real production scenarios pulled from anonymized logs. That got us from essentially 0 to ~55% coverage and gave us a safety net. Then I started incremental refactors: extracted small service objects for tax calculation and proration, added clear interfaces, and documented edge cases in code comments and ADRs. Each change was behind feature flags and shipped in small PRs under 300 lines, reviewed by at least one other engineer. Over about two months, we doubled coverage to ~80%, cut billing-related tickets by ~60%, and on‑call time for the team dropped noticeably. The code is now understandable enough that new hires ramp on it within a week.

Keywords

Started with characterization tests to lock in existing behaviorRefactored incrementally with small, reviewed PRs and feature flagsIntroduced clear boundaries (service objects/interfaces) and documentationMeasured impact via test coverage and reduced support tickets