Walk Me Through Your First Change in a Large Codebase You Didn't Write
Behavioral and LeadershipLearning and GrowthAsked atJuniorMidUpdated
“Walk me through the first time you had to make a change in a large codebase you didn't write.”
They're checking whether you read before you write. Strong answers show how you found your way around, kept the change small, and proved it worked.
Rubric by Level
Junior
- Explains how you oriented: reading tests, tracing one request, or finding a similar past PR.
- Kept the change small and close to existing conventions.
- Ran the tests or the app locally before opening the PR.
- Names one thing that confused you at first and what it turned out to be.
Mid
- Found the owners of the code and checked your plan with them before writing much.
- Matched existing patterns and called out any spot where you chose to differ.
- Added a test that pins the behavior you changed.
- Left the area a little clearer, like a comment or a doc fix.
Senior
- Mapped the blast radius of the change: callers, data, and deploy order.
- Judged which conventions were worth keeping and which were debt, and said so in review.
- Shipped behind a flag or in steps so it could roll back cleanly.
- Turned what you learned into onboarding material for the next engineer.
Principal
- Uses the story to show how you assess an unfamiliar system's health quickly.
- Connects what you found to an investment decision, like a rewrite, an ownership change, or a platform.
- Explains how you earned trust from the owning teams as an outsider.
Red Flags
- Edited code without understanding what called it.
- Avoided reading existing tests or conventions and reinvented patterns.
Follow-Ups to Expect
- How did you know your change didn't break a caller you hadn't seen?
- What would you tell the next new person to read first?
A Story Shape That Holds Up
The Change
What you had to do, and which part of the code it touched.
Finding Your Way
How you traced the code and who you asked.
Proving It Worked
The tests or checks you ran before and after merging.
What You Know Now
One thing about the codebase that clicked later.