Question 2Coding
You get a codebase you've never seen and one hour to fix a bug. Where do you start?
FDEs work inside customer and product code they didn't write.
“Engineering background. You're a genuine builder. Proficient in Python and at least one of TypeScript/JavaScript, Java, or Go; comfortable across data plumbing, APIs, and a bit of front-end when a demo needs it.”Forward Deployed Engineer · NinjaTech AI
The answerFour steps
- 1
Reproduce the bug with the smallest example
Say it like this
“First ten minutes: I take one real order it happened to and rebuild that cart locally until I see the double discount myself.”
Why it scores
Reproduces before reading. Without a repro you can't prove the fix.
- 2
Find where the data goes wrong (search, logs, debugger)
Say it like this
“Then I search for where discounts get applied. Say grep finds three call sites, and a breakpoint shows the second one runs again when the cart is re-saved after a coupon change.”
Why it scores
Search plus a debugger, not a tour of the whole codebase.
- 3
Make the smallest fix and add a test that fails without it
Say it like this
“The fix is two lines: skip it if the line item already has the discount. I write a test that builds that exact cart, watch it fail, then watch it pass.”
Why it scores
Smallest change and a test that fails first. That's proof, not hope.
- 4
Note anything risky you saw but didn't change
Say it like this
“I don't touch the other two call sites in the hour. I list them in the PR as a risk for whoever owns this code.”
Why it scores
Knows the limits of an hour in someone else's code. Flags it, doesn't rewrite it.
StrongAll together
About a minute, out loud. The marked parts are the four steps; hover one to find it above.
Say the bug is “discount applied twice on some orders” in a Django app I've never opened. First ten minutes: I take one real order it happened to and rebuild that cart locally until I see the double discount myself.1 Then I search for where discounts get applied. Say grep finds three call sites, and a breakpoint shows the second one runs again when the cart is re-saved after a coupon change.2 The fix is two lines: skip it if the line item already has the discount. I write a test that builds that exact cart, watch it fail, then watch it pass.3 I don't touch the other two call sites in the hour. I list them in the PR as a risk for whoever owns this code.4
WeakThe answer that fails
Sounds fine. Common. Scores low.
“I'd spend the first part reading through the code to understand the architecture, the main modules and how they connect. Once I get the big picture, I'd find the bug and fix it.”
Interviewer's note
Half the hour gone before touching the bug. No repro, so no way to show the fix worked.
PracticeTwo minutes, then they push back
Your turn
You get a codebase you've never seen and one hour to fix a bug. Where do you start?
Answer like you're in the room: about two minutes, no notes. Say it out loud, even alone. It feels different from thinking it. Then check your answer against what interviewers score.