Step 5 — Principal Engineer Review

Answer in writing. Several questions are about refusal, which is most of this module's practical value.

1. An engineer proposes a sprint normalising object shapes in a table row renderer, citing a 2× microbenchmark improvement. Price the proposal. What do you need to know to answer, what is your answer, and what would change it?

2. The same engineer is right — it is a 10M-cell virtualized grid, and the row renderer is the hot path. What changes? What do you ask them to measure before and after, and what would make you reject the work even so?

3. A style guide bans delete. Make the case for and against. Where would you actually enforce it, and how (lint rule, review convention, nothing)? What does your answer depend on?

4. A teammate says "we should use plain objects instead of Map, objects are faster." What is the steelman of their position, where is it true, and where is it exactly backwards?

5. Your benchmark shows a 3× regression after a dependency upgrade, isolated to property access. List four explanations other than shape changes, and the cheapest experiment that distinguishes them.

6. You measured a result you cannot explain (Step 3). Write the two-sentence version you would put in a PR description, and the version you would put in a team-wide performance guide. Should they differ?

7. A junior engineer asks whether they should worry about hidden classes. Give the answer that is useful rather than the one that is complete. What single heuristic do you leave them with?

8. Your organisation wants a "JavaScript performance" section in its engineering standards. Draft the outline. What is in it, what is deliberately not, and how do you stop it becoming a folklore repository within a year?

9. These are V8 numbers. Your product supports Safari and Firefox. What is your policy for engine-specific optimisation, and what evidence would justify an exception?

10. Reflect on the module's structure: it is short, largely negative, and explicitly bounded. Was that the right call for a Principal curriculum? Argue the other side — what does an engineer lose by not going deeper here, and when would that loss actually bite?

Record

Answers in RESULTS.md. Question 8 is a natural ADR — file it in ../fe-00-roadmap/docs/decisions/ with the reversibility and migration sections filled in seriously.