JavaScript & TypeScript / Quiz
Coverage: The Number That Lies Politely
Branches vs lines, the files coverage never saw, threshold ratchets, and v8 vs istanbul. Make the metric mean something.
- Difficulty
- Medium
- Format
- Quiz
- Points
- 150
- Estimate
- 9 min
// MISSION BRIEF
Your Mission
"We have 92% coverage" can mean a battle-tested codebase or a suite that imports everything and asserts nothing. The difference lives in configuration details most teams never read.
This quiz covers Jest and Vitest coverage mechanics: metric kinds, collectCoverageFrom and its silent-omission trap, thresholds (global, per-path, and negative), and the two provider engines.
Speed bonus active.
// FIRST CONTACT
Battle teaser
A function has 100% line coverage but its error branch (a ternary fallback) was never exercised. Which metric exposes that?
- AFunction coverage, which counts branches per function
- BBranch coverage: it counts each side of every decision point (if/else, ternary, optional chaining, logical short-circuits), not just whether the line executed
- CStatement coverage, which is stricter than branch
- DNo metric can see untaken branches
Answers, scoring, hints, and the full battle stay sealed.
// SKILL TAGS
javascriptcoveragejestvitestci