Modern test runners like Vitest and Jest wow us with next-level features. Think in-file concurrency, HMR, embedded Chromium, performance benchmarks - you name it. Yet behind all these goodies lies a labyrinth of runtime mechanics that tunnel deep into the JavaScript runtime. These sweet perks can quickly turn into a pitfall, from subtle mocking glitches to serious performance issues. Understanding these machines is not just a geeky tour into the heart of clever libraries - but also an insightful understanding of the V8 and code inference that will save you hours of head-scratching and help you craft more thoughtful tests. Don't like theoretical talks? Great, this one is for you.
This talk has been presented at Node Congress 2025, check out the latest edition of this JavaScript Conference.