Try the Tech Radar #5 — Mutation Testing in 500 Lines of Vanilla JS (and Why High Coverage Lies)

Thoughtworks Technology Radar Vol 34 (April 2026) put Mutation testing in the Trial ring with a sharp note: in an era of LLM-generated tests, coverage numbers stopped meaning what they used to. Mutation testing is what tells you whether your tests actually assert anything. I built a 500-line vanilla JS playground that runs entirely in-browser — paste a function and its tests, see how many mutants the suite kills.

100% coverage with 0% mutation score

Consider isAdult(age) { return age >= 18; } with these tests:

expect(isAdult(25)).toBe(true);

expect(isAdult(10)).toBe(false);