Pick features you actually use: "Optional chaining and nullish coalescing eliminated hundreds of lines of defensive null checking. structuredClone replaced our hacky JSON deep-copy pattern with proper semantics."
Impactful features: optional chaining (?.), nullish coalescing (??), destructuring, template literals, array methods (flatMap, at()), structuredClone, top-level await, private class fields (#), and the Temporal API (Stage 3). Strong candidates describe real refactoring: replacing nested null checks with optional chaining, using nullish coalescing instead of || for falsy defaults, or adopting structuredClone to replace JSON.parse(JSON.stringify()). They should mention browser/Node.js compatibility considerations.
Tests whether candidates keep current with the language. Those who still write verbose null checks or use lodash.cloneDeep when native alternatives exist are behind. Ask about compatibility — do they use Babel/TypeScript to target older environments?