Round 1 — Mock Interview

Hits: present → past → why-here in ~60–90s. Now (5 yrs, Bluesapling, BestWe = team-effectiveness platform, React/TS), one proud thread (Genius streaming), what you care about (architecture & performance), why this role. Trap: narrating the whole résumé chronologically. Follow-up: "Only one company?" → frame it as depth & ownership, then pivot to the range within BestWe. See §1.1.

Hits: lead with the tension, not a feature list — "Genius had to work in four surfaces with different UI but identical behaviour." Then the design (one shared SSE hook + core/wrapper split), then the payoff (reuse, consistent UX, killed duplication). Follow-up: "Why SSE over WebSockets?" and "Where does the shared logic end and per-surface UI begin?" — have both ready. See §1.3.

Hits: pick a surprising root cause — the stream-executor races are perfect (a dead stream's watchdog killing its successor; abort-vs-timeout ambiguity). Walk the method: reproduce → isolate → hypothesis → confirm → fix → add a guard. Trap: telling the bug without the method. See §1.4-B.

Hits: the active-round scoping call — Context + localStorage over the codebase's default Redux, because the requirement was "local, only here, only now," and global state you must remember to tear down is a leak. "Smallest scope that satisfies; the scope is the cleanup." Show you weighed the familiar option and rejected it on merit. See §1.4-A.

Hits: forward-facing — scope, ownership, a new domain, setting standards. One or two sentences, then pivot to what excites you about this team. Trap: criticising a current/past employer, even mildly.

Hits: function + the lexical scope it was created in, kept alive after the outer returns. Uses: data privacy, factories, hooks. Bite: closures capture variables, not values — the for (var i…) setTimeout logging 3 3 3; fix with let. See §2.

Hits: the Promise. Sync stack drains → all microtasks (Promise callbacks) → one macrotask (setTimeout) → microtasks again. Consequence to name: a runaway microtask chain starves rendering. See §2.

Hits: debounce = wait for activity to stop (search-as-you-type); throttle = at most once per interval during continuous activity (scroll). "Debounce waits for quiet; throttle rate-limits." Be ready to write debounce in ~5 lines. See §2.

Hits: any = off switch (avoid); unknown = safe top type, must narrow before use; never = bottom type, return of an always-throw, and the exhaustiveness-check tool in a switch. See §3.

Hits: a discriminated union with a literal status tag, so a switch narrows each case and a never default catches unhandled variants at compile time. Mention types are erased at runtime, so validate data crossing trust boundaries. See §3.

Hits: its own state/reducer update, its parent re-rendering, or a consumed context value changing. Props changing = "the parent re-rendered." Nuance: re-render ≠ DOM change — React still diffs; you optimise the wasted render work. See §4.

Hits: keys give list items stable identity so React can match/reorder and preserve state. Index-as-key breaks on reorder/filter — state and inputs attach to the wrong row. See §4.

Hits: missing deps → stale closure; no cleanup → leaks/duplicate subscriptions; setting state after unmount → use AbortController for fetches. StrictMode's dev double-run exists to surface missing cleanup. Trap: disabling the deps lint rule instead of fixing the dep. See §4.

Hits: by default. They cost memory + comparisons; memoising a callback is pointless unless the child is React.memo'd. Add them when you've measured a real re-render cost or need stable identity for a dep array. "Measure before memo." See §4.

Hits: every consumer re-renders when the value changes, and a fresh object literal each render counts as changed. Fixes: memoise the value, split contexts (state vs dispatch, or by domain), keep fast-changing state out of a wide context, or use a store with selectors. See §4.

Hits: SSE for one-way server→client token streaming (plain HTTP, auto-reconnect); WebSockets only for true bidirectional. Keep the transport in a custom hook that owns request/stream/routing, batch incremental updates, clean up with AbortController, and handle partial streams + retries + resume. This is your Genius design — see §4 & §1.3.

Hits: automatic batching (across promises/timeouts/native handlers), concurrent rendering (useTransition/useDeferredValue), the new createRoot API, Suspense SSR streaming, StrictMode effect double-invoke. Migration pain: batching + concurrency change when updates flush → ordering bugs in old sync-ish code. See §5.3.

Hits: dev serves native ESM unbundled + esbuild pre-bundles node_modules once, so start/rebuild stop scaling with app size; prod bundles with Rollup/Rolldown. Name the trade-offs: different dev/prod engines, and esbuild doesn't type-check (so you add a checker plugin + isolatedModules). See §5.1-B.

Hits: incremental — build a bridge/compat layer so old and new coexist (react-router v5-compat), keep the branch merge-able from main, change one axis at a time (Node bump ≠ React bump), take debt knowingly and visibly (ratchets not gates), and rebuild critical-path coverage before deleting the old test suite. "Version bump is 1%." See §5.2.

Hits: bundler resolves imports the way a bundler does (no required extensions, respects exports) vs node16 emulating Node's runtime rules. isolatedModules guarantees each file transpiles alone with no cross-file type info — which is how esbuild is fast, and why you must export type when re-exporting types. See §5.3.

Narrate: "feature-detect first — it throws in private mode and doesn't exist in SSR", "JSON parse/stringify in try/catch so a corrupt value can't crash the app", "always return a fallback so callers never get null", "keys typed against a schema so you can't read a value at the wrong type." Full implementation in §7.1.

Narrate: role → allowed-action set, with a function rule for ownership cases (an editor deletes only their own doc). Mention RBAC vs ABAC, and that permissions are a UX affordance, not security — the server must re-check. Full implementation in §7.2.

Narrate: a closed-over timer, clearTimeout on each call, apply to preserve this/args. Extension: expose .cancel() (clear the timer) and .flush() (fire immediately). Full implementation in §2.

Always have 2–3. "Biggest frontend pain point you'd want a senior to fix first?" · "How do technical decisions get made and code get reviewed?" · "What does success look like at 3 and 6 months?" · "What's your testing/release confidence like?" Asking nothing reads as disengaged. See §9.