Advanced interview questions in Frontend Dev, page 3
Using CSS Grid, how do repeat, auto-fit, and minmax create responsive columns?
Tests intrinsic sizing and responsive tracks without media queries. A strong answer gives repeat(auto-fit, minmax(250px, 1fr)), noting repeat generates tracks, auto-fit collapses empties, and minmax sets a floor with fractional growth.
How would you use DocumentFragment to optimize adding 1,000 list items?
Tests DOM reflow/repaint costs and off-DOM batching. A strong answer: create a DocumentFragment, build the 1,000 nodes off-DOM, then append once to trigger a single reflow. Red flag: claiming it saves memory or confusing it with innerHTML batching.
Implement IntersectionObserver in TypeScript for lazy-loading images
Tests precise DOM typing and observer lifecycle. A strong answer types the callback as receiving IntersectionObserverEntry[], narrows entry.target to HTMLImageElement, swaps data-src to src, and calls unobserve.
Explain Shallow Routing in Next.js Pages Router
This tests URL updates without re-running getServerSideProps or getStaticProps in the Pages Router. A strong answer defines shallow: true, cites query-state UI like filters or modals, and notes same-page and App Router limits.
Write a generic TypeScript handler for mixed form inputs?
Tests TypeScript DOM narrowing. Strong answer: union-type ChangeEvent, narrow target with instanceof or tag checks, branch on element.type to read .checked for checkboxes or .value otherwise. Red flag: casting to any or assuming all inputs use .value.
How do you handle 404s in React Router and Next.js App Router?
Tests client versus server routing architecture. React Router uses a wildcard route returning HTTP 200 by default; Next.js App Router uses not-found.js for server-rendered 404s. Red flag: confusing the two routers or ignoring status code semantics.
How do you programmatically sync CSS animation progress to scroll?
This tests scroll-driven APIs beyond legacy scroll listeners. A strong answer names CSS animation-timeline and scroll-timeline declaratively, plus the Web Animations API with ScrollTimeline in JS. A red flag is using only requestAnimationFrame style mutations.
How should you adapt animations for prefers-reduced-motion?
Scope motion to no-preference, swap scaling for opacity or instant cuts, keep functional changes visible.
Compiler vs runtime reactivity: Svelte, Vue, Angular?
Svelte compiles away reactivity (smaller bundles, zero-cost), Vue/Angular use runtime Proxies/zone.js (flexible but heavier).
How do you structure and tokenize animation and transition values?
This tests semantic token architecture for motion. Structure duration and easing into tiered tokens: primitive values, semantic aliases like feedback or navigation, and component bindings. Avoid hardcoding values or using only raw milliseconds without meaning.
Implement auto-cleanup reactive logic in Vue's Composition API and Svelte
Vue pairs onMounted with onUnmounted or watchEffect cleanup; Svelte uses $store auto-subscription or onDestroy.
How would you prevent search-as-you-type race conditions with AbortController?
Abort the previous request before each new keystroke, pass the fresh signal into fetch, and ignore the AbortError.
Implement inSequence(tasks) to execute promise-returning functions sequentially
Tests async/await control flow versus Promise.all parallelism. A strong answer uses a for-of loop with await inside an async function, accumulates results in order, and stops cleanly on rejection. Red flag: suggesting Promise.all or forEach with await.
Should a component own its outer spacing?
Components own internal padding but should not own external margin; the parent or layout owns spacing between siblings, keeping components context-independent.
Optimize a large form with frequent state updates
Tests render propagation and memoization discipline. Strong answers note parent updates re-render children, extract inputs to localize state, useMemo for derived values, and stable callbacks. Red flag: useMemo everywhere without component boundaries.
Utility-first versus semantic CSS trade-offs
Utility-first gives speed, small purged bundles, and no naming overhead but verbose markup; semantic BEM gives readable markup and clear boundaries but naming and dead-CSS risk.
Design a multi-step wizard form pattern in React
Shared state, step validation, and resilient navigation. A great answer uses one form instance with per-step Zod schemas, a context stepper, and localStorage persistence. Red flag: isolated forms per step that lose data on navigation.
Using :is() and :where() to manage specificity
:is() groups selectors and takes its most specific argument's specificity, :where() does the same grouping but contributes zero specificity, ideal for easily overridable defaults.
Shadow DOM vs build-time scoping for third-party widgets
Shadow DOM gives true isolation but blocks global theming and complicates SSR; build-time scoping keeps theming and SSR simple but needs tooling.
How does Vue's v-bind() CSS function work in style blocks?
Tests Vue SFC CSS transforms and browser primitives. Strong answer: Vue compiles v-bind() to scoped CSS custom properties, letting reactive values work with pseudo-selectors and media queries.
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