Interview questions in Frontend Dev, page 12
What is PostCSS and how it differs from Sass
PostCSS is a plugin-driven tool that parses CSS into an AST and transforms it; it is not a fixed language like Sass but a platform for plugins like autoprefixer.
How does Vue v-model work syntactically under the hood?
Tests if you know v-model is syntax sugar over prop and event pairs. Strong answers map text to :value/@input, checkboxes to :checked/@change, selects to value/@change, and note JS state wins. Red flag: calling it magic or true two-way binding.
How do you cancel pending fetches using AbortController?
Keep one AbortController, abort before each fetch, pass its signal, and swallow AbortError.
How does Next.js routing auto-implement code splitting?
Tests route-based code splitting versus manual setup. Strong answers note each page/route becomes its own chunk automatically, while client-only apps need explicit React.lazy and Suspense.
Build a DRY button with Sass maps and mixins
A base mixin for shared button styles, a variants map of name to colors looped to generate modifiers, and a sizes map looped for sizing.
Key differences between Template-Driven and Reactive Forms in Angular
This tests Angular form architecture tradeoffs. Contrast explicit synchronous reactive models with implicit asynchronous template-driven ones; assign reactive to scalable dynamic forms and template-driven to simple inputs.
When does fetch trigger a CORS preflight, and what POST is complex?
Tests whether you know the simple-request boundary. A strong answer names the three safe POST content-types and gives a cross-origin POST with application/json plus a custom header like Authorization.
React Profiler long render duration: causes and next steps?
Tests interpreting actualDuration vs baseDuration. Outline: close values on updates mean broken memoization—add memo or useMemo; high baseDuration alone means an inherently slow subtree. Red flag: proposing fixes before comparing these two Profiler metrics.
What does autoprefixer do and why?
Autoprefixer is a PostCSS plugin that adds vendor prefixes to CSS based on a browserslist target using compatibility data, so you write standard CSS and it handles support.
Bind an input to a variable and validate with reactivity
Tests Svelte reactivity: connecting DOM state to validation logic. Good answer: $state for the variable, $effect to run validation when it changes, cleanup if needed. Red flag: using effects where event handlers suffice or creating infinite loops.
Download a large file via fetch and track ReadableStream progress
It tests streaming I/O and backpressure beyond Promise-based fetch. Acquire a reader from response.body, loop read() until done, sum chunk lengths against Content-Length, and emit progress.
How would you avoid new functions per list item without useCallback?
Tests stable handler patterns for large lists. Answer: define one handler outside the map, attach it to every item, and read the id from a data-id attribute via currentTarget.dataset.id. Or use event delegation on the parent. Red flag: useCallback in loop.
Build a scalable Sass theming system
Store themes as maps, emit values as CSS custom properties so runtime switching needs no recompile, generate per-theme scopes via mixins.
Validate matching Vue passwords: computed property or watcher?
Use a computed boolean for caching and dependency tracking; reserve watchers for side effects only.
Refactor a function using generics to accept any array type
It tests generic type parameters for preserving element types through a function boundary. Answer: declare T, accept T[], return T, and let the compiler infer the element type from the call site.
Explain list virtualization and when it beats React.memo
This tests whether you know React.memo skips re-renders but leaves off-screen nodes in the DOM. A strong answer describes virtualization mounting only visible items to cut memory use. A red flag is claiming memoization fixes jank or saves memory for big lists.
Diagnose and shrink a large CSS bundle
Purge unused selectors, minify and merge with cssnano, split critical CSS and code-split per route.
How do you debounce input for API validation in Vue or Angular?
Tests reactive stream control and cancellation. A strong answer uses debounceTime at 300ms, switches with switchMap, and handles loading and error states. Red flag: using setTimeout without cleanup or ignoring race conditions.
Explain generic constraints and provide a length-constrained generic example
Explain that extends requires properties; show T extends { length: number }; give a generic function signature.
When would you choose Redux or Zustand over React's built-in hooks?
Choose global state when many components share rapidly changing state, logic is complex, or updates must be traceable.
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