Intermediate interview questions in Frontend Dev, page 9
How does Svelte surgically update the DOM without VDOM?
Tests VDOM overhead vs compile-time optimization. A strong answer explains that Svelte compiles reactive dependencies into direct DOM API calls, skipping tree creation and diffing. Red flag: claiming VDOM is slow because the DOM itself is slow.
Implement virtual scrolling for a large list in Vue
Tests DOM cost awareness and viewport culling. Strong answer: mount only visible nodes plus overscan, recycle DOM on scroll, and preserve total scroll height with a spacer. Red flag: suggesting pagination or naive v-for without keys.
GET caching in Route Handlers and how to opt out
Tests App Router GET Route Handler caching and dynamic opt-outs. Hit: export const dynamic = 'force-dynamic', revalidate = 0, dynamic APIs like cookies(), and unstable_noStore(). Red flag: confusing this with Pages Router API routes, dynamic by default.
How do you create a smooth Canvas 2D loop with requestAnimationFrame?
Tests render pipeline timing. Outline: queue rAF per refresh; derive delta from timestamp for frame-rate independence; recurse each frame; cancel via cancelAnimationFrame. Red flag: claiming rAF locks 60fps, ignoring timestamp, or setInterval is fine.
How do you create a dynamic API route and access its parameter?
This tests Next.js routing conventions and dynamic parameter extraction. Create /api/products/[productId]/route.js, export a GET handler, and read productId from params. Red flag: mixing App Router params with Pages Router req.query.
Which video event signals completion, and how do you track playback time?
Tests HTMLMediaElement events and UI performance. Answer: use ended for replay; read currentTime on timeupdate but throttle updates or use requestAnimationFrame. Red flag: setInterval polling, currentTime===duration checks, or unthrottled state writes.
How do you handle multiple HTTP methods in a single route.ts file?
Tests App Router Route Handler REST conventions. A strong answer exports named GET, PUT, DELETE handlers using NextRequest and NextResponse.json, plus 405 for unsupported methods.
Explain the roles of vertex and fragment shaders in WebGL
Vertex shaders write gl_Position per vertex; fragment shaders write gl_FragColor per pixel.
How do you read headers and search params in a Route Handler?
Tests fluency with the Web Standard Request API in Next.js App Router. A strong answer uses request.headers.get() for headers and request.nextUrl.searchParams for query params, avoiding Express-style req.query or req.headers.authorization.
Use MediaRecorder to capture a canvas stream and download as WebM
Tests MediaRecorder lifecycle and blob assembly. Answer: canvas.captureStream() into new MediaRecorder with video/webm, start(), collect chunks via dataavailable, stop(), then build a Blob and object URL for download.
When container queries beat media queries
Media queries react to viewport, container queries react to a parent's size, ideal for reusable components; set container-type on an ancestor then query it.
How do you test emitted events using Vue Test Utils?
Tests Vue Test Utils event inspection. Strong answer: mount, trigger('click'), then check wrapper.emitted('increment') for the payload array. Remember it returns an array of arrays, one per emission.
Styling a parent with the :has() selector
:has() matches an element that contains or relates to a given selector, enabling upward styling; figure:has(figcaption) targets captioned figures.
How do you mock an async service call in an Angular component test?
This tests Angular unit isolation and HTTP mocking. A strong answer covers HttpTestingController setup, flushing a mock response, asserting UI changes, and notes mocking avoids flaky networks. Red flag: using real HTTP calls in unit tests.
Why Oklch beats HSL for color variations
HSL lightness is not perceptually uniform so equal steps look uneven, Oklch keeps perceived lightness consistent across hues, giving predictable hover and palette scales.
How do you test a Svelte component updates a writable store?
Import the store with the component, render, await fireEvent.click, then assert the store value.
Describe the role of a spy in testing with a framework example
This tests whether you know a spy records calls without replacing original implementation. A strong answer defines the tracking role, picks a concrete Vue or Angular handler, and asserts with toHaveBeenCalled.
How would you use tsconfig.json to enable shorter path aliases?
This tests TypeScript module resolution. A strong answer sets compilerOptions.baseUrl, maps paths like @/* to src/*, and warns bundlers need matching aliases. A red flag is omitting baseUrl or claiming paths alone changes runtime resolution.
How BEM tames specificity and !important
BEM names block, element, modifier as flat single classes, keeping specificity low and equal, removing the need for deep selectors and !important.
What problem does esModuleInterop solve for CommonJS imports?
Tests CommonJS-to-ESM interop. A strong answer covers: default imports for CommonJS modules; runtime helpers checking __esModule and wrapping exports; and implicit allowSyntheticDefaultImports. Red flag: claiming it is purely type-level with no emit impact.
We are hiring for this. Every open role lists the topics its interview covers, so you can prepare for the real thing rather than guessing.
See open roles