Everything in Frontend Dev, page 28
Accessible React Forms: Labels, Focus, and Errors
Accessible React forms need real labels, keyboard focus, and error announcements, not just ARIA. Screen reader users must perceive inputs and validation without visual cues. The common footgun is placeholder text or divs instead of label elements with htmlFor.
HashRouter: routing without server configuration
HashRouter traps routes after a # the server never sees, letting SPAs run on static hosts like GitHub Pages without rewrite rules. The tradeoff is ugly URLs and SEO blind spots since search engines may ignore whatever follows the hash.
BrowserRouter: Real URLs in React SPAs
BrowserRouter turns the address bar into a client-side navigation system, using real URLs without reloads. It is the default choice for SPAs that need shareable links.
BEM: Self-Documenting CSS Class Names
BEM turns classes into a mini filesystem: Block__Element--Modifier. It prevents CSS collisions in large React codebases where many teams share a global stylesheet. The footgun is turning every nested tag into an element and creating comically long names.
Server vs. Client Components in Next.js
Next.js forces a render boundary. Server Components generate HTML without sending JavaScript, while Client Components ship JS for interactivity. The footgun is importing a heavy library into a Server Component via a child, bloating the client bundle.
Rules of Hooks: Call Order Is Sacred
Hook calls must keep the same order every render because React maps them to state by array index. Only call hooks at the top level of React functions. Break the order and React desyncs, producing stale state or crashes.
React Reconciliation: The Virtual DOM Diff
React diffs a lightweight in-memory tree against the next render to apply only surgical changes to the real DOM. It fires on every state or prop change. Missing keys in lists force React to rebuild more nodes than needed, destroying performance.
Yup: Schema Builder for Runtime Validation
Yup is a validation pipeline: schemas parse, coerce, and assert untrusted data in one pass. Use it for API payloads or forms while TypeScript infers types automatically.
The act Utility in React Testing Library
React Testing Library re-exports everything from DOM Testing Library and explicitly lists act as a core top-level API method alongside render and renderHook, making it available as a named export from the framework package without requiring a separate wrapper.
Protected Routes: Server Gates, Not Hidden Links
A protected route is a server-enforced gate, not a hidden link. In Next.js, middleware or server components validate sessions before HTML ships, which matters for dashboards and billing.
Lists and Keys in React
React turns arrays into UI with map, yet each child needs a unique key prop or you get console warnings. Filter first, then map. The footgun: forgetting keys or failing to include stable unique IDs in your source data.
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.
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.
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.
CSS will-change: purpose, appropriate use, and overuse consequences
Tests whether you know will-change is a last-resort hint, not a default. Strong answers: hint imminent changes, toggle via script, and warn that overuse wastes memory. Red flag: leaving it in CSS permanently or applying it preemptively to many elements.
Create a staggered list fade-in using only CSS
Tests CSS animation orchestration via keyframes and delay strategies. Great answers use :nth-child or --index variables for scalable staggering, set fill-mode forwards, and honor prefers-reduced-motion.
Why is animating transform preferred over top or margin-left?
Grasp of the rendering pipeline and frame rate. A strong answer says transform skips layout and runs on the compositor, while top and margin-left force layout and drop frames.
How do you trigger a CSS transition? Provide two methods.
Set transition on the element, then change a property via :hover or by toggling a class with JavaScript.
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.
Implement fluid typography from 16px to 24px across 400px–1200px in one declaration?
This tests clamp() fluency for viewport-relative type scaling without media queries. A strong answer states clamp(16px, calc(16px + 8 * (100vw - 400px) / 800), 24px) and explains the slope math. A red flag is multiple media queries or calc() without clamp().
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