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Compare HOCs and custom hooks for sharing logic
Tests your grasp of React's shift from wrapper-based to function-based logic reuse. Hooks eliminate wrapper hell and implicit props by extracting stateful behavior into plain functions without nesting components. Red flag: calling hooks just sugar.
useImperativeHandle: Expose a Custom Ref API
useImperativeHandle exposes a custom API via a ref, letting a parent call specific methods on a child. Instead of exposing an entire DOM node, you can provide a handle with methods like reset() or focus().
Describe combining useContext and useReducer for scalable feature state
This tests replacing prop drilling with a context-reducer pattern. A strong answer covers two contexts for state and dispatch, a provider wrapping the subtree, and a pure reducer. A red flag is one global context or async logic inside the reducer.
Pass server-fetched data from Server Components to nested Client Components
Tests RSC boundary awareness. Strong answer: fetch in the async Server Component, pass serializable props to nested Client Components. Red flag: proposing Context or useEffect inside the Server Component.
React's Controlled vs. Uncontrolled Components
A controlled component is a form input whose value is driven by React state; an uncontrolled component lets the DOM manage its own state. Use controlled for instant validation or conditional logic.
What is a stale closure in React hooks?
Tests closures and hook dependencies. A strong answer defines stale closure as capturing an outdated variable, shows a useEffect interval with stale state, and fixes it with dependencies and cleanup. Red flag: omitting cleanup or using refs blindly.
How do CSS Modules solve global scope conflicts in React?
This tests build-time hashing and local scope. An answer explains that CSS Modules compile classes into unique hashes, scoping styles to the component, and shows the React import pattern styles.primary.
Where and how to import global stylesheets in Next.js
This tests Next.js style entry points. In the App Router, import global CSS in the root layout.js; in the Pages Router, import it in _app.js. Never import global styles inside pages or components, since Next.js restricts global CSS to root-level files.
How would you create a styled Button accepting a variant prop
It tests prop interpolation for dynamic styling in CSS-in-JS. Good answers use styled.button with a function reading props.variant to map CSS rules in one component. A red flag is splitting into separate components per variant or using inline styles.
Compound Components: Build Flexible APIs via Shared State
Think of HTML's <select> and <option>. The Compound Component pattern lets a parent manage state while children act as declarative configuration. It's used for flexible UI like tabs or accordions.
Implement a light/dark theming system with CSS-in-JS and React Context
This tests React Context architecture with CSS-in-JS theme injection. Strong answers include: a Theme Context with mode and toggle; ThemeProvider at the app root; consumption via useTheme or props.theme.
Next.js Layouts: Shared UI That Survives Navigation
Think of a Next.js Layout as a picture frame that stays put while you swap the pictures (Pages) inside. It defines shared UI like headers that persist across routes, preserving state.
Runtime CSS-in-JS performance pitfalls in SSR Next.js
Runtime libs serialize styles per render, need style collection on the server, risk FOUC and double work in App Router.
next/dynamic: Defer Loading Heavy Components
next/dynamic splits a component into its own file, loading it only when needed to shrink your initial bundle. Use it for heavy components below the fold or browser-only libraries. The footgun: forgetting a loading state causes layout shifts.
CSS Custom Properties vs JS Theme Object in React
Tests render overhead vs CSS cascade. Strong answers: CSS variables update styles without re-renders, while JS theme objects in Context force subtree re-renders. Mention scoping, SSR FOUC, and setProperty interop.
useTransition: Keep Your UI Responsive During State Changes
useTransition lets you update state without blocking the UI, keeping your app responsive. Use it for slow renders like filtering large lists, while the isPending flag shows a loading state.
What is the fundamental difference between client-side and server-side routing?
Server routes return full documents and reload; client routes swap JS views and use the History API without server requests.
useDeferredValue: Keep UI Responsive During Renders
useDeferredValue keeps your UI responsive by showing a stale value while a new, expensive-to-render value is prepared in the background. Use it for search inputs filtering large lists.
Primary roles of BrowserRouter, Routes, and Route, and basic route setup
Tests v6 routing hierarchy. BrowserRouter provides history; Routes picks one match; Route maps path to element. Wrap Routes in BrowserRouter with nested Route elements for / and /about. Red flag: omitting the router or confusing Routes with Switch.
useSyncExternalStore: Safely Read from External State
useSyncExternalStore lets your React components safely subscribe to data outside of React's state. Use it to connect to state libraries like Zustand or browser APIs. The main footgun: your getSnapshot function must return an immutable, cached value.