Easy interview questions in React & Next.js
What is JSX and how does the browser run it?
JSX is XML-like syntax compiled to React.createElement calls; it differs from HTML via className and camelCase; browsers never see JSX, only transpiled JS.
Explain props in React and how parent components pass data to children
Define props as the single object argument; show destructuring; stress immutability and downward flow; note defaults.
What are the two main ways to define a component in React?
Tests fluency in function and class syntax. Outline: show a function returning an h1; show a class with render() returning an h1; note functions are the React 19 default. Red flag: offering arrow vs function declaration as the two ways, or omitting render().
How do you use useState to track user input?
Initialize with useState at top level, bind input value to state, and update via onChange using setter.
Explain useEffect dependency array behavior for [], [deps], and omitted
It tests reactive dependency tracking. Omitting re-runs every render; [] runs on mount with cleanup on unmount; [deps] re-runs when Object.is detects change. Red flag: claiming [] means 'run once' without mentioning cleanup or stale values.
What is prop drilling in React?
Define it as threading props through middle layers; give a three-layer example like a theme toggle; cite useContext.
How does children work, and why is it fundamental to composition?
Tests React composition knowledge. Good answer: children is a prop holding nested JSX; a Card wrapper renders {children} to let parents inject markup; contrast with prop-driven text. Red flag: calling children magic syntax instead of a standard prop.
What is a custom hook? Write a simple useToggle example.
What it tests: Whether you see hooks as stateful logic extraction. Outline: Define a use-prefixed function calling hooks; write useToggle with useState and callback; explain extraction vs duplication. Red flag: Helpers without hooks or missing use prefix.
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.
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.
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.
Why use Link over <a> for internal navigation?
This tests client-side routing vs full page reloads. A strong answer covers click interception, History API URL updates, and route prefetching. A red flag is claiming Link is purely stylistic or identical to a plain anchor.
Explain controlled vs uncontrolled React form inputs and trade-offs
Tests state versus DOM ownership. Controlled inputs bind to React state via onChange; uncontrolled inputs read from DOM via refs. Trade-offs: reactivity, validation, complexity. Red flag: saying uncontrolled is easier while ignoring lost live validation.
How do you handle a basic form submission in React?
Tests declarative form handling via controlled state. Nail it with onSubmit on the form, preventDefault to stop native reload, and state as the single source of truth. Red flag: reading values through refs or querySelector.
Explain the difference between React.memo and useMemo
React.memo skips re-renders when props match; useMemo caches a computed value between renders.
Child re-renders with unchanged props: function identity and useCallback fix
This tests reference equality and React memoization. A strong answer says inline functions recreate on every parent render, breaking React.memo, and uses useCallback with a correct dependency array. A red flag is ignoring identity or omitting dependencies.
How would you use React Profiler to find unnecessary re-renders?
This tests the Profiler for wasted renders. A good answer covers wrapping a subtree in Profiler, comparing actualDuration to baseDuration on updates, and checking for missing memoization. Red flag: mentioning only the browser extension without timing metrics.
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.
How do Redux actions, reducers, and click-to-render data flow work?
Tests Redux unidirectional flow and immutability. Strong answer: actions are plain event objects, reducers are pure functions returning new state, and flow goes dispatch to store to UI. Red flag: actions mutating state or reducers with side effects.
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