React.js & Frontend Framework Questions Interview Questions and Answers

React.js & Frontend Framework Questions Interview Questions and Answers

Practice every question in this topic using expandable answers and interview tips. Answer aloud first, then open the explanation.

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Q1. What is React.js and what are its key features? [Easy]

Answer: React.js is an open-source JavaScript library developed by Facebook for building interactive user interfaces, particularly single-page applications. Key features include component-based architecture, Virtual DOM, declarative UI syntax using JSX, unilateral data flow, and rich ecosystem support.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Q2. What is the Virtual DOM in React and how does reconciliation work? [Easy]

Answer: The Virtual DOM is an in-memory lightweight copy of the real DOM. When component state changes, React creates a new Virtual DOM tree, compares it with the previous tree using a diffing algorithm called reconciliation, and batches minimal necessary updates to the real DOM for high performance.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Q3. What is JSX? [Easy]

Answer: JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows developers to write HTML-like markup directly inside JavaScript code. Babel transpiles JSX into React.createElement() function calls.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Q4. What is the difference between Functional Components and Class Components in React? [Easy]

Answer: Functional components are plain JavaScript functions that accept props and return JSX. They use React Hooks for managing state and lifecycle effects. Class components extend React.Component, use render() method, and manage local state via this.state and lifecycle methods like componentDidMount.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Q5. Explain React Hooks. What are useState and useEffect? [Medium]

Answer: React Hooks are functions that let functional components tap into React state and lifecycle features. useState declares state variables that trigger re-renders when updated. useEffect manages side effects like fetching data, subscriptions, or DOM manipulation, replacing componentDidMount, componentDidUpdate, and componentWillUnmount.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Q6. What is the Purpose of useMemo and useCallback? [Medium]

Answer: useMemo memoizes the computed value of an expensive calculation so it only recalculates when dependency values change. useCallback memoizes callback function instances to prevent unnecessary re-creations and unnecessary re-renders of child components wrapped in React.memo.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Q7. What is Props Drilling and how can you prevent it? [Medium]

Answer: Props drilling is passing data down through multiple nested child components that do not need the data themselves just to reach a lower component. It can be prevented using the React Context API, Redux, Zustand, or Component Composition.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Q8. Explain Redux Toolkit and how state flow works in Redux. [Hard]

Answer: Redux is a centralized state management library for JavaScript applications. The state flow follows a strict unidirectional pattern: UI dispatches an Action -> Action passes through Middleware to Reducers -> Reducer functions compute the new immutable State -> Store updates UI components via Selectors. Redux Toolkit simplifies Redux setup with createSlice and createAsyncThunk.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Q9. What are React Server Components (RSC) and Next.js App Router? [Hard]

Answer: React Server Components run exclusively on the server, allowing components to fetch data, render to HTML, and bundle zero JavaScript sent to the client. Next.js App Router leverages RSC to deliver streaming HTML, server-side rendering (SSR), static site generation (SSG), and improved initial page load speeds.

Interview tip: Define it clearly, give a small example, and mention one practical use or limitation.

Study checklist

Explain the answer without reading.
Write one small example.
Compare framework or database version differences in official documentation.

React.js & Frontend Framework Questions Interview Questions and Answers

Practice one question at a time. Read the question, answer aloud, then compare your explanation with the answer.

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Study method: define the concept, give a small example, and mention one practical use or limitation.