Question 1

What is React and what are its key features?

Accepted Answer

React is a JavaScript library for building user interfaces, particularly single-page applications. Its key features include a virtual DOM for efficient updates, component-based architecture for reusable UI elements, declarative programming style, unidirectional data flow, JSX syntax for writing HTML-like code in JavaScript, and React Native for mobile app development. React focuses on the view layer and is often used with other libraries for state management and routing.

Question 2

Explain the difference between functional and class components in React.

Accepted Answer

Functional components are JavaScript functions that return React elements, while class components are ES6 classes that extend React.Component. Functional components have simpler syntax and no 'this' binding issues, while class components have built-in state and lifecycle methods. With the introduction of Hooks in React 16.8+, functional components can now handle state and side effects, making them the preferred choice for new code.

Question 3

What are React Hooks? Explain the most commonly used hooks.

Accepted Answer

React Hooks are functions that allow functional components to use state and lifecycle features. The most common hooks include useState for managing state, useEffect for side effects, useContext for accessing context values, useReducer for complex state logic, useCallback for memoizing functions, useMemo for memoizing values, and useRef for creating persistent references. Hooks were introduced in React 16.8 to enable stateful logic in functional components without using classes.

Question 4

What is the virtual DOM in React and how does it work?

Accepted Answer

The virtual DOM is a lightweight copy of the real DOM that React maintains in memory. When state changes, React creates a new virtual DOM representation, compares it with the previous one using a diffing algorithm, and then efficiently updates only the changed parts of the real DOM. This approach is more efficient than directly manipulating the DOM because it minimizes expensive DOM operations and batches updates together.

Question 5

Explain React's component lifecycle methods and their purpose.

Accepted Answer

React class components have three main lifecycle phases: mounting, updating, and unmounting. The mounting phase includes constructor, getDerivedStateFromProps, render, and componentDidMount. The updating phase includes getDerivedStateFromProps, shouldComponentUpdate, render, getSnapshotBeforeUpdate, and componentDidUpdate. The unmounting phase includes componentWillUnmount. In functional components, the useEffect hook replaces these lifecycle methods, with dependency arrays controlling when effects run.

Question 6

What is JSX? Why is it used in React?

Accepted Answer

JSX is a syntax extension for JavaScript that allows writing HTML-like code in JavaScript files. It makes React components more readable and expressive by providing a familiar syntax for developers. JSX is transpiled to React.createElement() calls and allows embedding JavaScript expressions within markup. It also provides security benefits by escaping values before rendering and improves the developer experience with better error messages.

Question 7

How does state management work in React? When would you use Redux or Context API?

Accepted Answer

React offers multiple state management approaches. Component state (useState or this.state) is used for local state. The Context API is built into React for sharing state across components without prop drilling, ideal for global state that changes infrequently. Redux is a separate library for complex applications with extensive state logic, offering features like time-travel debugging and middleware. The choice depends on the application's complexity and state management needs.

Question 8

What are keys in React lists and why are they important?

Accepted Answer

Keys in React are special attributes used to identify which items in a list have changed, been added, or been removed. They help React efficiently update the UI by tracking which components need to be updated, added, or removed. Keys should be unique among siblings, stable across renders, and typically come from data IDs rather than array indices. Using proper keys is crucial for maintaining component state and optimizing performance.

Question 9

What is prop drilling in React and how can you avoid it?

Accepted Answer

Prop drilling is when data is passed through multiple levels of components that don't need the data but only pass it down to deeper components. This creates unnecessary coupling and makes code harder to maintain. To avoid prop drilling, you can use the Context API for sharing global state, state management libraries like Redux, component composition techniques, or custom hooks to share stateful logic between components.

Question 10

Explain React's error boundaries and how they work.

Accepted Answer

Error boundaries are React components that catch JavaScript errors in their child component tree, preventing the entire app from crashing. They display a fallback UI when errors occur and can log error information. Error boundaries catch errors during rendering, in lifecycle methods, and in constructors, but not in event handlers or asynchronous code. They're implemented as class components with getDerivedStateFromError and componentDidCatch methods.

Question 11

What are React's performance optimization techniques?

Accepted Answer

React offers several performance optimization techniques including memoization (React.memo, PureComponent), virtualization for large lists, code splitting with React.lazy and Suspense, and hooks optimization (useMemo, useCallback). Other techniques include using proper keys in lists, avoiding inline function definitions, debouncing/throttling frequent events, and using web workers for CPU-intensive tasks. Production builds also optimize performance by removing development warnings and minifying code.

React JS Interview Questions and Answers (2025)

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Table of Contents

Top React Interview Questions

1. What is React and what are its key features?

2. Explain the difference between functional and class components in React.

3. What are React Hooks? Explain the most commonly used hooks.

4. What is the virtual DOM in React and how does it work?

5. Explain React's component lifecycle methods and their purpose.

6. What is JSX? Why is it used in React?

7. How does state management work in React? When would you use Redux or Context API?

8. What are keys in React lists and why are they important?

9. What is prop drilling in React and how can you avoid it?

10. Explain React's error boundaries and how they work.

11. What are React's performance optimization techniques?

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