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Top 50 React Interview Questions and Answers

React is one of the most popular JavaScript libraries for building user interfaces. If you're preparing for a React job interview or simply want to sharpen your skills, this comprehensive list of 50 important React interview questions will help you master the key concepts.

1. What is the difference between Virtual DOM and Real DOM?

Feature	Virtual DOM	Real DOM
Changes	Easier to update	Expensive to update
Memory Usage	Minimal memory wastage	High memory consumption
Updates	JSX element updates efficiently	Creates a new DOM every time
Performance	Faster updates	Slower updates

2. What is React?

React is an open-source JavaScript library developed by Facebook in 2011. It is used for building fast and scalable UI components. React follows a component-based architecture, making it highly reusable and efficient.

3. What is Virtual DOM in React?

Virtual DOM is a lightweight JavaScript representation of the actual DOM. Instead of updating the entire UI, React compares the Virtual DOM with the Real DOM and updates only the necessary changes using the Diffing Algorithm.

4. What are the key features of React?

Uses a single-direction data flow
Efficient updates with Virtual DOM
Component-based structure for reusability
Supports server-side rendering
Uses JSX (JavaScript XML) for easier coding

5. What is JSX?

JSX (JavaScript XML) is a syntax extension that allows writing HTML inside JavaScript. Example:

const element = <h1>Hello, React!</h1>;

6. Can browsers read JSX files?

No, browsers cannot read JSX files directly. JSX must be transpiled into JavaScript using Babel before execution.

7. Why is React widely used today?

Fast rendering with Virtual DOM
Reusable components
Easy UI testing
Integrates with other frameworks (like Angular)

8. What are the disadvantages of React?

Uses JSX, which may be complex for beginners
Steep learning curve due to various tools and libraries
Not a full framework, requiring additional dependencies

9. React vs Angular: Key Differences

Feature	React	Angular
Developer	Facebook	Google
DOM	Virtual DOM	Real DOM
Architecture	Only View (MVC)	Full MVC support
Data Binding	One-way	Two-way

10. What is a Component in React?

Components are reusable UI elements that make up a React application. There are two types:

Functional Components – Written as JavaScript functions
Class Components – Uses ES6 classes with lifecycle methods

11. How does rendering work in React?

React uses the render() function to update the UI dynamically. The function returns a React element that represents the DOM.

12. What are States in React?

State is a JavaScript object that stores dynamic data for a component. It allows React to reactively update the UI. Example:

this.state = { count: 0 };

13. What are Props in React?

Props (short for properties) allow components to pass data from parent to child. Props are read-only and cannot be modified by child components.

14. What is an Arrow Function in React?

Arrow functions are used for concise syntax and automatic binding of this. Example:

<MyInput onChange={(e) => this.handleChange(e)} />

15. What are Higher-Order Components (HOCs)?

HOCs are functions that wrap another component to reuse logic. Example:

const EnhancedComponent = withLogging(MyComponent);

16. What is create-react-app?

create-react-app is a CLI tool that sets up a React project with zero configuration. Example:

npx create-react-app my-app

17. Advantages of create-react-app

Supports JSX, ES6, and CSS preprocessing
Fast interactive testing
Built-in live development server

18. What is Redux?

Redux is a state management library that stores application states in a single store, making debugging easier.

19. Difference Between Props and States

Feature	Props	State
Changeable?	No	Yes
Managed By	Parent component	Component itself
Usage	Data passing	Dynamic updates

20. What are the Three Phases of the React Lifecycle?

Mounting – Component creation
Updating – State/prop changes
Unmounting – Component removal

21. What are Events in React?

Events in React work like browser events but use camelCase instead of lowercase. Example:

<button onClick={handleClick}>Click Me</button>

22. What is Routing in React?

React Router is used for navigation. Example:

<Route path="/home" component={Home} />

23. What is Flux vs Redux?

Feature	Flux	Redux
Stores	Multiple stores	Single store
State	Mutable	Immutable

24. What are Synthetic Events?

Synthetic events normalize browser events across different platforms in React.

25. What are Controlled and Uncontrolled Components?

Controlled Components – React controls the state
Uncontrolled Components – DOM controls the state

26. What are Refs in React?

Refs allow accessing DOM elements directly in React.

27. What are Hooks in React?

Hooks allow functional components to use state and lifecycle methods. Example:

const [count, setCount] = useState(0);

28. What is React Fiber?

React Fiber is an optimized rendering engine introduced in React 16 to improve performance.

29. What is Strict Mode in React?

Strict Mode detects potential issues in React applications.

30. How to Speed Up Rendering in React?

Use React.memo() to prevent unnecessary renders
Use PureComponent for class components

31-50: Additional Advanced Topics

31. What are Keys in React?

Keys are unique identifiers used in lists to help React efficiently update and render components. They improve performance by identifying which elements changed, were added, or removed.

const listItems = items.map((item) => <li key={item.id}>{item.name}</li>);

32. What is React.memo()?

React.memo() is a higher-order component that prevents unnecessary re-renders of functional components by memoizing the result.

const MemoizedComponent = React.memo(MyComponent);

33. How do you conditionally add attributes in JSX?

You can conditionally add attributes using JavaScript expressions inside JSX.

const button = <button disabled={isDisabled}>Click me</button>;

34. What is the difference between cloneElement and createElement?

React.cloneElement() copies an existing element and allows modification.
React.createElement() creates a new React element from scratch.

const newElement = React.cloneElement(oldElement, { className: 'new-class' });

35. Why use useEffect Hook in React?

useEffect is used to perform side effects like data fetching, DOM manipulation, or subscriptions in function components.

useEffect(() => {
  console.log('Component mounted');
}, []);

36. What is Context API?

Context API allows state to be shared globally without prop drilling.

const MyContext = React.createContext();

37. What is Lazy Loading?

Lazy loading delays component rendering until it’s needed.

const LazyComponent = React.lazy(() => import('./LazyComponent'));

38. How do you optimize performance in React?

Use React.memo() for optimization.
Use useCallback() and useMemo() to prevent unnecessary re-renders.

39. What is Server-Side Rendering (SSR)?

SSR pre-renders React components on the server for better SEO and performance.

40. How does Suspense work in React?

Suspense allows React to handle asynchronous loading of components.

<Suspense fallback={<div>Loading...</div>}>
  <LazyComponent />
</Suspense>

41. What is Hydration in React?

Hydration is the process of attaching event handlers to server-rendered HTML in React apps.

42. Difference between useState and useReducer?

useState is simpler and used for basic state logic.
useReducer is better for managing complex state logic.

43. What are Fragments in React?

Fragments allow multiple elements to be returned without adding an extra DOM node.

<>
  <h1>Title</h1>
  <p>Description</p>
</>

44. How to debug React applications?

Use React Developer Tools.
Use console.log() and breakpoints in Chrome DevTools.
Utilize Error Boundaries to catch JavaScript errors in components.

45. What is the significance of Error Boundaries?

Error Boundaries catch JavaScript errors in components and display a fallback UI instead of crashing the entire app.

46. What is useMemo Hook?

useMemo is used to memoize values to avoid unnecessary computations.

const memoizedValue = useMemo(() => computeExpensiveValue(data), [data]);

47. What are Portals in React?

Portals allow rendering a child component outside its parent hierarchy.

ReactDOM.createPortal(child, container);

48. What is Concurrent Mode?

Concurrent Mode allows React to work on multiple tasks simultaneously to improve UI responsiveness.

49. How to implement a custom Hook?

A custom Hook is a JavaScript function that starts with use and reuses logic.

function useCustomHook() {
  const [state, setState] = useState(0);
  return [state, setState];
}

50. What are Render Props in React?

Render Props is a pattern for sharing logic between components using a function as a prop.

<DataFetcher render={(data) => <DisplayData data={data} />} />

TOP 50 React JS Interview Question and Answer by Shiva Sir | React JS Inteview Question and Answer