React Interview Questions

React is a JavaScript library for building user interfaces, developed and maintained by Facebook. It allows developers to create reusable UI components that efficiently update in response to data changes. React employs a declarative approach to building UIs, where developers describe how the UI should look and behave based on the application’s state, and React takes care of efficiently updating the DOM to reflect those changes. One of its key features is the virtual DOM, which enables React to optimize rendering performance by only updating the parts of the DOM that have changed, rather than re-rendering the entire page.

React is commonly used in single-page applications, where a smooth and responsive user experience is crucial. It promotes the concept of a unidirectional data flow, making it easier to reason about the state of an application and manage the flow of data within the components. React’s popularity has grown significantly since its introduction, and it is widely adopted in the development community for building interactive and dynamic web applications.

React Interview Questions For Freshers

1. What is React?

React is a JavaScript library for building user interfaces, particularly single-page applications where UI updates dynamically based on data changes.

import React from 'react';

// Define a functional component
function HelloWorld() {
  return (
    <div>
      <h1>Hello, React!</h1>
    </div>
  );
}

// Render the component
ReactDOM.render(<HelloWorld />, document.getElementById('root'));

2. What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript recommended by React for describing what the UI should look like.

// Filename - App.js

import React from "react";

const name = "Learner";

const element = (
 <h1>
  Hello,
  {name}.Welcome to GeeksforGeeks.
 </h1>
);

ReactDOM.render(element, document.getElementById("root"));

3. Explain Virtual DOM in React?

The virtual DOM is an in-memory representation of the actual DOM elements. React uses it to optimize updates by calculating the difference (diffing) between the virtual DOM and the real DOM, updating only the changed parts.

4. What are React Components?

Components are reusable, self-contained building blocks in React that define how a part of the user interface should appear and behave.

5. How do you create a functional component in React?

You can create a functional component by writing a JavaScript function that returns JSX. For example:jsx

function MyComponent() { return <div>Hello, React!</div>; }

6. Explain the purpose of state in React?

State in React is used to manage and store data that may change over time, triggering UI updates when the state changes.

7. What is the difference between props and state?

Props are used to pass data from a parent component to a child component, while state is used to manage data within a component.

8. What is the significance of keys in React lists?

Keys are used to uniquely identify and differentiate between elements in a React list. They help React optimize rendering and update only the necessary components when the list changes.

9. What is the significance of the useEffect hook?

useEffect is a hook in React used for handling side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM.

10. How does React handle forms?

React forms use controlled components where form elements are controlled by React state, allowing dynamic manipulation and validation of form data.

11. Explain the concept of lifting state up?

Lifting state up refers to moving the state from a child component to its parent, making it a shared state among multiple components. This pattern helps in managing and syncing the state between components.

12. What is the purpose of the useContext hook?

useContext is a React hook that allows functional components to consume values from the React context, providing a way to share values like themes or authentication status across the component tree.

import React from 'react';

// Creating the context object and passing the default values.
const authContext = React.createContext({status:null,login:()=>{}});

export default authContext;

13. What is Redux, and why might you use it with React?

Redux is a state management library for JavaScript applications. It can be used with React to manage the state of the entire application in a predictable and centralized manner.

14. Explain the concept of Higher Order Components (HOC) in React?

HOC is a pattern where a function takes a component and returns a new component with additional features or props. It is used for code reuse, logic abstraction, and sharing functionalities across multiple components.

15. What is the significance of React Router?

React Router is a library for handling navigation in React applications. It enables the creation of single-page applications with multiple views by managing the URL and rendering different components based on the URL.

16. What is the purpose of the useMemo hook?

useMemo is a hook in React used for memoization, optimizing the performance of expensive calculations by caching the result and only recomputing when the dependencies change.

// Filename - App.js

import React, { useState } from "react";

function App() {
 const [number, setNumber] = useState(0);
 const squaredNum = squareNum(number);
 const [counter, setCounter] = useState(0);

 // Change the state to the input
 const onChangeHandler = (e) => {
  setNumber(e.target.value);
 };

 // Increases the counter by 1
 const counterHander = () => {
  setCounter(counter + 1);
 };
 return (
  <div className="App">
   <h1>Welcome to Geeksforgeeks</h1>
   <input
    type="number"
    placeholder="Enter a number"
    value={number}
    onChange={onChangeHandler}
   ></input>

   <div>OUTPUT: {squaredNum}</div>
   <button onClick={counterHander}>
    Counter ++
   </button>
   <div>Counter : {counter}</div>
  </div>
 );
}

// Function to square the value
function squareNum(number) {
 console.log("Squaring will be done!");
 return Math.pow(number, 2);
}

export default App;

17. What are React Hooks?

React Hooks are functions that enable functional components to use state, lifecycle methods, and other React features. They were introduced in React 16.8 to allow stateful logic in functional components.

18. Explain the concept of controlled components in forms?

Controlled components in forms are those where React state controls the input elements, making React the source of truth for the input’s value. It enables dynamic updates and validation of form data.

19. What is the significance of the useRef hook?

useRef is a hook in React used for accessing and interacting with a DOM element or a mutable value without triggering re-renders. It is often used for imperative operations like focusing on an input element.

20. What is the purpose of the useCallback hook?

useCallback is a hook in React used for memoizing functions to prevent unnecessary re-creation of function instances, optimizing performance by memoizing functions when dependencies don’t change.

import React, { useState, useCallback } from 'react';

function ClickCounter() {
  // State to track the number of clicks
  const [count, setCount] = useState(0);

  // Regular function that increments the count
  const increment = () => {
    setCount(count + 1);
  };

  // Using useCallback to memoize the increment function
  const memoizedIncrement = useCallback(increment, [count]);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={memoizedIncrement}>Increment</button>
    </div>
  );
}

export default ClickCounter;

21. Explain the concept of code splitting in React?

Code splitting is the technique of breaking down a large JavaScript bundle into smaller chunks, loaded on demand. React.lazy and Suspense are features used for implementing code splitting in React applications.

22. How does React handle error boundaries?

Error boundaries in React are special components that catch JavaScript errors anywhere in their child component tree, log errors, and display a fallback UI. They help prevent the entire application from crashing due to a single component’s error.

23. What is the purpose of the setState function in React?

setState is a function in React used to update the state of a component. It triggers a re-render and updates the DOM to reflect the new state.

24. Explain the concept of forwardRef in React?

forwardRef is a feature in React that allows a component to pass its ref attribute to a child component. It is particularly useful when working with higher-order components and libraries that require direct access to a component’s instance.

25. What is the significance of the componentDidCatch lifecycle method?

componentDidCatch is a lifecycle method in React used for handling errors that occur during the rendering of a component. It is part of the error boundary concept.

26. How does React differ from Angular and Vue.js?

React is a JavaScript library for building user interfaces, while Angular and Vue.js are full-fledged frameworks. React focuses on building UI components and follows a one-way data flow, whereas Angular provides a more comprehensive framework with features like two-way data binding, dependency injection, and a more opinionated structure.

27. Explain the concept of context in React?

Context in React is a mechanism for passing data through the component tree without having to pass props manually at every level. It provides a way to share values like themes or user authentication status with components at different levels of the hierarchy.

28. What is the purpose of the shouldComponentUpdate lifecycle method?

shouldComponentUpdate is a lifecycle method in React that allows a component to control whether it should re-render when the state or props change. It is used for performance optimization by preventing unnecessary renders.

29. What are the differences between functional components and class components in React?

Functional components are simpler, use functional syntax, and are stateless by default. Class components, on the other hand, have access to lifecycle methods, manage state, and handle more complex logic. With the introduction of hooks in React 16.8, functional components can now handle state and lifecycle methods as well.

30.How does React handle routing?

React handles routing through libraries like React Router, which enables the creation of single-page applications with multiple views. It manages the URL, renders components based on the URL, and provides navigation between different views.

React Interview Questions For Experience

1. Explain the purpose of React.memo?

React.memo is a higher-order component that memoizes the result of a functional component, preventing unnecessary renders when the component receives the same props.

import React from 'react';

// Regular functional component
const MyComponent = ({ data }) => {
  console.log('Rendering MyComponent');
  return (
    <div>
      <p>{data}</p>
    </div>
  );
};

// Memoized version of MyComponent using React.memo
const MemoizedMyComponent = React.memo(MyComponent);

// Parent component
class ParentComponent extends React.Component {
  constructor() {
    super();
    this.state = {
      value: 'Initial Data',
    };
  }

  handleClick = () => {
    // Simulating a change in data
    this.setState({ value: 'Updated Data' });
  };

  render() {
    console.log('Rendering ParentComponent');
    return (
      <div>
        <button onClick={this.handleClick}>Update Data</button>
        {/* Using the MemoizedMyComponent */}
        <MemoizedMyComponent data={this.state.value} />
      </div>
    );
  }
}

export default ParentComponent;

2. How does React Router handle navigation in a React application?

React Router is a library that handles navigation in React applications by managing the URL and rendering different components based on the URL. It provides components like BrowserRouter and Link for navigation.

3. What is the significance of code splitting in React applications?

Code splitting is the technique of breaking down a large JavaScript bundle into smaller chunks, loaded on demand. It helps improve application performance by reducing initial loading times.

4. What is the role of the Redux library in React applications?

Redux is a state management library for JavaScript applications, often used with React. It provides a predictable and centralized state container, making it easier to manage and update application state.

5. What is server-side rendering (SSR) in React?

Server-side rendering is the process of rendering React components on the server before sending the HTML to the client. It can improve performance and is beneficial for SEO.

6. How does React handle forms, and what are controlled components?

React handles forms using controlled components, where form elements are controlled by React state. This allows dynamic manipulation and validation of form data.

7. What is the purpose of the useRef hook in React?

useRef is a hook in React used for accessing and interacting with a DOM element or a mutable value without triggering re-renders. It is often used for imperative operations like focusing on an input element.

8. Explain the concept of lazy loading in React?

Lazy loading is the technique of loading components or resources only when they are needed, typically used with React’s React.lazy and Suspense to implement code splitting.

9. What is the significance of the useMemo hook in React?

useMemo is a hook in React used for memoization, optimizing the performance of expensive calculations by caching the result and only recomputing when the dependencies change.

10. How does error handling work in React?

Error handling in React is achieved through error boundaries, which are special components that catch JavaScript errors anywhere in their child component tree, log errors, and display a fallback UI.

import React, { Component } from 'react';

class ErrorBoundary extends Component {
  constructor(props) {
    super(props);
    this.state = {
      hasError: false,
      error: null,
      errorInfo: null,
    };
  }

  componentDidCatch(error, errorInfo) {
    this.setState({
      hasError: true,
      error: error,
      errorInfo: errorInfo,
    });

    // You can also log the error to an error reporting service here
    // For example: logErrorToMyService(error, errorInfo);
  }

  render() {
    if (this.state.hasError) {
      // You can render a fallback UI here
      return (
        <div>
          <h2>Something went wrong</h2>
          <p>{this.state.error && this.state.error.toString()}</p>
          <p>Component Stack Trace:</p>
          <pre>{this.state.errorInfo && this.state.errorInfo.componentStack}</pre>
        </div>
      );
    }

    // If no error occurred, render the child components as usual
    return this.props.children;
  }
}

class MyComponentWithError extends Component {
  render() {
    // Simulating an error in the render method
    throw new Error('Error in render method of MyComponentWithError');
  }
}

class App extends Component {
  render() {
    return (
      <ErrorBoundary>
        {/* ErrorBoundary will catch errors thrown by MyComponentWithError */}
  

11. Explain the purpose of the useReducer hook in React?

useReducer is a hook in React used for managing more complex state logic. It takes a reducer function and an initial state, returning the current state and a dispatch function.

12. What is the significance of the PureComponent in React?

PureComponent is a class component in React that performs a shallow comparison of its props and state to determine whether it should re-render. It can be useful for optimizing performance.

13. How does React handle routing in a single-page application (SPA)?

React handles routing in SPAs using libraries like React Router. It enables the creation of multiple views, manages the URL, and renders components based on the URL.

14. Explain the role of the key prop in React lists?

The key prop is used to uniquely identify and differentiate between elements in a React list. It helps React optimize rendering and update only the necessary components when the list changes.

15. What are the advantages of using functional components with hooks over class components in React?

Functional components with hooks are simpler, promote code reusability, and provide a more concise syntax. Hooks, like useState and useEffect, enable functional components to manage state and side effects.

16. How does React handle context?

React handles context through the React.createContext API, allowing values to be shared across components without explicitly passing props through each level of the component tree.

17. How can you optimize performance in React applications?

Performance optimization in React can be achieved through techniques such as code splitting, memoization (using React.memo and useMemo), using lazy loading, and minimizing unnecessary renders.

18. Explain the concept of the Portal in React?

Portals in React allow rendering children into a DOM node that exists outside the hierarchy of the parent component. It is useful for rendering components like modals that need to be positioned outside the usual parent-child relationship.

19. What is the significance of the render prop pattern in React?

The render prop pattern involves passing a function as a prop to a component, enabling dynamic content rendering. It is a flexible pattern often used for sharing code and logic between components.

20. How does React handle the uncontrolled component?

Uncontrolled components in React are those where the form data is handled by the DOM itself, rather than by React state. Refs are commonly used to interact with uncontrolled components.

21. What is the significance of the React DevTools extension?

React DevTools is a browser extension that provides tools for inspecting and debugging React components. It allows developers to visualize the component hierarchy, inspect props and state, and profile performance.

22. How can you improve the SEO of a React application?

Server-side rendering (SSR) can be used to improve the SEO of a React application by rendering the initial HTML on the server before sending it to the client. This ensures that search engines can crawl and index the content effectively.

React Interview Questions For 5 Years Experience

1. Explain the significance of React Hooks and how they differ from class components?

React Hooks are functions that enable functional components to use state and lifecycle features. Unlike class components, hooks allow for better code organization, reuse, and make it easier to manage component logic.

2. What are the benefits of using Redux in a React application, and when might you consider using it?

Redux is beneficial for managing complex state in larger applications, providing a predictable state container. It allows for centralized state management, easy debugging, and enables better separation of concerns. Consider using Redux when state management becomes challenging with just React’s local state.

3. Explain the purpose of React Portals and provide an example scenario where they might be useful?

React Portals allow rendering a child component into a DOM node outside of its parent component’s hierarchy. They are useful for scenarios like modals, tooltips, or any UI element that needs to break out of its container.

4. What is the Virtual DOM in React, and how does it contribute to performance optimization?

The Virtual DOM is an in-memory representation of the actual DOM. React uses it to optimize rendering by calculating the difference between the virtual and real DOM and updating only the changed parts, reducing unnecessary re-renders and improving performance.

5. How does React handle routing, and what are some common React Router features?

React Router is a library for handling navigation in React applications. It provides components like BrowserRouter for setting up routes and Link for navigation. Features include nested routes, route parameters, and code-splitting for better performance.

6. Explain the role of context in React, and in what scenarios might you use it?

Context in React allows the sharing of values like themes or authentication status across the component tree. It’s useful in scenarios where prop drilling becomes cumbersome, and you need to pass data to components at different levels.

7. What is the significance of the useEffect hook, and how does it handle side effects?

useEffect is used for handling side effects in functional components. It runs after each render and is often used for data fetching, subscriptions, or any code that involves interactions outside the component.

8. Explain the concept of code splitting in React and how it can improve application performance?

Code splitting involves breaking down a large JavaScript bundle into smaller chunks loaded on demand. This improves performance by reducing initial loading times, as only the necessary code is loaded when a specific feature is needed.

9. How does React handle forms, and what are controlled components?

React handles forms using controlled components, where form elements are controlled by React state. It allows for dynamic manipulation and validation of form data.

10. What are memoization and Pure Components in React, and how do they contribute to performance optimization?

Memoization involves caching the result of a function to avoid unnecessary recalculations. Pure Components perform shallow comparisons of props and state, preventing unnecessary renders and contributing to performance optimization.

11. Explain the purpose of the useRef hook in React, and provide an example scenario where it might be used?

useRef is used for accessing and interacting with a DOM element or a mutable value without triggering re-renders. It can be used for imperative operations like focusing on an input element or storing a mutable variable across renders.

12. What is server-side rendering (SSR) in React, and what are its advantages?

SSR is the process of rendering React components on the server before sending HTML to the client. Advantages include improved performance, better SEO, and faster initial page loads.

13. Explain the role of the useMemo hook in React, and in what scenarios might you use it?

useMemo is used for memoization, optimizing the performance of expensive calculations by caching the result. It’s useful when you want to avoid recomputing values in every render and only update them when dependencies change.

14. What is the purpose of the useReducer hook in React, and how does it differ from useState?

useReducer is used for managing more complex state logic. It takes a reducer function and an initial state, returning the current state and a dispatch function. It’s useful when state transitions are more intricate than what useState can handle.

15. Explain the concept of Higher Order Components (HOC) in React and provide a use case?

HOCs are functions that take a component and return a new component with additional features. A use case might be creating an HOC to handle authentication, wrapping components that require authentication checks.

16. How can you optimize the performance of a React application, and what tools might you use for profiling?

Performance optimization can involve code splitting, lazy loading, using React.memo and useMemo, and minimizing unnecessary renders. Tools like React DevTools and browser performance profiling tools can help identify performance bottlenecks.

17. Explain the purpose of the forwardRef function in React?

forwardRef is a feature in React that allows a component to pass its ref attribute to a child component. It is particularly useful when working with higher-order components or when a parent component needs access to a child component’s instance.

18. What are some considerations when choosing between using functional components and class components in React?

Functional components with hooks are preferred for simplicity, readability, and code organization. Class components might be necessary when working with lifecycle methods, third-party libraries, or when you need access to the this context.

19. Explain the significance of the render prop pattern in React and provide an example of its use?

The render prop pattern involves passing a function as a prop to a component, enabling dynamic content rendering. It’s useful for sharing code and logic between components. An example could be a DataProvider component that takes a render function as a prop to provide data to its children.

React Developers Roles ans Responsibilities

React developers play a crucial role in building modern and interactive user interfaces for web applications. Their responsibilities involve a range of tasks related to the development, optimization, and maintenance of React-based applications. Here are common roles and responsibilities for React developers:

1. Design and Implementation: Collaborate with UI/UX designers to implement responsive and visually appealing user interfaces. Translate design mockups and wireframes into functional React components.
2. Component Development: Develop reusable and modular React components following best practices. Implement state management using React Hooks or other state management libraries (e.g., Redux).
3. Application Architecture: Participate in the architectural design of React applications. Ensure code scalability, maintainability, and adherence to established coding standards.
4. Data Fetching and API Integration: Fetch data from APIs and integrate it into React applications. Handle asynchronous operations using technologies like Axios or the Fetch API.
5. Routing: Implement client-side routing using React Router or other routing libraries.
6. State Management: Manage component state effectively to ensure data consistency and synchronization. Implement state management solutions like Redux for complex applications.
7. Performance Optimization: Identify and implement performance optimization techniques, such as lazy loading, code splitting, and memoization. Optimize rendering to enhance application speed and responsiveness.
8. Testing: Write unit tests using testing libraries like Jest and testing utilities provided by React. Conduct and participate in code reviews to ensure code quality and catch potential issues.
9. Debugging: Debug and troubleshoot issues efficiently using browser developer tools and React DevTools. Identify and fix bugs or performance bottlenecks in a timely manner.
10. Collaboration: Work closely with backend developers to integrate frontend and backend functionality. Collaborate with cross-functional teams, including designers and product managers.
11. Version Control and Git: Use version control systems like Git to manage and track changes in the codebase. Collaborate effectively with team members using Git workflows.
12. Continuous Learning: Stay updated on React best practices, emerging trends, and new features. Share knowledge with the team and contribute to the improvement of development processes.
13. Documentation: Document code, APIs, and architectural decisions to facilitate collaboration and future maintenance.
14. Security Considerations: Implement security best practices to protect against common web application vulnerabilities.
15. Deployment: Deploy React applications to hosting platforms (e.g., AWS, Netlify, Vercel) and configure deployment pipelines.
16. Monitoring and Maintenance: Monitor application performance and respond to incidents or issues. Perform maintenance tasks, updates, and improvements to ensure long-term stability. Stay informed about security updates and patches relevant to React applications.

React developers may also specialize in certain areas, such as mobile development with React Native or server-side rendering with Next.js. The specific responsibilities can vary based on the project’s requirements and the developer’s expertise.

Frequently Asked Questions