Back

Top 40+ React Interview Questions and Answers

23 Oct 2024
5 min read

In the fast-paced world of web development, React has emerged as a popular library for building user interfaces. Mastering React coding interview questions is essential for developers looking to excel in technical interviews. This article provides a comprehensive overview, including basic concepts, common questions, and tips for success.

Overview of React

React is a JavaScript library developed by Facebook for creating dynamic web applications. It enables developers to create reusable UI components, efficiently manage state, and update the user interface effectively. Understanding the core principles of React, such as components, props, state, and lifecycle methods, is crucial for React interview coding questions.

Top 32 React Interview Questions with Answers

Here are the React questions asked in the interviews:

1. What is React?

React is a JavaScript library for building user interfaces, primarily for single-page applications. It utilizes a component-based architecture, allowing developers to create reusable UI components that manage their state.

2. What are the Components in React?

Components serve as the fundamental elements of a React application. They encapsulate the UI and the logic needed to manage that UI, making them reusable and modular. Components can be either class-based or functional. Class components manage their state and lifecycle through methods, while functional components utilize hooks for similar functionality. This architecture enhances maintainability, scalability, and collaboration among developers.

3. What is JSX?

JSX (JavaScript XML) is a syntax extension that enables developers to manage HTML-like code directly within JavaScript. It improves code readability and enables a clearer visualization of the UI structure directly in the code. JSX is transformed into JavaScript function calls that create React elements, making it a widely adopted approach for building UIs in React.

4. What are Lifecycle Methods?

Lifecycle methods are specific functions in class components that allow developers to hook into key moments in a component's lifecycle, such as when it mounts, updates, or unmounts. For example, componentDidMount() is used for operations like data fetching after the component is added to the DOM. It is useful for many developers who are transitioning to functional components with hooks like useEffect() for managing side effects.

5. What is State in React?

A state in react is an object that holds dynamic data within a react component. They are immutable and passed from parent to child. It is managed internally and changes responses for user interactions or events. When the state changes, react re-renders the component to reflect these updates in the UI, which is crucial for building interactive applications.

6. What is the Purpose of useEffect()?

The useEffect() hook is crucial for handling side effects in functional components. It allows developers to perform actions like data fetching or subscriptions after every render or based on specific dependencies. Separating side-effect logic from rendering logic, leads to clear and more maintainable code.

7. What is Redux?

Redux is a state management library commonly used with React for managing complex application states. It centralizes the state in a single store to utilize actions, and reducers to modify that state. This structured approach simplifies global state management and enhances debugging, especially in larger applications.

8. What is Context in React?

Context offers a method to share data throughout the component tree without the need to manually pass props at each level. It’s especially useful for global data such as themes or authentication states. By creating a context provider, developers can encapsulate data and make it accessible to any component that needs it, reducing the complexity of prop drilling.

9. How do you Handle Errors in React?

Error boundaries are components that capture JavaScript errors from their child components, allowing the user interface to degrade smoothly. They enable developers to display fallback UIs or log errors, preventing the entire application from crashing and enhancing the user experience.

10. What is server-side rendering?

Server-side rendering (SSS) is a technique is a technique where the server renders the initial HTML of a React application. This improves performance and SEO by providing fully rendered pages to users quickly, reducing the time to enhance the user experience, especially on slower networks.

11. What is the Purpose of useMemo?

The useMemo hook is used to optimize performance in functional components by memoizing expensive calculations. It returns a memoized value that is recalculated only when its dependencies change, preventing unnecessary recalculations during re-renders, which is beneficial for performance, especially with complex computations.

12. What is Prop Drilling?

Prop drilling occurs when data is passed down through multiple layers of components, which can complicate the code and make it harder to manage. This often leads to a situation where a component needs to pass value in proportions to its child components just to get data to a deeper nested component. The Context API can help to solve these issues by allowing data to be shared directly among components without the need to pass react components through an interface.

13. How do you Achieve Lazy Loading in React?

Lazy loading can be implemented using react.lazy and Suspense to load components only when they are needed, improving initial load time and performance.

14. What are Fragment Components?

Fragments allow developers to group several elements without inserting extra nodes into the DOM. This helps to keep the DOM tree clean and optimize rendering.

15. How do you Handle API Calls in React?

API calls are typically managed using the useEffect() hook to fetch data when a component mounts. This ensures that the component reflects the latest data from the server and can trigger updates based on user interactions or state changes. Using async/await within useEffect() allows for clearer, more readable code when dealing with asynchronous operations.

16. What are React DevTools?

React DevTools is a browser extension that provides developers with tools to inspect the React component hierarchy, analyze performance, and debug applications more efficiently. It allows users to view component props and state, profile component rendering, and track updates, making it an invaluable tool for React development.

17. What is the Significance of the React Key Prop?

The key prop is essential for helping React identify which items in a list have changed, been added, or removed. Keys must be unique among them and should be stable to ensure efficient updates. Proper use of keys enhances performance by minimizing unnecessary rendering, allowing React to handle changes efficiently in the DOM.

18. What is a Higher-Order Component (HOC)?

A higher-order component (HOC) is a function that takes a component and returns a new component with enhanced functionality. HOCs are commonly used for multiple concerns such as data fetching, authentication, or logging. This pattern promotes code reusability by allowing shared behavior to be abstracted away from the individual components.

19. What is the Purpose of useReducer()?

The useReducer() hook is used for managing complex logic in functional components. It is similar to Redux. This hook is particularly useful for managing state transitions that depend on multiple values or require complex logic. By using useReducer, developers can keep their state management organized and predictable.

20. What is the Purpose of useMemo?

The useMemo hook is used to memoize expensive computations, preventing unnecessary recalculations during re-renders and optimizing performance.

21. What is the Significance of the React Key Prop?

The key prop is essential for helping React identify which items in a list have changed, ensuring efficient updates, and minimizing unnecessary rendering.

22. What is the purpose of React Fragment?

React Fragment allows you to group a list of children without adding extra nodes to the DOM. It helps avoid unnecessary wrapper elements, leading to cleaner markup and better performance. You can use syntax <>...</> for fragments as well.

23. What are PropTypes in React?

PropTypes are a way to validate the types of props passed to a component. They help catch bugs by ensuring that the correct data types are used and providing warnings in development mode.

24. How does React differ from Angular?

Here is the comparison of React and Angular:

  • Angular is developed by Google, while React is developed by Facebook.
  • Angular is a complete MVC framework, whereas React focuses on the view layer.
  • Angular uses two-way data binding, while React uses one-way data binding.

25. How do you write comments in React?

In React, you can write comments using two formats:

  • Single-line comments: // This is a comment
  • Multi-line comments: {/* This is a multi-line comment */}

26. . What do you mean by an arrow function, and how is it used in React?

In Javascript, an arrow function is a concise way to write functions. In React, they are particularly useful because they do not bind their own, and preventing issues in callbacks.

27. How is React different from React Native?

Here is the comparison of react and react native:

  • React is for web applications, while React Native is for mobile applications (iOS and Android).
  • React uses HTML, whereas React Native uses native components.
  • React uses CSS, while React Native uses a styling system that resembles CSS but with different properties.

28. Explain how lists work in React.

Lists in React are created using the map() function to iterate over an array of data, generating a set of elements. Each element in the list should have a unique key prop to help React identify and manage updates efficiently.

29. Why is there a need for using keys in lists?

Keys are essential for identifying which items in a list have changed, been added, or removed. They help React optimize rendering by allowing it to re-render only the changed components rather than the entire list.

30. What are synthetic events in React?

Synthetic events are React’s cross-browser wrapper for native events. They provide a consistent API and help in normalizing event properties across different browsers, ensuring compatibility.

31. How do you create a React app?

  • Install Node.js to use npm.
  • Install the create-react-app package via the command prompt.
  • Set up a text editor like VS Code or Sublime Text.
  • Create a new app using the command npx create-react-app my-app.

32. How do you create an event in React?

You create an event in React by defining an event handler function and attaching it to an element using JSX. For example:

<button onClick={this.handleClick}>Click me</button>

Advanced React Interview Questions

Here are the advanced questions of react asked in interviews:

33. What is the Difference Between Controlled and Uncontrolled Components?

Controlled components depend on the state to manage their values, meaning that form inputs are controlled by React's state. This approach provides better data flow, validation, and easier management of form state. While uncontrolled components store their values in the DOM, allowing the input elements to maintain their state.

34. How to modify the counter to include a reset button that sets the count back to zero?

function Counter() {
  const [count, setCount] = useState(0);
  
  return (
    <div>
      <p>{count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <button onClick={() => setCount(count - 1)}>Decrement</button>
    </div>
  );
}

35. Explain the difference between Props and State.

Props are read-only data passed from parent components to child components, allowing for data flow and communication. While the state is mutable and managed within a component. It is used to track local component data that can change over time, triggering re-renders when updated.

36. How to implement a feature to mark a to-do item as completed and style it differently?

function TodoApp() {
  const [todos, setTodos] = useState([]);
  const [input, setInput] = useState("");

  const addTodo = () => {
    setTodos([...todos, input]);
    setInput("");
  };

  return (
    <div>
      <input value={input} onChange={(e) => setInput(e.target.value)} />
      <button onClick={addTodo}>Add</button>
      <ul>
        {todos.map((todo, index) => (
          <li key={index}>{todo}</li>
        ))}
      </ul>
    </div>
  );
}

37. How to handle authentication in a React application?

Authentication can be handled using context or state management libraries like Redux to store user data. You can use higher-order components to wrap protected routes, checking for authentication before rendering components. Implementing JWT tokens for API requests is also a common practice.

38. What is the difference between static and dynamic routing in React Router?

Static routing refers to predefined routes that do not change, while dynamic routing allows routes to be defined at runtime based on data. Dynamic routing can be achieved using route parameters and can change based on application state or API responses.

39. What is the React Synthetic Event System?

React's synthetic event system is a cross-browser wrapper around the browser's native event system. It normalizes events so that they have consistent properties across different browsers. Synthetic events are pooled for performance, meaning that their properties are reused, which is why you cannot access event properties asynchronously without calling event.persist().

40. How to handle loading and error states when fetching data from the API?

function FetchData() {
  const [data, setData] = useState([]);

  useEffect(() => {
    fetch("https://jsonplaceholder.typicode.com/posts")
      .then(response => response.json())
      .then(data => setData(data));
  }, []);

  return (
    <ul>
      {data.map(item => (
        <li key={item.id}>{item.title}</li>
      ))}
    </ul>
  );
}

41. What are custom hooks, and how are they created?

Custom hooks are JavaScript functions that use React hooks and encapsulate reusable logic. They can take arguments and return values. You create a custom hook by defining a function that starts with "use" and inside it, you can use other hooks as needed. For example, function useFetch(url) { ... }.

Tips for Cracking React Coding Interview Questions

Here are some effective strategies to help you succeed in your React coding interviews:

  • Make sure you have a strong grasp of the basics of React and its core principles.
  • Use platforms like LeetCode or CodeChef to practice react coding interview questions and react interview coding challenges.
  • Create small projects to solidify your understanding of concepts and showcase your skills.
  • Familiarize yourself with advanced concepts and react js technical interview questions to stand out.
  • Attend mock interviews to improve your confidence and presentation.

Conclusion

In conclusion, React coding interviews can be challenging, but thorough preparation and practice can significantly increase your chances of success. Focus on understanding both basic and advanced concepts, and don’t hesitate to build projects that highlight your skills.

Read More Articles

Chat with us
Talk to career expert