Deloitte Top 50 React JS Interview Questions and Answers

Prepare for your Deloitte React JS interview with these top 50 questions and answers, covering fundamental to advanced topics like hooks, state management, and performance optimization.

Basic Concepts

1. Define React and its purpose.

   • React is an open-source JavaScript library developed by Facebook for building user interfaces, primarily focusing on single-page applications where a seamless and dynamic user experience is required. It allows developers to construct UIs using reusable components that manage their own state. React simplifies the process of building and maintaining complex applications by breaking down the interface into smaller, manageable pieces (components). It promotes a component-based architecture, making it easy to develop, maintain, and scale user interfaces effectively.

2. What are the key characteristics of React?

   • Component-Based Architecture: Encourages the creation of encapsulated components that manage their own state.
   • Declarative Nature: Simplifies the process of designing interactive UIs by allowing developers to define simple views for each state.
   • Virtual DOM: Optimizes rendering performance through a lightweight representation of the actual DOM.
   • Unidirectional Data Flow: Ensures that data flows in a single direction, making it easier to understand and debug applications.

3. What is the virtual DOM, and how does it function?

   • The virtual DOM is a lightweight copy of the actual DOM. When changes occur in the UI, React first updates the virtual DOM, then compares it with the real DOM to identify the minimal changes needed for rendering, enhancing performance.

4. What are components in React?

   • Components are the fundamental building blocks of a React application, which can be either functional or class-based. They are reusable pieces of code that render a React element to the DOM.

5. Differentiate between functional components and class components.

   • Functional Components: Written as plain functions, can use hooks for state and lifecycle features, and are simpler to read.
   • Class Components: Require extending from React.Component, include lifecycle methods, and manage their own state.

6. What is JSX?

   • JSX (JavaScript XML) is a syntax extension for JavaScript that looks similar to HTML and is used in React to define UI structure within JavaScript code. It allows developers to write HTML-like elements directly within JavaScript, making it easier to visualize and build React components. JSX is not valid JavaScript, so it must be compiled (using tools like Babel) into JavaScript. Under the hood, JSX is converted into React.createElement() calls, creating the actual JavaScript objects that represent the component structure in the virtual DOM.

7. How can you set up a new React application?

   • A new React application can be created using Create React App by running:

   npx create-react-app my-app

8. Explain the concept of props in React.

   • Props (short for properties) enable data transfer from parent to child components. They are immutable and help in making components reusable by allowing dynamic values to be passed.

9. What is state in React, and how is it different from props?

   • State is an object that holds data that can change over the lifecycle of a component, whereas props are immutable and passed from parent to child components.

10. How can you manage state in a React application?

    • State can be managed locally within components using useState, or globally using the Context API or libraries like Redux.

Advanced Concepts

11. What are React Hooks?

    • React Hooks, introduced in React 16.8, are functions that let developers use state and other React features in functional components, removing the need for class components to manage state or lifecycle events. Hooks like useState and useEffect enable state management and side effects in a way that is simpler and more modular, promoting cleaner code and improved reusability. Hooks have revolutionized how React applications are built, allowing developers to leverage functional components without sacrificing features.

12. Describe the useEffect hook.

    • The useEffect hook handles side effects in functional components, replacing lifecycle methods. It runs after every render and can be configured with dependencies to control when it executes.

13. What is the purpose of useState?

    • The useState hook allows you to add state to functional components. It returns an array with two elements: the current state value and a function to update that value. This makes it possible to track and manage dynamic data within a functional component. useState is versatile, allowing you to initialize state with any data type, including objects and arrays. When the state is updated via the provided function, React triggers a re-render of the component, ensuring the UI reflects the latest state.

14. How are forms handled in React?

    • Forms in React are typically managed as controlled components, where form inputs are synchronized with the component’s state. By controlling form elements with state, you can enforce validation rules, modify input behavior dynamically, and handle form submissions more effectively. In a controlled form, each input field’s value is bound to a state variable, and every change triggers an update in the component’s state through an onChange event handler. This setup enables precise control over the form’s data and can be useful for tasks like real-time validation and conditional formatting.

15. What is the Context API in React?

    • The Context API allows for sharing values between components without manually passing props at every level, making it useful for global data like themes.

16. How can you enhance performance in a React application?

    • Performance can be improved by using React.memo, implementing lazy loading, optimizing images, and using techniques like shouldComponentUpdate.

17. What are higher-order components (HOCs)?

    • Higher-order components (HOCs) are functions that take a component as an argument and return a new component with enhanced functionality. HOCs add extra behavior or props to the wrapped component, promoting code reuse without altering the original component. For example, an HOC could handle authorization checks, inject props, or provide data fetching capabilities. Commonly used HOCs include tools like connect in Redux. Though React Hooks have reduced the need for HOCs, they are still useful in cases where hooks aren’t sufficient for reuse across components

18. Explain the render props pattern.

    • Render props is a technique for sharing code between components using a prop that is a function, allowing for greater flexibility and reusability.

19. Differentiate between controlled and uncontrolled components.

    • Controlled Components: Input elements are controlled by React's state.
    • Uncontrolled Components: Input elements maintain their own state, accessed via refs.

20. How do you implement routing in a React application?

    • React Router is commonly used for routing, allowing navigation between different views. Set it up by installing react-router-dom and wrapping your application in a Router.

Lifecycle Methods

21. What are the lifecycle methods in React?

• Lifecycle methods in React are special methods that allow you to hook into specific phases of a component’s life: mounting, updating, and unmounting. These methods give developers control over what happens when a component is created, updated, or destroyed.

  There are three main lifecycle phases:

  • Mounting: When the component is being inserted into the DOM (constructor, componentDidMount).
  • Updating: When the component is re-rendered due to state or props changes (shouldComponentUpdate, componentDidUpdate).
  • Unmounting: When the component is removed from the DOM (componentWillUnmount).

           With React hooks, lifecycle methods are replaced by useEffect.

22. Explain the componentDidMount method.

• The componentDidMount method is invoked immediately after a component is mounted. It is often used for tasks such as fetching data from an API or setting up subscriptions.

23. What is the purpose of shouldComponentUpdate?

• The shouldComponentUpdate method optimizes performance by controlling whether a component should re-render when its state or props change. It allows you to return true (to re-render) or false (to prevent re-rendering).

24. Describe the componentWillUnmount method.

• The componentWillUnmount method is called just before a component is removed from the DOM. It is commonly used for cleanup operations, such as clearing timers or unsubscribing from events.

25. How do you fetch data in a React component?

• Data can be fetched using the componentDidMount lifecycle method in class components or the useEffect hook in functional components, often utilizing the native fetch API or libraries like Axios.

Testing and Debugging

26. How do you test React components?

• React components can be tested using libraries like Jest and Enzyme, which provide tools for unit testing and component testing.

27. What is Jest?

• Jest is a JavaScript testing framework developed by Facebook, known for its simplicity, powerful mocking capabilities, and snapshot testing.

28. Explain the purpose of Enzyme.

• Enzyme is a testing utility for React that allows for shallow rendering of components, making it easier to test their behavior.

29. How do you handle errors in React applications?

• Errors can be handled using try-catch blocks, error boundaries, or error reporting tools like Sentry.

30. What tools do you use for debugging React applications?

• Several tools can be used for debugging, including React Developer Tools, browser developer tools, console.log for logging, and Redux DevTools for state management.

Performance Optimization

31. What is memoization in React?

• Memoization is an optimization technique that caches the results of expensive function calls and returns the cached result when the same inputs occur again, enhancing performance.

32. Explain lazy loading in React.

• Lazy loading delays the loading of components or resources until they are needed, optimizing performance by reducing initial load time.

33. How do you prevent unnecessary re-renders?

• Unnecessary re-renders can be prevented by using React.memo, memoizing calculations with useMemo, and avoiding inline functions in render methods.

34. What is the purpose of React.memo?

• React.memo is a higher-order component that prevents functional components from re-rendering when their props do not change, optimizing performance.

35. How do you use the useCallback hook?

• The useCallback hook memoizes callback functions in functional components, returning a memoized version that only changes if one of the dependencies has changed.

Styling and Theming

36. What are the different ways to style React components?

• Various methods include CSS stylesheets, inline styles, CSS Modules, styled-components, Emotion, and utility-first frameworks like Tailwind CSS.

37. Explain CSS Modules.

• CSS Modules allow for modular and scoped styles, ensuring that class names are unique to the component, preventing conflicts.

38. How do you implement theming in a React application?

• Theming can be implemented using the Context API, styled-components, or CSS variables to provide theme values throughout the component tree.

39. What is styled-components?

• Styled-components is a library for styling React applications using tagged template literals, allowing for scoped styles and dynamic styling based on props.

40. How do you handle responsive design in React?

• Responsive design can be managed using CSS media queries, utility-first libraries like Tailwind CSS, or responsive design libraries like React-Grid-Layout.

Ecosystem and Tools

41. What is Redux, and how does it work with React?

• Redux is a state management library that provides a central store for application state, using actions and reducers to manage state changes in a predictable way.

42. Explain the concept of middleware in Redux.

• Middleware enhances the Redux store by allowing you to intercept actions before they reach the reducer, useful for logging or handling asynchronous actions.

43. What is React Router?

• React Router is a library for routing in React applications, enabling navigation and rendering of different components based on the URL.

44. How do you manage side effects in React applications?

• Side effects in React can be managed using the useEffect hook for functional components, Redux Thunk for asynchronous actions, or custom hooks to encapsulate side effect logic.

45. What are some popular libraries used with React?

• Popular libraries include Redux for state management, React Router for navigation, Axios for HTTP requests, styled-components for CSS-in-JS, Formik for form handling, and React Query for data fetching and caching.

Miscellaneous

46. What are the differences between React and Angular?

• Library vs. Framework: React is a library focused on the view layer, while Angular is a full-fledged framework providing a complete solution for building applications.
• Data Binding: React uses one-way data binding, whereas Angular supports two-way data binding.
• State Management: React relies on component state and external libraries, while Angular has built-in services for state management.
• Learning Curve: React has a steeper learning curve due to its reliance on JavaScript, while Angular offers a more structured approach.
• Performance: React generally performs better in dynamic applications due to its virtual DOM, while Angular may face challenges in larger applications.
• Ecosystem: React has a larger ecosystem with many third-party libraries, while Angular provides a more cohesive experience with built-in features.

47. How do you handle authentication in a React application?

• Authentication can be managed using libraries like Auth0, Okta, or Firebase, or by implementing custom authentication logic.

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

• Server-Side Rendering (SSR) generates HTML for a web page on each request, improving performance and SEO by delivering fully rendered pages to the client.

49. Explain the concept of progressive web apps (PWAs) in relation to React.

• Progressive Web Apps (PWAs) combine the best of web and mobile apps, providing features like offline access, push notifications, and fast loading times, enhancing user experience.

50. What are some common performance pitfalls in React applications?

• Common pitfalls include unnecessary re-renders, large component trees, inefficient state management, heavy computation in render methods, frequent updates, blocking the main thread, inefficient event handlers, and not utilizing React's concurrent features.