Top 40 React Interview Questions and Answers for 2024

React.js has become one of the most popular JavaScript libraries for building user interfaces, and with its widespread adoption, React developers are highly sought after. As 2024 approaches, many companies continue to prioritize front-end developers skilled in React, especially those who have a deep understanding of its core concepts like hooks, lifecycle methods, and state management. If you’re preparing for a React interview, this comprehensive guide will equip you with the top 40 interview questions and answers for React in 2024.

What is React.js?

React.js is a JavaScript library developed by Facebook, which is used to build user interfaces, particularly single-page applications. It allows developers to create reusable UI components that efficiently update and render data when the underlying state changes. Its component-based architecture and virtual DOM optimization are two key reasons why React has gained popularity.

1. What are the core features of React?

Some of the core features of React include:

• JSX: JavaScript syntax extension that allows writing HTML-like code within JavaScript.
• Components: Building blocks of React applications, making code reusable and modular.
• Virtual DOM: React uses a virtual DOM to optimize updates and improve performance by reducing direct manipulations of the real DOM.
• Unidirectional Data Flow: Data flows in one direction, making state management predictable.
• Hooks: Functions that allow you to “hook into” React state and lifecycle features in functional components.

2. What is the Virtual DOM in React, and how does it work?

The Virtual DOM is an in-memory representation of the actual DOM elements in the browser. React uses the Virtual DOM to minimize the number of changes made to the real DOM, improving performance. When the state of an application changes, React first updates the Virtual DOM. It then compares the Virtual DOM with the previous version, finds the differences (diffing), and only updates the changed elements in the actual DOM.

3. What is JSX, and why is it used in React?

JSX (JavaScript XML) is a syntax extension for JavaScript, which allows developers to write HTML-like code directly inside JavaScript. JSX makes it easier to visualize the UI structure in React components. Under the hood, JSX gets compiled into JavaScript functions (React.createElement), making it more readable and easier to manage complex UIs.

4. What is the difference between a functional component and a class component in React?

• Functional Components: These are stateless components, which are written as JavaScript functions and focus solely on rendering UI based on props.
• Class Components: Class components extend React.Component and have access to state and lifecycle methods. They are more powerful but have become less common since the introduction of Hooks, which allow functional components to use state and other React features.

5. What are React Hooks, and why were they introduced?

React Hooks are functions that let you use state and lifecycle features in functional components. Hooks were introduced in React 16.8 to allow developers to use state and other React features without needing to write class components. The most commonly used hooks are:

• useState: Manages state in functional components.
• useEffect: Manages side effects like fetching data or updating the DOM.
• useContext: Provides a way to pass data through the component tree without manually passing props.

React State Management Questions

State management is a crucial aspect of React development, ensuring that changes in data reflect in the UI.

6. What is the difference between state and props in React?

• State: Represents dynamic data that can change over time. It is managed within a component and can change based on user interaction or other factors.
• Props: Short for properties, props are read-only and are passed from parent components to child components to configure or display dynamic content. Unlike state, props cannot be modified by the receiving component.

7. How do you manage state in functional components?

State in functional components is managed using the useState Hook. This hook returns an array with two elements: the current state value and a function to update the state. Here’s a simple example:

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

The first value, count, holds the state, and setCount is the function to update it.

8. What is the Context API, and when would you use it?

The Context API in React provides a way to share values like state or functions between components without passing props down manually at every level (also known as “prop drilling”). It is useful for managing global data, such as theme settings or user authentication information, across the entire application.

9. How do you manage side effects in React?

Side effects in React, such as fetching data or interacting with the browser’s DOM, are managed using the useEffect Hook. This hook runs after the component renders and can be configured to run only when certain dependencies change, or it can run only once after the initial render.

10. What is Redux, and how does it integrate with React?

Redux is a state management library that provides a centralized store for managing application state. In Redux, state is stored in a single object, and components can access or update this state by dispatching actions. Redux integrates with React through the react-redux library, which provides the Provider component to pass the Redux store to React components and the connect function or useSelector hook to access state.

Lifecycle Methods and useEffect Hook

React components go through various phases in their lifecycle, and understanding these phases is crucial to handling side effects efficiently.

11. What are React lifecycle methods?

Lifecycle methods are special methods in class components that are invoked at different stages of a component’s life in React. The three main phases are:

• Mounting: When the component is first created (e.g., componentDidMount).
• Updating: When the component is re-rendered due to state or prop changes (e.g., componentDidUpdate).
• Unmounting: When the component is removed from the DOM (e.g., componentWillUnmount).

12. How does the useEffect Hook differ from lifecycle methods in class components?

The useEffect Hook in functional components combines the behavior of multiple lifecycle methods found in class components. It is used to handle side effects like fetching data, updating the DOM, and managing timers. The useEffect Hook can mimic componentDidMount, componentDidUpdate, and componentWillUnmount by controlling its dependencies.

13. Can you explain the useEffect dependency array?

The dependency array in useEffect is the second argument passed to the hook. It allows you to specify when the effect should re-run. If no dependencies are provided, the effect runs after every render. If an empty array ([]) is provided, it runs only once after the component mounts, simulating the behavior of componentDidMount. When specific values are added to the array, the effect will re-run only when those values change.

14. How do you clean up side effects in React using useEffect?

To clean up side effects, useEffect can return a function, which React will call before the component unmounts or before the effect re-runs in subsequent renders. This is useful for cleaning up timers, subscriptions, or event listeners. For example:

useEffect(() => {
  const timer = setInterval(() => {
    console.log("Interval running");
  }, 1000);
  
  return () => clearInterval(timer); // Cleanup function
}, []);

15. What are controlled and uncontrolled components in React?

• Controlled Components: In a controlled component, the form data is handled by the React component’s state. For example, an input field’s value is controlled via a value prop and an onChange event handler.
• Uncontrolled Components: Uncontrolled components use the browser’s default behavior for form elements. You can access their values through refs instead of controlling them with state.

React Hooks and Advanced Concepts

In 2024, Hooks continue to play a critical role in React development. Expect several questions on Hooks, including custom Hooks and performance optimizations.

16. What is useMemo, and how does it optimize performance?

The useMemo Hook is used to memoize the result of a computation to avoid recalculating it on every render unless its dependencies change. It is useful for performance optimization, especially when dealing with expensive computations or large datasets. Here’s an example:

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

17. What is the useCallback Hook, and when should you use it?

useCallback is similar to useMemo, but instead of memoizing a value, it memoizes a function. It is used to prevent the recreation of functions on every render, which can help optimize performance when passing callbacks to child components.

const memoizedCallback = useCallback(() => doSomething(), [dependency]);

18. What are custom Hooks, and how do you create them?

Custom Hooks are functions that allow you to extract and reuse stateful logic across multiple components. A custom Hook is simply a JavaScript function whose name starts with use, and it can call other hooks within it. For example:

function useWindowWidth() {
  const [width, setWidth] = useState(window.innerWidth);

  useEffect(() => {
    const handleResize = () => setWidth(window.innerWidth);
    window.addEventListener('resize', handleResize);
    
    return () => window.removeEventListener('resize', handleResize);
  }, []);
  
  return width;
}

This useWindowWidth custom Hook can now be reused across components that need access to the window’s width.

19. What is the purpose of the useReducer Hook?

The useReducer Hook is an alternative to useState for managing complex state logic. It is especially useful when the state depends on multiple actions or when the next state depends on the previous state. useReducer takes a reducer function and an initial state and returns the current state and a dispatch function to update it.

20. What is the difference between useEffect and useLayoutEffect?

While both hooks are used for handling side effects, useLayoutEffect fires synchronously after all DOM mutations but before the browser paints, making it ideal for reading layout information or performing DOM measurements. In contrast, useEffect runs asynchronously after the render and the browser has painted, making it better for tasks like data fetching or updating external systems.

Advanced React Interview Questions

For experienced React developers, interviews often include more in-depth questions that test knowledge of advanced concepts, performance optimizations, and handling specific use cases. As you prepare for a React interview in 2024, expect questions related to state management strategies, rendering patterns, and more sophisticated hooks.

21. What are Pure Components in React?

A Pure Component in React is a class component that implements shouldComponentUpdate() with a shallow comparison of the component’s state and props. This means that the component only re-renders if the shallow comparison detects changes. Pure Components are used to improve performance by preventing unnecessary re-renders.

Functional components can achieve similar behavior using React.memo, which performs a similar shallow comparison to decide whether a component needs to re-render.

const MyComponent = React.memo((props) => {
  // Component code
});

22. What is the significance of React.memo in functional components?

React.memo is a higher-order component that optimizes functional components by preventing unnecessary re-renders. It memoizes the rendered output of a functional component and re-renders it only if its props change. This is similar to how PureComponent works for class components.

Here’s an example of using React.memo:

const MemoizedComponent = React.memo((props) => {
  return <div>{props.value}</div>;
});

React.memo is particularly useful when components rely heavily on props that do not frequently change, reducing the number of unnecessary renders and improving performance.

23. What are Error Boundaries in React, and how do they work?

Error boundaries are special components in React that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of crashing the entire application. Error boundaries can only catch errors during rendering, lifecycle methods, and constructors in the child components.

An error boundary can be created using a class component by implementing componentDidCatch and getDerivedStateFromError.

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError(error) {
    // Update state to show the fallback UI
    return { hasError: true };
  }

  componentDidCatch(error, errorInfo) {
    // Log the error or send it to an error reporting service
    console.error("Error caught by ErrorBoundary:", error, errorInfo);
  }

  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }
    return this.props.children;
  }
}

24. How does server-side rendering (SSR) work in React, and why would you use it?

Server-Side Rendering (SSR) is a technique in React where the HTML for a component is generated on the server and sent to the client. This can improve performance and SEO because the content is immediately available on page load. Tools like Next.js provide built-in support for SSR in React applications.

With SSR, when a user requests a page, the server renders the React components to HTML and sends the fully rendered HTML to the client. The browser then downloads the JavaScript, and React takes over to manage any interactions on the client-side, a process called “hydration.”

25. What is code splitting in React, and how does it improve application performance?

Code splitting allows you to break your React application into smaller bundles that can be loaded on demand. By splitting code, you can avoid loading the entire application at once, reducing the initial load time and improving performance, especially in large applications.

React implements code splitting using dynamic import() statements and tools like React.lazy and Suspense.

Here’s an example of how code splitting works with React.lazy:

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

function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <LazyComponent />
    </Suspense>
  );
}

The Suspense component renders a fallback UI while the LazyComponent is being loaded.

26. How do you handle forms in React?

Handling forms in React typically involves two approaches: controlled components and uncontrolled components.

• Controlled Components: In a controlled component, form data is handled by the component’s state. The value of form elements like <input> or <textarea> is tied to the component’s state, and any changes trigger an onChange handler that updates the state.

const [inputValue, setInputValue] = useState('');

const handleChange = (e) => {
  setInputValue(e.target.value);
};

return <input value={inputValue} onChange={handleChange} />;

• Uncontrolled Components: Uncontrolled components rely on the DOM to manage form data. You access the form data using refs, which allows you to retrieve form values without keeping them in the component state.

const inputRef = useRef(null);

const handleSubmit = () => {
  console.log(inputRef.current.value);
};

return <input ref={inputRef} />;

27. What is reconciliation in React?

Reconciliation is the process React uses to update the DOM. When a component’s state or props change, React compares the new Virtual DOM with the previous one. This process, known as diffing, helps React determine the minimal number of changes required to update the actual DOM efficiently.

React uses a heuristic algorithm to make this process fast by assuming that components of different types produce different trees. Therefore, React will destroy and recreate components if their types change, but will re-use the existing DOM nodes if the component type remains the same.

28. What is useRef, and how is it different from useState?

useRef is a hook in React that allows you to create a mutable object whose .current property persists across renders. Unlike useState, changes to the .current property of useRef do not cause re-renders, making it useful for storing references to DOM elements or mutable values that don’t require updates to the UI.

const inputRef = useRef(null);

const handleFocus = () => {
  inputRef.current.focus();
};

return <input ref={inputRef} />;

In this example, inputRef is used to directly access the DOM element without causing a re-render when it is updated.

29. How do you handle performance optimization in React applications?

Optimizing performance in React applications can be done in several ways:

• Memoization: Use React.memo to prevent unnecessary re-renders of components.
• Lazy loading: Dynamically import components using React.lazy and Suspense.
• useMemo and useCallback: Use these hooks to memoize values and functions to avoid unnecessary re-calculations or function creations.
• Key Prop: Always provide a unique key when rendering lists of elements, which helps React optimize re-renders.
• Code splitting: Split the application code into smaller bundles to reduce initial load time.
• Avoiding anonymous functions in render: Refrain from declaring inline anonymous functions inside JSX to prevent re-rendering of child components.

30. What are Portals in React?

Portals allow you to render components outside of the DOM hierarchy defined by the parent component. They are useful for rendering components like modals or tooltips that need to escape the boundaries of their parent component for styling or positioning purposes.

Here’s an example:

ReactDOM.createPortal(
  <div>Modal Content</div>,
  document.getElementById('modal-root')
);

This will render the modal content into a separate DOM node, even though it appears in the context of the React component tree.

Testing in React

Testing is an important part of React development, and questions about tools and strategies for testing React applications are commonly asked.

31. How do you test a React component?

React components can be tested using libraries like Jest and React Testing Library. Jest is a testing framework that provides utilities for running unit tests, while React Testing Library focuses on testing the component’s behavior from the user’s perspective rather than implementation details.

Example using Jest with React Testing Library:

import { render, screen } from '@testing-library/react';
import MyComponent from './MyComponent';

test('renders component with text', () => {
  render(<MyComponent />);
  expect(screen.getByText(/hello world/i)).toBeInTheDocument();
});

32. What is snapshot testing in React?

Snapshot testing captures the rendered output of a React component and saves it as a snapshot file. Each time the component renders, Jest compares the current output to the saved snapshot. If the output changes unexpectedly, the test fails, alerting developers of the change.

Here’s an example of snapshot testing:

test('renders component correctly', () => {
  const tree = renderer.create(<MyComponent />).toJSON();
  expect(tree).toMatchSnapshot();
});

33. What is Enzyme, and how does it differ from React Testing Library?

Enzyme is a testing utility for React that allows you to manipulate, traverse, and simulate behavior in React components. It offers more control over component internals, such as shallow rendering, and is often used for unit testing of individual components.

In contrast, React Testing Library encourages testing components as the user would interact with them, focusing more on behavior than implementation details. React Testing Library is gaining popularity due to this “user-first” approach.

Frequently Asked Questions (FAQs)

How does useState differ from useReducer for managing state?

What are controlled and uncontrolled components in React?

How do React Hooks simplify functional component development?

What is the Virtual DOM, and why does React use it?

How does code splitting with React.lazy improve performance?

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

Conclusion

Preparing for a React interview in 2024 requires a solid understanding of the core concepts, as well as advanced topics like state management, hooks, and performance optimization. The top 40 React interview questions in this guide provide a comprehensive overview of the most important topics you’ll encounter, giving you the edge you need to succeed. By mastering these concepts, you’ll be ready to confidently tackle any React interview question that comes your way.