Top 10 React Engineer Interview Questions & Answers in 2024

1. How does React's virtual DOM contribute to the efficiency of web applications, and what are the key steps in the React rendering process?

The virtual DOM in React is an in-memory representation of the actual DOM. It optimizes rendering by minimizing direct manipulation of the DOM. When state or props change, React creates a new virtual DOM, compares it with the previous one, and updates only the changed elements in the real DOM. This process reduces unnecessary reflows and repaints, improving application performance.

2. Explain the significance of React Hooks and provide examples of when to use useState and useEffect in functional components.

React Hooks are functions that enable the use of state and lifecycle features in functional components. useState is used to declare state variables, while useEffect handles side effects in functional components. For example, useState might be used to manage a component's local state, and useEffect can handle actions like data fetching or subscriptions.

3. Discuss the differences between controlled and uncontrolled components in React. Provide scenarios where you might choose one approach over the other.

Controlled components in React are controlled by state, where their behavior is determined by React. Uncontrolled components rely on the DOM and handle state externally. Use controlled components when you need to manage component state in React, ensuring a single source of truth. Uncontrolled components are useful for integrating React with non-React code or when you want to leverage the existing DOM state.

4. How can you optimize the performance of a React application? Mention tools, techniques, and best practices for improving rendering speed.

Performance optimization in React involves using tools like React DevTools and auditing with Lighthouse for profiling. Techniques include code splitting, lazy loading, and optimizing the critical rendering path. Best practices include minimizing unnecessary renders, using PureComponent, and optimizing images. Tools like Webpack Bundle Analyzer help analyze and optimize the bundle size.

5. Explain the concept of Higher Order Components (HOCs) in React. Provide an example of when you might use an HOC.

Higher Order Components (HOCs) are functions that take a component and return a new enhanced component. They enable code reuse, logic extraction, and can modify the behavior of components. An example use case for an HOC is creating an authentication HOC that adds authentication logic to a component, restricting access based on user authentication status.

6. Discuss the benefits and challenges of using React context for state management in comparison to Redux.

React context provides a way to share values like themes or authentication status across components without prop drilling. However, it may not be suitable for complex state management needs. Redux offers a centralized store and actions, suitable for large-scale applications. Choose context for simpler scenarios, and Redux for more advanced state management requirements.

7. How does React Router work, and how can you implement client-side routing in a React application?

React Router enables client-side routing in React applications. It uses the history API to manipulate the browser's URL without triggering a full page reload. Components like BrowserRouter or HashRouter provide routing functionality. Use the Route component to define routes, and Link for navigation between them. This allows for seamless, single-page application navigation.

8. Discuss the concept of lazy loading in React. How can you implement lazy loading for components, and what benefits does it offer?

Lazy loading involves loading only the necessary parts of an application when needed, reducing the initial load time. In React, lazy loading can be achieved using the React.lazy function, which dynamically imports components. This is particularly useful for optimizing large applications by deferring the loading of less critical components until they are actually required, improving overall performance.

9. Explain the purpose of React Fragments and when you might use them in your components.

React Fragments allow grouping multiple elements without adding unnecessary nodes to the DOM. They are especially useful when you need to return adjacent JSX elements from a component without introducing extra container elements. Fragments improve the structure and cleanliness of the JSX code without affecting the rendered HTML structure.

10. How can you integrate React with external libraries or non-React code? Provide examples and considerations for such integrations.

Integrating React with external libraries or non-React code involves using refs to interact with DOM elements, lifecycle methods for initialization and cleanup, and using dangerouslySetInnerHTML for rendering raw HTML. For instance, embedding a D3 chart or integrating with a third-party JavaScript library requires careful consideration of the component lifecycle and potential side effects.