React2

React Developer Interview Questions (10+ Years Experience)

1. Can you explain the virtual DOM and how React uses it for performance optimization?

The virtual DOM is a lightweight representation of the actual DOM. React creates a virtual DOM to minimize the number of changes to the actual DOM. When a state or props change, React first updates the virtual DOM. It then compares the virtual DOM with the previous version (reconciliation) and calculates the most efficient way to update the real DOM. This reduces the number of direct DOM manipulations, which can be expensive in terms of performance.

2. What are the key differences between React class components and functional components?

Class components are ES6 classes that extend React.Component and have lifecycle methods like `componentDidMount`, `shouldComponentUpdate`, etc. Functional components are simpler and were traditionally stateless but with the introduction of hooks, they can now manage state and lifecycle events. Functional components are preferred due to their simplicity and ease of testing, whereas class components can be more verbose.

3. How does React's reconciliation algorithm work?

React's reconciliation algorithm, also known as "Diffing," compares the new virtual DOM with the previous one to determine the minimal set of changes to be applied to the real DOM. React uses a heuristic approach by assuming that elements of the same type will have similar structures. It optimizes performance by updating only the parts of the DOM that have changed, rather than re-rendering the entire tree.

4. What is the purpose of hooks in React, and can you give an example of useState and useEffect?

Hooks are functions that let you "hook into" React's state and lifecycle features from functional components. `useState` allows you to add state to a functional component, and `useEffect` lets you perform side effects like fetching data or manually manipulating the DOM. Example:

                    const [count, setCount] = useState(0);
                    useEffect(() => {
                        document.title = `You clicked ${count} times`;
                    }, [count]);
                

5. What are controlled and uncontrolled components in React?

A controlled component is one where React is responsible for managing the component's state (e.g., form inputs). The value of the input is controlled by React, and any changes to it are handled by state updates. An uncontrolled component manages its own state, typically using the `ref` API, and React does not control the input's value directly.

6. How would you optimize a large React application for performance?

Performance optimizations in React include: 1. **Code splitting**: Load parts of the app only when needed using dynamic imports. 2. **React.memo**: Prevent unnecessary re-renders of functional components by memoizing them. 3. **useCallback & useMemo**: Memoize functions and values that don't need to change on every render. 4. **Lazy loading**: Load images or components only when they are visible on screen (e.g., using the `React.lazy()` function). 5. **Throttling and Debouncing**: Reduce the frequency of state updates, especially for user input or scroll events.

7. Can you explain the Context API and its usage in React applications?

The Context API is a way to pass data through the component tree without having to manually pass props down to each level. It's useful for global state management where data needs to be accessible by multiple components at different levels. Context is created using `React.createContext()` and accessed using `useContext()` hook or `Context.Consumer` in class components.

8. How do you manage state in a large-scale React application?

For large-scale applications, I typically use state management libraries such as Redux or Context API for global state management. For localized component state, I use the `useState` hook or `useReducer` for complex state logic. In addition, I adopt patterns like "lifting state up" to share state between components and consider tools like Redux Toolkit or Recoil for scalability and easier management.

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

Server-side rendering (SSR) involves rendering the React components on the server and sending the fully rendered HTML to the client. This improves the initial load time, SEO, and accessibility since search engines can crawl the fully rendered content. Frameworks like Next.js provide an easy way to implement SSR with React.

10. Can you explain the concept of higher-order components (HOCs) in React?

A higher-order component (HOC) is a function that takes a component and returns a new component with additional props or functionality. HOCs are used to reuse component logic without changing the original component. For example, an HOC could be used for adding authentication checks or managing state.

11. How does React's error boundary work? Can you explain the use of componentDidCatch?

An error boundary is a React component that catches JavaScript errors anywhere in its child component tree, logs those errors, and displays a fallback UI. This is done using the `componentDidCatch` lifecycle method in class components. Functional components can achieve the same using the `ErrorBoundary` pattern and the `useErrorBoundary` hook in libraries like `react-error-boundary`.

12. How would you handle authentication and authorization in a React app?

For authentication, I typically use token-based authentication (e.g., JWT) where the server generates a token that the client stores (e.g., in localStorage or sessionStorage). This token is sent with each request to authenticate the user. For authorization, I handle role-based access by checking the user's role and rendering components conditionally or redirecting to appropriate routes using React Router.

13. What are React’s lifecycle methods and how do you use them in class components?

React class components have lifecycle methods such as `componentDidMount`, `componentDidUpdate`, and `componentWillUnmount`. `componentDidMount` is used for actions after the component mounts, such as fetching data. `componentDidUpdate` is called after each re-render, useful for reacting to prop or state changes. `componentWillUnmount` is called before the component is removed from the DOM, often used for cleanup tasks.

14. Can you explain the concept of "lifting state up" in React?

Lifting state up in React refers to moving the state from a child component to the nearest common ancestor so that multiple components can share that state. This is usually done by passing down state and setter functions as props to child components, allowing them to update the state and propagate changes to other components that need it.

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

Redux is a predictable state container for JavaScript apps. It helps manage the application's state globally and ensures that components can access and update state in a predictable manner. In React, Redux is often used to manage global state through actions, reducers, and a central store. React components connect to the Redux store using the `connect` function or the `useSelector` and `useDispatch` hooks.

16. What are the key differences between Redux and React's Context API?

Redux is a more advanced and specialized state management solution for large-scale applications, supporting features like middleware (e.g., for async actions) and devtools integration. The Context API is more lightweight and built into React for passing state down the component tree, but it lacks some of the features of Redux, such as middleware and fine-grained control over updates.

17. How would you handle routing in a React application? Can you describe how React Router works?

React Router is a popular library for routing in React applications. It allows you to navigate between different components based on the URL. The `Router` component wraps the entire app, and routes are defined using the `Route` component. React Router uses a declarative approach to map components to specific routes, allowing for nested and dynamic routing.

18. What tools and strategies do you use for testing React components?

I use Jest for unit testing React components, along with React Testing Library for rendering components and interacting with the DOM. Jest provides built-in mocking and assertions, while React Testing Library encourages testing components from the user's perspective. For end-to-end testing, I use Cypress or Selenium.

19. Can you explain how code splitting works in React and why it's important?

Code splitting is the process of breaking up your JavaScript bundle into smaller chunks that can be loaded on demand. This reduces the initial load time of the app. React supports code splitting through `React.lazy` and `Suspense`, which allow components to be loaded asynchronously, ensuring that the app is faster to load and improves performance.

20. How do you approach accessibility (a11y) in React applications?

I approach accessibility by ensuring that components are keyboard navigable, using proper ARIA roles and attributes, and ensuring color contrast for readability. I also use tools like Axe or Lighthouse to audit my app and make improvements. It’s important to consider screen readers and other assistive technologies to make the app accessible to all users.