React.js Interview Questions and Answers for 5 years experience

1. What is React.js and why is it used?

   • Answer: React.js is a JavaScript library for building user interfaces (UIs), primarily for single-page applications. It's used because of its component-based architecture, which promotes reusability and maintainability. Its virtual DOM allows for efficient updates, resulting in faster performance. Its large community and extensive ecosystem provide abundant resources and support.

2. Explain the concept of JSX.

   • Answer: JSX (JavaScript XML) is a syntax extension to JavaScript that allows you to write HTML-like code within your JavaScript. It makes React code more readable and easier to understand, especially for developers familiar with HTML. JSX is not required to use React, but it's highly recommended and widely used.

3. What are components in React?

   • Answer: Components are reusable building blocks of a React application. They can be functional components (simple JavaScript functions) or class components (ES6 classes extending React.Component). They encapsulate UI elements and their associated logic, making code modular and maintainable.

4. Describe the difference between functional and class components.

   • Answer: Functional components are simpler, typically used for presentational logic and often use hooks for state and lifecycle management. Class components, using ES6 classes, offer lifecycle methods (like `componentDidMount`, `componentDidUpdate`, `componentWillUnmount`) for managing side effects and complex state updates. Functional components with hooks are now the preferred approach for most use cases.

5. Explain the concept of props and state in React.

   • Answer: Props (properties) are immutable values passed from a parent component to a child component. They are used to configure the child component. State, on the other hand, is mutable data managed within a component. Changes to state trigger re-renders of the component and its children.

6. How does state management work in React?

   • Answer: In simple applications, state can be managed directly within components using `useState` hook (functional components) or `this.setState` (class components). For larger applications, dedicated state management libraries like Redux, Zustand, Context API, or Recoil are used to manage complex state efficiently across multiple components.

7. What is the virtual DOM?

   • Answer: The virtual DOM is a lightweight in-memory representation of the actual DOM. React uses it to efficiently update the real DOM only when necessary. By comparing the virtual DOM with the previous version, React identifies minimal changes and applies only those updates, significantly improving performance.

8. Explain the lifecycle methods of a class component.

   • Answer: Class components have several lifecycle methods that allow you to perform actions at various stages of a component's existence (mounting, updating, and unmounting). Key methods include `componentDidMount` (after initial render), `componentDidUpdate` (after an update), `componentWillUnmount` (before unmounting), `render` (responsible for rendering the UI), and others. Functional components use hooks for similar functionality.

9. What are React hooks?

   • Answer: React Hooks are functions that let you “hook into” React state and lifecycle features from within functional components. They provide access to features previously only available in class components, such as state management (`useState`), side effects (`useEffect`), and context (`useContext`).

10. Explain `useEffect` hook.

    • Answer: The `useEffect` hook allows you to perform side effects in functional components. Side effects include data fetching, DOM manipulations, subscriptions, and timers. It takes a function as an argument and an optional array of dependencies. The function runs after every render unless dependencies are specified, in which case it only runs when those dependencies change.

11. Explain how to optimize React application performance.

    • Answer: Optimizing React app performance involves various techniques like using `React.memo` for memoizing components, using `useCallback` to prevent unnecessary re-renders, implementing code splitting for lazy loading, optimizing images, using a virtualized list for large datasets, profiling the application using React DevTools, and minimizing unnecessary state updates.

12. Describe different ways to handle routing in React.

    • Answer: Popular routing libraries in React include React Router. It allows you to define routes, manage navigation, and render different components based on the URL. It offers features like nested routes, route parameters, and programmatic navigation.

13. How to handle forms in React?

    • Answer: Controlled components are commonly used to handle forms in React. In this approach, form values are stored in the component's state, and changes are updated using event handlers. Uncontrolled components use native DOM attributes to manage form values.

14. What are Higher-Order Components (HOCs)?

    • Answer: HOCs are functions that take a component as an argument and return a new enhanced component. They're used for code reuse, adding features like authentication, logging, or data fetching to multiple components without modifying their original code.

15. What are Render Props?

    • Answer: Render props are a technique for sharing code between React components using a prop whose value is a function. This function is called by the component receiving the prop, allowing it to render children based on its own state or context.

16. Explain Context API in React.

    • Answer: The Context API provides a way to pass data through the component tree without having to pass props down manually at every level. This is useful for sharing global state like user authentication or theme settings.

17. What are some popular state management libraries for React?

    • Answer: Redux, Zustand, Jotai, Recoil, and Context API are popular choices, each with its strengths and weaknesses depending on the project's complexity and requirements.

18. How do you handle errors in React applications?

    • Answer: Error boundaries catch errors during rendering, in lifecycle methods, and in constructors of the whole tree below them. They prevent the entire application from crashing and allow you to display a fallback UI.

19. Describe your experience with testing React components.

    • Answer: (This answer should be tailored to the candidate's experience. It should mention testing frameworks like Jest and React Testing Library, and describe techniques like snapshot testing, unit testing, and integration testing.)

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