Alright, let’s dive into some ReactJS questions! To give you the most helpful answers, I’ll cover a range of topics from basic to more advanced.

Basic React Questions:

1. What is ReactJS?
   • ReactJS is a JavaScript library for building user interfaces. It allows developers to create reusable UI components.¹
2. What are components in React?
   • Components are independent and reusable pieces of UI. They allow you to divide the UI into smaller, manageable parts.
3. What is JSX?
   • JSX (JavaScript XML) is a syntax extension for JavaScript that looks similar to HTML. It’s used in React to describe what the UI should look like.²
4. What are props?
   • Props (properties) are used to pass data from parent components to child components. They are read-only.
5. What is state?
   • State is a JavaScript object that represents the internal data of a component. It can be changed within the component, causing the UI to re-render.
6. What is the difference between props and state?
   • Props are read-only and passed from parent to child, while state is mutable and managed within the component.
7. What is the virtual DOM?
   • The virtual DOM is a lightweight copy of the real DOM. React uses it to efficiently update the UI by only changing the parts that have changed.
8. What are React hooks?
   • Hooks are functions that let you “hook into” React state and lifecycle features from function components.³ They allow you to use state and other React features without writing⁴ class components.

Intermediate React Questions:

9. What are some commonly used React hooks?
   • useState, useEffect, useContext, useReducer, useMemo, useCallback, and useRef.
10. What is the useEffect hook used for?
    • The useEffect hook is used for performing side effects in function components, such as data fetching, subscriptions, or manually changing the DOM.⁵
11. What is the purpose of keys in React lists?
    • Keys help React identify which items in a list have changed, added, or removed. This improves performance and prevents unexpected behavior.
12. What is context API?
    • The Context API provides a way to share values like themes, user authentication, or preferred languages between components without having to pass them explicitly through every level of the component tree.
13. What are controlled and uncontrolled components?
    • Controlled components have their form data handled by React state, while uncontrolled components have their form data handled by the DOM itself.
14. What is React Router?
    • React Router is a library that enables navigation between different views or pages in a single-page application (SPA).

Advanced React Questions:

15. What is code splitting?
    • Code splitting is a technique that allows you to break your application into smaller chunks, loading them on demand. This improves performance by reducing the initial load time.
16. What is server-side rendering (SSR) in React?
    • SSR renders React components on the server and sends the HTML to the client. This improves SEO and initial load time.
17. What are React performance optimization techniques?
    • Using useMemo and useCallback to memoize expensive calculations and functions.
    • Using React.memo to prevent unnecessary re-renders of functional components.
    • Code splitting.
    • Virtualizing long lists.
18. What is Redux?
    • Redux is a state management library that provides a predictable way to manage application state. It’s often used in large and complex React applications.
19. What is a Higher-Order Component (HOC)?
    • A higher-order component is a function that takes a component and returns a new component with enhanced functionality.⁶
20. What are React custom hooks?
    • Custom hooks are JavaScript functions that you create to reuse stateful logic. They allow you to extract component logic into reusable functions.