Top 50 ReactJS Technical Interview Questions

What is ReactJS

ReactJS is an open-source JavaScript library developed by Facebook for building dynamic and interactive user interfaces. It is widely used for creating single-page applications (SPAs) and mobile applications through React Native. React’s component-based architecture allows developers to break down complex UIs into reusable, self-contained components, making code more modular and maintainable. Its virtual DOM (Document Object Model) ensures efficient updates and rendering, significantly improving performance compared to traditional DOM manipulation. With its declarative syntax and rich ecosystem, React has become one of the most popular front-end libraries in the web development community.

One of React’s standout features is its unidirectional data flow, which simplifies state management and debugging. Developers can manage application state using built-in hooks like useState and useEffect or external libraries like Redux and Context API. React also supports server-side rendering (SSR) through frameworks like Next.js, enhancing SEO and performance for web applications. Its extensive ecosystem includes tools like React Router for navigation, Jest for testing, and Webpack for bundling. React’s flexibility and scalability make it suitable for projects of all sizes, from small personal websites to large-scale enterprise applications.

React’s popularity is further fueled by its active community, frequent updates, and compatibility with modern JavaScript features like ES6+. Its learning curve is relatively gentle, making it accessible to beginners while offering advanced features for experienced developers. Whether you’re building a simple UI component or a complex application, React provides the tools and flexibility to create fast, responsive, and user-friendly interfaces. Its cross-platform capabilities, combined with frameworks like React Native, also enable developers to build mobile apps using the same principles, making React a versatile choice for modern web and mobile development.

Top 50 ReactJS Technical Interview Questions with Answers

1. What is ReactJS?
   A JavaScript library for building user interfaces using reusable components.
2. Virtual DOM?
   A lightweight copy of the real DOM that improves performance by minimizing direct DOM manipulation.

1. JSX?
   JavaScript XML: A syntax extension that allows writing HTML-like code in JavaScript.
2. Components in React?
   Reusable, self-contained pieces of UI. Can be functional or class-based.
3. Functional vs. Class Components?
   Functional components are simpler and use hooks; class components use lifecycle methods.
4. State?
   An object that holds data affecting a component’s rendering and behavior.
5. Props?
   Read-only properties passed from parent to child components.
6. State vs. Props?
   State is internal and mutable; props are external and immutable.
7. Hooks?
   Functions like useState and useEffect that enable state and lifecycle features in functional components.
8. useState?
   A hook for managing state in functional components: const [state, setState] = useState(initialValue);.
9. useEffect?
   A hook for side effects (e.g., API calls) in functional components. Runs after render.
10. Lifecycle Methods?
    Methods like componentDidMount, componentDidUpdate, and componentWillUnmount in class components.
11. React Router?
    A library for handling navigation and routing in React applications.
12. Conditional Rendering?
    Displaying components based on conditions: {isLoggedIn ? <Welcome /> : <Login />}.
13. Keys in Lists?
    Unique identifiers for list items to help React identify changes and optimize rendering.
14. Controlled Components?
    Form elements whose values are controlled by React state.
15. Uncontrolled Components?
    Form elements managed by the DOM itself, using ref.
16. Higher-Order Components (HOC)?
    Functions that take a component and return a new component with additional props or behavior.
17. Context API?
    A way to share data across the component tree without passing props manually.
18. Redux?
    A state management library for managing global state in large applications.
19. Actions in Redux?
    Objects that describe what happened (e.g., { type: 'ADD_TODO', payload: 'Learn React' }).
20. Reducers?
    Pure functions that specify how the state changes in response to actions.
21. Middleware in Redux?
    Functions like redux-thunk or redux-saga that handle async actions.
22. React Fragments?
    A way to group multiple elements without adding extra nodes: <></> or <React.Fragment>.
23. Error Boundaries?
    Components that catch JavaScript errors in their child component tree.
24. Portals?
    Render children outside the DOM hierarchy: ReactDOM.createPortal(child, container).
25. React.memo?
    A higher-order component for optimizing functional components by memoizing them.
26. useCallback?
    A hook for memoizing functions to prevent unnecessary re-renders.
27. useMemo?
    A hook for memoizing expensive calculations.
28. Lazy Loading?
    Loading components only when needed using React.lazy and Suspense.
29. Server-Side Rendering (SSR)?
    Rendering React components on the server to improve SEO and performance.
30. Next.js?
    A React framework for SSR, static site generation, and API routes.
31. Prop Drilling?
    Passing props through multiple layers of components. Solved using Context API or Redux.
32. React Strict Mode?
    A tool for highlighting potential problems during development.
33. React DevTools?
    Browser extension for debugging React applications.
34. Synthetic Events?
    Cross-browser wrappers around native browser events for consistency.
35. React Fiber?
    A reimplementation of React’s core algorithm for better performance and rendering.
36. Code Splitting?
    Dividing code into smaller bundles to improve load times.
37. React Testing Library?
    A library for testing React components in a user-centric way.
38. Jest?
    A JavaScript testing framework often used with React.
39. PropTypes?
    A way to validate the types of props passed to components.
40. Default Props?
    Default values for props if not provided: Component.defaultProps = { name: 'Guest' }.
41. React Hooks Rules?
    Hooks must be called at the top level, not inside loops or conditions.

1. Custom Hooks?
   Reusable functions that encapsulate logic using built-in hooks.
2. React Portals Use Case?
   Rendering modals, tooltips, or dropdowns outside the parent DOM hierarchy.
3. React Performance Optimization?
   Use React.memo, useCallback, useMemo, and code splitting.
4. React vs. Angular?
   React is a library focused on UI; Angular is a full-fledged framework.
5. React vs. Vue?
   React has a larger ecosystem; Vue is simpler and more beginner-friendly.
6. React Native?
   A framework for building mobile apps using React.
7. Key Benefits of React?
   Reusable components, virtual DOM, strong community, and rich ecosystem.

Conclusion

Embrace yourself for the interview, be confident and learn more about ReactJS. Perhaps more practical example would set you up for destined success. Good Luck!

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