Title:Interview Questions on React JS – Top 50+ Most Asked in 2025
Description:Interview Questions on React JS: Prepare with the top 50+ react js interview questions for freshers in 2025. Get simple answers, coding examples, and expert tips for javascript interview questions for freshers in 2025.
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Interview Questions on React JS – Top 50+ Most Asked in 2025
Interview Questions on React JS are crucial for freshers and entry-level developers preparing for technical interviews in 2025. This guide compiles the most asked react js interview questions and answers, including coding examples and practical explanations. Use this resource to ace your next React JS interview and boost your confidence.
Top 50 Interview Questions on React JS (2025)
1. What is React JS and why is it used?
React JS is an open-source JavaScript library focused on building user interfaces, especially for single-page applications. It enables developers to create reusable UI components, manage state efficiently, and update the DOM quickly using its virtual DOM system.
2. What are the key features of React JS?
Key features include:
Virtual DOM
Component-based architecture
One-way data binding
JSX syntax
Performance optimization
Strong community support
3. Explain the virtual DOM in React.
The virtual DOM is a lightweight JavaScript object representing the real DOM. React creates a virtual DOM tree, compares it with the previous version (diffing), and updates only the changed parts in the actual DOM, making updates fast and efficient.
4. What are React components?
Components are the building blocks of React applications. They are reusable, independent pieces of UI, defined as JavaScript functions or classes that return JSX.
5. Difference between functional and class components.
Functional components are simple JavaScript functions that accept props and return JSX. Class components are ES6 classes that extend React.Component and can hold state and lifecycle methods.
6. What are React Hooks?
Hooks are functions that let you use React state and lifecycle features in functional components. Common hooks include useState, useEffect, and useContext.
7. Explain useState and useEffect hooks.
useStatemanages state in functional components.useEffecthandles side effects like data fetching, subscriptions, or manual DOM updates.
8. What is JSX?
JSX (JavaScript XML) is a syntax extension that allows writing HTML-like code within JavaScript. It makes code more readable and easier to write React elements.
9. How does React handle events?
React uses camelCase syntax for event handlers (e.g., onClick). Event handlers are passed as functions and receive a synthetic event object, ensuring consistency across browsers.
10. What is state and props in React?
State: Local data maintained by a component, can change over time.
Props: Read-only data passed from parent to child components.
11. Explain component lifecycle methods.
Lifecycle methods are special functions in class components that run at specific points (mounting, updating, unmounting). Examples: componentDidMount, componentDidUpdate, componentWillUnmount.
12. What is the purpose of keys in React lists?
Keys help React identify which items have changed, are added, or are removed. They optimize rendering by giving each element a unique identity.
13. How do you optimize React performance?
Use React.memo and PureComponent
Lazy load components with React.lazy
Use keys in lists
Avoid unnecessary re-renders
14. What is React Router?
React Router is a library for handling navigation in React apps. It enables single-page application routing using components like <Route>, <Link>, and <Switch>.
15. Explain context API in React.
The Context API allows passing data through the component tree without manually passing props at every level. Useful for global data like themes and authentication.
16. How do you handle forms in React?
Use controlled components where form data is managed by React state. Use onChange handlers to update state and submit handlers to process data.
17. What is Redux and why is it used?
Redux is a state management library for JavaScript apps. It centralizes application state, making it easier to manage and debug complex apps.
18. What is the difference between state and props?
State is managed within the component and can change. Props are passed from parent to child and are read-only.
19. How do you pass data between components?
Parent to child: via props
Child to parent: via callback functions
Sibling to sibling: via shared parent state or context
20. What is prop drilling and how do you avoid it?
Prop drilling is passing props through many layers of components. Avoid it using the Context API or state management libraries like Redux.
21. What is useCallback and useMemo?
useCallback: Memoizes a callback function.useMemo: Memoizes a computed value.
Both optimize performance by preventing unnecessary recalculations.
22. How do you fetch data in React?
Use useEffect for data fetching in functional components, often with fetch or axios.
23. What is server-side rendering (SSR) in React?
SSR renders React components on the server, sending HTML to the client. Improves SEO and initial load performance.
24. What is React Fragment?
<React.Fragment> lets you group multiple elements without adding extra nodes to the DOM.
25. How do you handle errors in React?
Use error boundaries (class components with componentDidCatch) to catch and handle errors in the component tree.
26. What is React Portal?
Portals render children into a DOM node outside the parent component hierarchy, useful for modals and tooltips.
27. How do you update state based on previous state?
Use a function in setState or useState updater:
setCount(prevCount => prevCount + 1);
28. What is the difference between controlled and uncontrolled components?
Controlled components are managed by React state, while uncontrolled components manage their own state via the DOM.
29. What is reconciliation in React?
Reconciliation is the process React uses to update the DOM by comparing the new virtual DOM with the previous one and applying the minimal set of changes.
30. What is the use of shouldComponentUpdate?
It lets you prevent unnecessary re-renders by returning false when an update isn’t needed.
31. What is HOC (Higher-Order Component)?
A HOC is a function that takes a component and returns a new component, often used for code reuse and logic abstraction.
32. What is the difference between useEffect and componentDidMount?
useEffect can replicate componentDidMount, componentDidUpdate, and componentWillUnmount behaviors in functional components.
33. How do you optimize re-rendering in React?
Use React.memo, PureComponent, useCallback, and useMemo
Avoid changing references unless necessary
34. What is lazy loading in React?
Lazy loading delays loading components until needed, improving app performance. Use React.lazy and Suspense.
35. What is the difference between React and Angular?
React is a library focused on UI, uses virtual DOM, and is component-based. Angular is a full-fledged framework with two-way data binding and built-in solutions for routing and state.
36. What is the use of keys in React lists?
Keys provide a unique identity to list items, helping React optimize rendering and updates.
37. What is useRef and when would you use it?
useRef returns a mutable ref object. Use it to access DOM elements or persist values across renders without causing re-renders.
38. What is a pure component?
A pure component only re-renders if its props or state change, optimizing performance.
39. How does React handle conditional rendering?
Use JavaScript conditional statements or ternary operators inside JSX to render components based on conditions.
40. What is the difference between mount, update, and unmount phases?
Mount: Component is created and inserted into the DOM.
Update: Component is re-rendered due to state/props change.
Unmount: Component is removed from the DOM.
41. What is the use of defaultProps?
defaultProps provide default values for props if none are supplied.
42. How do you test React components?
Use testing libraries like Jest and React Testing Library for unit and integration testing.
43. What is the use of PropTypes?
PropTypes validate the types of props passed to a component, helping catch bugs.
44. What is the Context API?
Context API provides a way to share values between components without passing props manually at every level.
45. What is the difference between useEffect and useLayoutEffect?
useLayoutEffect fires synchronously after all DOM mutations, while useEffect runs asynchronously after painting.
46. What is React StrictMode?
<React.StrictMode> helps identify potential problems in an application by activating additional checks and warnings.
47. What is the use of React DevTools?
React DevTools is a browser extension for inspecting React component hierarchies and state.
48. What is the difference between stateful and stateless components?
Stateful components manage state; stateless components do not.
49. What is the use of React.Children?
React.Children provides utilities for dealing with the children prop, such as mapping, counting, and validating.
50. How do you deploy a React application?
Build the app using npm run build and deploy the static files to platforms like Netlify, Vercel, or traditional servers.
React JS Interview Preparation Guide for Freshers
Master the basics of React JS including components, state management, hooks, and routing. Practice coding challenges and build small projects to strengthen your understanding. Stay updated with the latest React features and best practices. For more, explore:
Top 50 react js Interview Questions and Answers for Freshers
Most Asked react js Interview Questions in 2025 (With Simple Answers)
React js Coding Interview Questions and Answers for Beginners
Top react js Interview Questions You Must Prepare
React js Interview Questions for Freshers, Interns, and Entry-Level Jobs
Top react js Questions for College Placements and Freshers
Crack Your First React JS Interview
FAQs
What are the best resources to prepare for React JS interviews?
A: Use Letcode , w3schools for practice and coding challenges.
What topics should freshers focus on for React JS interviews?
A: Focus on React fundamentals, hooks, state and props, component lifecycle, and routing