React Interview Questions and Answers are essential for anyone preparing for React job interviews. React is a widely used JavaScript library for building user interfaces, and understanding its core concepts can help you secure a role as a frontend developer.
In this guide, we have compiled a list of React Interview Questions and Answers covering fundamental and advanced topics such as JSX, Virtual DOM, state and props, React hooks, performance optimization, and state management with Redux. Whether you’re a beginner or an experienced developer, this resource will help you confidently tackle React-related interview questions and improve your understanding of best practices.
1. What are the key features of React?
Answer:
- React uses JSX for writing UI elements. Moreover, it has a Virtual DOM for efficient updates.
- Uses a component-based architecture.
- Follows a unidirectional data flow for predictable state management.
- Provides hooks for managing state and lifecycle in functional components.
Additionally, React supports server-side rendering (SSR) with frameworks like Next.js, improving performance and SEO.
2. What is the difference between functional and class components?
Answer:
- Functional components are simple JavaScript functions. In addition, they use hooks for state and side effects.
- Class components use lifecycle methods and this.state for state management.
- Functional components are preferred due to better performance and readability.
They also result in cleaner, more maintainable code and allow for easier testing compared to class components.
3. What is JSX and why is it used in React?
Answer:
- JSX is a syntax extension of JavaScript that allows writing HTML-like code inside JavaScript.
- It improves code readability and makes it easier to visualize UI structure.
JSX is compiled into React.createElement() calls, which return React elements for rendering.
4. What is the Virtual DOM and how does it work?
Answer:
- The Virtual DOM is a lightweight copy of the real DOM. As a result, React updates the Virtual DOM first.
- This process improves performance and minimizes unnecessary re-renders.
React’s reconciliation algorithm efficiently compares the new Virtual DOM with the old one and updates only the parts that have changed, resulting in faster rendering.
5. What are props in React?
Answer:
- Props (short for properties) are read-only data passed from a parent component to a child component.
- They allow components to be dynamic and reusable.
Props can include numbers, strings, arrays, functions, or even other React components.
6. What is state in React?
Answer:
- State is a mutable object that holds component-specific data.
- When the state changes, the component re-renders with the new data.
State can be managed locally using hooks like useState, or globally using tools like Context API or Redux.
7. What is the difference between state and props?
Answer:
- Props are immutable and used to pass data from a parent to a child component.
- State is mutable and managed within the component itself.
Props make components flexible, while state makes them interactive and dynamic.
8. What is React Router?
Answer:
- React Router is a navigation library that enables client-side routing in React applications.
- It allows switching between views without refreshing the page.
It provides components like <BrowserRouter>, <Route>, <Link>, and <Switch> to manage routes efficiently.
9. What is the difference between controlled and uncontrolled components?
Answer:
- Controlled components manage form data using React state (useState).
- Uncontrolled components store form data in the actual DOM (using ref).
Controlled components are preferred when you need real-time validation or instant UI updates.
10. What are React hooks?
Answer:
- Hooks allow functional components to use state and lifecycle features.
- Common hooks include useState, useEffect, useContext, useMemo, and useCallback.
Custom hooks can also be created to encapsulate reusable logic across components.
11. What is useEffect and how does it work?
Answer:
- useEffect is a hook that manages side effects in functional components, such as data fetching, event listeners, or subscriptions.
- It can run on mount, update, or unmount depending on dependencies.
If the dependency array is empty, useEffect runs only once after the initial render.
12. What is useMemo and when should you use it?
Answer:
- useMemo optimizes performance by memoizing expensive computations.
- It prevents unnecessary recalculations on every render.
It’s especially useful when dealing with large datasets or complex calculations.
13. What is useCallback and how is it different from useMemo?
Answer:
- useCallback memoizes a function so that it does not get re-created on every render.
- useMemo memoizes a computed value rather than a function.
Both hooks improve performance in components with frequent re-renders or child components depending on stable references.
14. What is React Context API?
Answer:
- The Context API is a state management solution that allows passing global data without prop drilling.
- It is useful for themes, authentication, and global settings.
It consists of React.createContext(), a Provider, and a Consumer (or useContext hook).
15. What is Redux and why is it used?
Answer:
- Redux is a state management library used for managing application-wide state.
- It follows a unidirectional data flow and consists of actions, reducers, and a store.
Redux Toolkit simplifies setup and reduces boilerplate code for Redux applications.
16. What is React.memo and how does it help performance?
Answer:
- React.memo is a higher-order component that prevents unnecessary re-renders by memoizing a component.
- It only re-renders if the component’s props change.
It is similar to PureComponent but used with functional components.
17. What are Higher-Order Components (HOCs)?
Answer:
- HOCs are functions that wrap components to add additional functionality.
- They enable code reuse, such as authentication handling and logging.
An example is withRouter() from React Router, which injects routing props into components.
18. What is lazy loading in React?
Answer:
- Lazy loading is a performance optimization technique that defers loading components until they are needed.
- It reduces the initial load time of an application.
React provides React.lazy() and <Suspense> for implementing lazy loading.
19. What is reconciliation in React?
Answer:
- Reconciliation is React’s diffing algorithm that updates only the changed parts of the real DOM efficiently.
- It compares the new Virtual DOM with the previous one and updates only necessary elements.
This process helps React maintain high performance even in large-scale applications.
20. What is Concurrent Mode in React?
Answer:
- Concurrent Mode is an experimental feature that improves rendering performance by allowing React to work on multiple UI updates at the same time.
- It enhances responsiveness and user experience.
It is part of React’s future architecture (React Fiber) and enables features like Suspense for data fetching.
Learn more about React and explore advanced concepts on our websitte DesignWithRehana platform while also checking out in-depth video tutorials on our YouTube channel.
Bonus Tip: React developers should also be familiar with modern tools like Next.js, TypeScript, and testing libraries such as Jest and React Testing Library for building production-ready applications.