TOP 70 REACT INTERVIEW QUESTIONS EVERY DEVELOPER MUST KNOW – PART 1 (1–35)

Preparing for a ReactJS interview? Whether you’re a fresher or an experienced developer, mastering these commonly asked questions can give you the edge you need. In this blog, we’ve curated the top 35 React interview questions with clear and concise answers to help you crack your next interview with confidence.

REACT INTERVIEW QUESTIONS & ANSWERS – PART 1

1. What is ReactJS?

ReactJS is an open-source JavaScript library developed by Facebook for building user interfaces, particularly for single-page applications (SPAs). It allows developers to create reusable UI components that manage their own state. React makes it easier to develop large applications that can update and render efficiently in response to data changes.

 2. Does React use HTML?

React does not directly use traditional HTML. Instead, it uses JSX (JavaScript XML), which is a syntax extension that allows HTML-like code to be written in JavaScript. JSX is later transpiled to React.createElement calls using tools like Babel.

3. When was React first released?

React was initially released in March 2013 by Facebook and was later made open-source. Since then, it has become one of the most widely used libraries for frontend development.

 4. What are two significant drawbacks of React?

1. Integration complexity: React does not follow the MVC architecture, so integrating it with other frameworks like Rails or Angular can be complicated.

2. Learning curve: React’s ecosystem includes tools like Redux, JSX, Babel, and Webpack, which may overwhelm beginners.

5. What is the difference between Real DOM and Virtual DOM?

• Real DOM manipulates the HTML document directly, which is slow and inefficient for frequent updates.

• Virtual DOM is a lightweight in-memory representation of the real DOM. React updates the virtual DOM first and compares it with the previous version (diffing), then only updates the real DOM where changes occurred — resulting in better performance.

 6. What is the Flux architecture in React?

Flux is a design pattern developed by Facebook to handle unidirectional data flow in client-side applications. It includes components like actions, dispatchers, stores, and views. Unlike traditional MVC, Flux eliminates cascading updates and makes the data flow more predictable.

 7. What is Redux in React?

Redux is a state management library inspired by Flux. It stores the entire application state in a single store and updates the state via dispatched actions. State is updated by pure functions called reducers. Redux improves maintainability and makes debugging easier with tools like Redux DevTools.

 8. What is the ‘Store’ in Redux?

The Redux store is a central place where the entire state of the application resides. It holds the current state and provides methods like dispatch() to send actions and getState() to access the state. Components can subscribe to the store to listen for updates.

 9. What is an Action in Redux?

An action is a plain JavaScript object with a type property and optionally a payload. It describes what happened but not how the state should change. Reducers handle the logic of changing the state based on the action type.

 10. What are the key features of React?

• Reusable components

• Virtual DOM for performance

• Unidirectional data flow

• Support for third-party libraries

• Strong community and Facebook support

• JSX syntax

• Tools like Hooks, Context, and lifecycle methods

11. What are stateless components in React?

Stateless components are functional components that do not manage or hold any state. They receive data and callbacks via props and render UI accordingly. They are simple, easier to test, and render faster than class components. With the introduction of Hooks, even stateless components can now use state.

12. What is React Router?

React Router is a standard library for routing in React applications. It allows developers to build single-page applications with navigation and dynamic routing. React Router keeps the UI in sync with the URL and provides components like <Route>, <Link>, and <BrowserRouter> for managing page views.

13. What are popular animation libraries in the React ecosystem?

1. React Transition Group – For simple enter/exit transitions of elements.

2. React Spring – For advanced physics-based animations.

3. Framer Motion – Powerful, easy-to-use animation library that supports gestures, layout transitions, and more.

4. GSAP – Though not React-specific, it integrates well and is widely used.

14. What is Jest?

Jest is a JavaScript testing framework developed by Facebook. It’s used to test React components and includes features like snapshot testing, mock functions, and code coverage. It’s easy to set up and works seamlessly with Babel and React projects.

15. What is a dispatcher in Flux architecture?

The dispatcher is a central hub in the Flux pattern. It receives all actions and broadcasts them to registered stores. It ensures a consistent and predictable data flow in the application.

16. What is a callback function in React?

In React, a callback function is passed as a prop from a parent component to a child component. When an event happens in the child, it invokes the callback to notify the parent. Also, setState can accept a callback as its second argument, which runs after the state update is complete.

17. What is a Higher-Order Component (HOC)?

A Higher-Order Component is a function that takes a component and returns a new component with additional functionality. It’s a pattern for code reuse in React. Example use cases include authentication checks, theming, and analytics tracking.

18. What is a Presentational Component?

A presentational component is concerned only with how things look. It receives data and functions via props and doesn’t manage state. It’s also known as a “dumb component” because it doesn’t handle any logic other than rendering.

19. What are props in React?

Props (short for properties) are the mechanism for passing data from a parent component to a child. They are read-only and help make components reusable and dynamic.

20. What is the super() keyword used for in React?

In a class component, super(props) is used inside the constructor to call the constructor of the parent class (React.Component) and access this.props. It’s required when defining a constructor in ES6 classes.

21. What is the yield keyword in JavaScript?

yield is used in generator functions to pause execution and return a value. The generator function can then resume from where it left off. It’s useful for writing asynchronous code in a synchronous style.

22. What are the two types of React components?

1. Functional Components – Basic functions that return JSX.

2. Class Components – ES6 classes that extend React.Component and support lifecycle methods and state.

23. What is a synthetic event in React?

Synthetic events are React’s wrapper around native browser events. They work identically across different browsers, providing consistent APIs like preventDefault() and stopPropagation().

24. What is React State?

State is a built-in object that holds dynamic data for a component. It determines how the component renders and behaves. When state changes, React re-renders the component to reflect the new data.

 25. How do you update state in React?

In class components, state is updated using this.setState(). In functional components, you use the useState() hook. Direct mutation of state is discouraged because it won’t trigger a re-render.

26. What is the use of arrow functions in React?

Arrow functions automatically bind this to the surrounding context, which avoids common issues in callbacks where this becomes undefined. They make event handling and functional components more concise.

27. What are the React component lifecycle steps?

1. Mounting – Component is being created and inserted into the DOM.

2. Updating – Component is being re-rendered due to changes in state or props.

3. Unmounting – Component is removed from the DOM.
   Lifecycle methods include componentDidMount, shouldComponentUpdate, and componentWillUnmount.

 28. What is the main difference between props and state?

• Props are immutable and passed by the parent.

• State is mutable and managed within the component itself.
  Props are used for configuration; state is used for internal data changes.

29. What are pure components in React?

Pure components are components that render the same output given the same props and state. They implement shouldComponentUpdate with a shallow comparison to prevent unnecessary re-renders, improving performance.

30. What controls a React component?

React components are controlled using:

• Props for receiving external data.

• State for managing internal data.
  Together, they determine how a component behaves and renders.

 31. What is create-react-app?

create-react-app is a CLI tool provided by Facebook to scaffold a new React application with a pre-configured build setup. It handles Babel, Webpack, and ESLint under the hood, allowing developers to focus on writing code.

 32. What is the use of key in React lists?

key is a special prop used to give elements a unique identity. It helps React efficiently update or re-order items in a list by identifying which items changed, were added, or were removed.

33. What is the children prop in React?

The children prop allows components to pass JSX elements or content between their opening and closing tags. It’s useful for creating wrapper or layout components that can render nested content.

34. What are Error Boundaries in React?

Error boundaries are React components that catch JavaScript errors in their child component tree and display fallback UIs instead of crashing the app. They are implemented using componentDidCatch and getDerivedStateFromError.

 35. Why are empty tags used in React?

Empty tags (or fragments) are used to group multiple elements without adding an extra node to the DOM. They are useful when returning multiple elements from a component:

 CONCLUSION

You’ve completed Part 1 of the React Interview Series! These 35 questions build a solid base in React’s structure and design patterns. Stay tuned for Part 2, where we’ll explore advanced topics like Hooks, Context API, server-side rendering, and more.

Don’t just read—try to practice these concepts by building small projects or experimenting with code. That’s the best way to learn.

Good luck with your preparation!

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