30 Must-Know React Interview Questions: From Fundamentals to Advanced Concepts

2025-02-08

Are you preparing for a React interview? Test yourself with these 30 must-know questions from fundamentals to advanced concepts!

Basic Questions

1. What is React, and why is it popular?
   React is a JavaScript library for building user interfaces, particularly single-page applications. It is popular due to its component-based architecture, virtual DOM, and strong community support.

2. What are the key features of React?

Key features include JSX, Virtual DOM, component-based architecture, and unidirectional data flow.

1. What is JSX? How is it different from HTML?
   JSX is a syntax extension for JavaScript that allows you to write HTML-like code in JavaScript. Unlike HTML, JSX allows embedding JavaScript expressions within curly braces {}.

2. What is the Virtual DOM, and how does it work?
   The Virtual DOM is a lightweight copy of the actual DOM. React uses it to optimize updates by comparing the Virtual DOM with the real DOM and applying only the necessary changes.

3. What are props in React? How do they work?
   Props (short for properties) are used to pass data from parent to child components. They are read-only and help in creating reusable components.

4. What is state in React? How is it different from props?
   State is used to manage data that changes over time within a component. Unlike props, state is mutable and managed within the component.

5. What is the difference between functional and class components?
   Functional components are simpler and use hooks for state and lifecycle methods, while class components use `this.state` and lifecycle methods like componentDidMount.

6. What is the role of the key prop in lists?
   The `key` prop helps React identify which items have changed, are added, or are removed, improving performance during list rendering.

9. How do you handle events in React?

Events in React are handled using camelCase syntax (e.g., onClick) and are passed as functions rather than strings.

1. What is the purpose of React Fragments (<>...</>)?
   React Fragments allow you to group multiple elements without adding extra nodes to the DOM.

Intermediate Questions

1. What is React Router, and why is it used?
   React Router is a library for routing in React applications, enabling navigation between different components without reloading the page.

12. What are controlled and uncontrolled components?

Controlled components manage form data via state, while uncontrolled components rely on the DOM to handle form data.

13. How does React handle forms?

React handles forms using controlled components, where form data is managed by the component’s state.

1. What is the `useState` hook? Give an example.
   The `useState` hook allows functional components to have state. Example:
   [javascript]
   const [count, setCount] = useState(0);
   [/javascript]

2. What is the `useEffect` hook, and how does it work?
   The `useEffect` hook allows you to perform side effects in functional components, such as fetching data or updating the DOM.

16. What are synthetic events in React?

Synthetic events are cross-browser wrappers around the browser’s native events, ensuring consistent behavior across different browsers.

1. What is the Context API? How does it work?
   The Context API provides a way to pass data through the component tree without manually passing props at every level.

2. What is prop drilling, and how do you avoid it?
   Prop drilling is the process of passing props through multiple levels of components. It can be avoided using the Context API or state management libraries.

19. What is lazy loading in React?

Lazy loading is a technique to load components only when they are needed, improving performance.

20. What are Higher-Order Components (HOCs)?

HOCs are functions that take a component and return a new component, enabling code reuse and logic sharing.

Advanced Questions

1. What is React Fiber, and how does it improve React performance?
   React Fiber is a reimplementation of React’s core algorithm, enabling better performance and more flexible rendering.

22. What is reconciliation in React?

Reconciliation is the process through which React updates the DOM by comparing the new Virtual DOM with the previous one.

1. How does memoization work in React (React.memo, useMemo, useCallback)?
   Memoization optimizes performance by caching the results of expensive calculations. `React.memo` memoizes components, `useMemo` memoizes values, and `useCallback` memoizes functions.

2. What is the difference between `useRef` and useState?
   `useRef` is used to persist values across renders without causing re-renders, while `useState` triggers re-renders when the state changes.

3. What are React Portals, and when should you use them?
   React Portals allow rendering children into a DOM node outside the parent component’s hierarchy, useful for modals or tooltips.

4. How do you handle performance optimization in React?
   Performance can be optimized using techniques like memoization, lazy loading, and avoiding unnecessary re-renders.

5. What is the difference between `useEffect` and useLayoutEffect?
   `useEffect` runs after the render is committed to the screen, while `useLayoutEffect` runs synchronously after all DOM mutations.

28. What are Server Components in React?

Server Components allow rendering components on the server, reducing the amount of JavaScript sent to the client.

1. How do you manage global state in a React app?
   Global state can be managed using Context API, Redux, or other state management libraries.

2. How does React handle hydration in SSR (Server-Side Rendering)?
   Hydration is the process of attaching event listeners to the server-rendered HTML, making it interactive.

What Undercode Say

React is a powerful library for building modern web applications, and mastering it requires a deep understanding of both fundamental and advanced concepts. Here are some Linux commands and tools that can aid in your React development journey:

1. Node.js Installation:

sudo apt update
sudo apt install nodejs

2. NPM Installation:

sudo apt install npm

3. Create a React App:

npx create-react-app my-app
cd my-app
npm start

4. Install React Router:

npm install react-router-dom

5. Install Axios for API Calls:

npm install axios

6. Run ESLint for Code Quality:

npx eslint src/

7. Build the React App for Production:

npm run build

8. Serve the Build Locally:

npm install -g serve
serve -s build

9. Install React Testing Library:

npm install @testing-library/react @testing-library/jest-dom

10. Run Tests:

npm test

React’s ecosystem is vast, and continuous learning is key. Practice by building small projects, and explore advanced topics like React Fiber, Server Components, and performance optimization. For further reading, check out the official React documentation: React Docs.

Remember, real-world experience matters more than just theory. Keep coding, keep experimenting, and you’ll ace your React interviews!

References:

Hackers Feeds, Undercode AI