10 Must-Know React.js Interview Questions (With Answers) ??

Preparing for a React.js interview? Understanding the core concepts of React can give you an edge. Here are 10 essential interview questions with concise answers to help you succeed.

1?? What is React.js?

React.js is an open-source JavaScript library developed by Facebook for building fast and scalable user interfaces, primarily for single-page applications (SPAs). It is component-based, uses a Virtual DOM, and follows a unidirectional data flow, making it efficient and flexible.

2?? What is JSX, and why is it used?

JSX (JavaScript XML) is a syntax extension that allows developers to write HTML-like code inside JavaScript files. It makes the code more readable and improves UI development. Under the hood, JSX is transpiled into JavaScript functions, making it compatible with browsers.

3?? What is the Virtual DOM?

The Virtual DOM is a lightweight copy of the real DOM that helps React update only the necessary parts of the UI. Instead of updating the entire DOM, React compares changes and efficiently updates only modified elements, improving performance.

4?? What is the difference between Props and State?

Props are read-only values passed from parent to child components, while State is mutable and managed inside a component. Props are used for passing data, whereas State is used to handle dynamic content within a component.

5?? What are React Hooks?

Hooks (introduced in React 16.8) allow functional components to use state and lifecycle methods without needing class components. Common hooks include useState() for managing state and useEffect() for handling side effects like API calls.

6?? What is the Context API?

The Context API helps manage global state without passing props down multiple levels (prop drilling). It is commonly used for theme settings, authentication, and language preferences, making data sharing across components more efficient.

7?? What are Controlled and Uncontrolled Components?

• Controlled components have their form inputs managed via React state, ensuring controlled updates.
• Uncontrolled components store their state inside the DOM, accessed via refs, useful for third-party integrations.

8?? What is the difference between Functional and Class Components?

Functional components are simpler, lightweight, and use Hooks for managing state, while class components require more boilerplate code and use lifecycle methods like componentDidMount(). Modern React favors functional components.

9?? What is React Router, and why is it used?

React Router enables client-side navigation in SPAs, allowing users to switch between pages without full-page reloads. It supports dynamic routing, nested routes, and lazy loading, improving the user experience.

?? How can you optimize React performance?

Performance optimizations in React include:

• React.memo() to prevent unnecessary re-renders
• Lazy loading to load components only when needed
• useCallback() and useMemo() to optimize function and value calculations
• Virtualized lists for rendering only visible elements in large datasets

?? Final Thoughts

React.js is widely used in frontend development, and mastering its key concepts can help you excel in interviews. Practice building small projects, stay updated with the latest React features, and keep refining your knowledge.

?? Which React interview question do you find the trickiest? Drop a comment below! ??