React.js has become the go-to library for building user interfaces, thanks to its flexibility, performance, and vibrant community. Whether you're a seasoned developer or just starting with React, it's crucial to be well-prepared for technical interviews. In this blog post, we'll explore the top 20 React.js interview questions that can help you land your dream job.

1. What is React.js?

React.js is an open-source JavaScript library for building user interfaces or UI components. It allows developers to create reusable UI components that update in real-time when data changes.

2. What are the key features of React.js?

React.js boasts features like virtual DOM, component-based architecture, one-way data binding, and JSX syntax, enabling efficient UI development.

3. Explain Virtual DOM in React.js.

Virtual DOM is a lightweight copy of the real DOM. React uses it to improve performance by reducing the number of direct manipulations to the actual DOM.

4. What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript recommended by React. It allows mixing HTML with JavaScript, enhancing readability and ease of writing components.

5. What are React Components?

React components are independent, reusable pieces of UI. They can be class components or functional components.

6. What is the difference between functional and class components?

Functional components are simpler and used for simple UI representation, while class components have additional features like state and lifecycle methods.

7. What is State in React?

State is an object that holds data which affects the component's behavior and renders when it changes. It is mutable and should be changed using the setState method.

8. What are Props in React?

Props (short for properties) are a way to pass data from parent to child components. They are read-only and help in maintaining the unidirectional flow of data.

9. Explain React Lifecycle Methods.

React components have lifecycle methods that get called at different phases of a component's life. They include componentDidMount, componentDidUpdate, and componentWillUnmount.

10. What is Redux?

Redux is a predictable state container for JavaScript applications. It helps manage the state of an entire application in a single immutable state tree.

11. What are React Hooks?

Hooks are functions that let developers "hook into" React state and lifecycle features from functional components. They include useState, useEffect, and useContext.

12. What is the significance of useEffect in React Hooks?

useEffect is a hook that performs side effects in functional components. It can mimic componentDidMount, componentDidUpdate, and componentWillUnmount lifecycle methods.

13. What is Context in React.js?

Context provides a way to pass data through the component tree without having to pass props down manually at every level. It's especially useful for passing theme data, user authentication, etc.

14. Explain Error Boundaries in React.

Error boundaries are React components that catch JavaScript errors anywhere in a component tree and log those errors, and display a fallback UI instead of crashing the component tree.

15. What is React Router?

React Router is a standard library for routing in React. It enables the navigation among views of various components in a React Application, allows changing the browser URL, and keeps UI in sync with the URL.

16. What is the significance of key prop in React lists?

The key prop is a special string attribute you need to include when creating lists of elements. It helps React identify which items have changed, are added, or are removed.

17. What is React Fiber?

React Fiber is a set of internal algorithms for rendering updates in React 16 and above. It enables React to interrupt rendering to work on other tasks and resume later, improving the user experience.

18. What is the Virtual DOM reconciliation process?

Reconciliation is the process through which React updates the real DOM to keep it in sync with the virtual DOM. It's a way of making changes to the UI without sacrificing performance.

19. How do you optimize React performance?

Performance optimization techniques in React include using production builds, minimizing state usage, utilizing shouldComponentUpdate, and lazy loading components.

20. What are the differences between React and Angular?

React is a JavaScript library focused on building user interfaces, while Angular is a complete web application framework. React uses a virtual DOM, while Angular uses a real DOM. React is maintained by Facebook, and Angular is maintained by Google.

These questions cover a wide range of topics in React.js and can serve as a valuable resource for your preparation. Understanding these concepts thoroughly will not only help you in interviews but also make you a more proficient React developer.

Best of luck with your interviews! Happy coding!