React JS is a powerful JavaScript library for building interactive user interfaces. With React, you craft reusable components that update efficiently, giving users a fast and smooth experience. Major companies like Facebook, Instagram, and Airbnb use React due to its flexibility and performance. Learning React opens doors to modern web development, letting you create both web and mobile apps with the same knowledge base.

To become a pro in React JS, focus on mastering:

1. Components and Props: Understand functional and class components and how to pass data using props.
2. State and Lifecycle Methods: Learn how state management works and the importance of lifecycle methods in class components.
3. Hooks: Grasp newer features like useState, useEffect, and useContext, which allow functional components to handle state and side effects.
4. React Router: Implement navigation in your single-page applications.
5. Context API and Redux: Delve into advanced state management solutions.
6. Higher Order Components (HOCs) and Render Props: Enhance component functionality and reusability.
7. Virtual DOM and Reconciliation: Understand the underlying magic that makes React fast.
8. JSX: Familiarize yourself with this syntax extension, which makes writing React components more readable.
9. Performance Optimization: Techniques like lazy loading, memoization, and PureComponent.
10. Testing with tools like Jest and React Testing Library: Ensure the reliability and stability of your React applications.

By thoroughly understanding these concepts and getting hands-on experience, you’ll be well on your way to becoming a React JS expert.