ReactJS

ReactJS is a JavaScript library created by Facebook in 2013 for building user interfaces. It allows developers to create reusable UI components and manage the state of their applications in an efficient way. React uses a virtual DOM (Document Object Model) which optimizes updates and improves the overall performance of the application. 

React is commonly used for building single-page applications and is often used in combination with other libraries and frameworks such as Redux (a library that helps manage the state of an application), Next.js (a framework for building server-rendered React applications), and React Router (used to create Single Page Web Apps and utilized to define various routes). React is also popular for building complex user interfaces in web and mobile applications.

Some main properties and advantages of React are:

• Declarative
• Component-based
• Supports server-side
• Mobile support
• Simple yet powerful
• Fast, Flexible and extensible
• Community support