React is a popular JavaScript library used for building user interfaces (UIs), particularly for single-page applications (SPAs).

react choose to separate view rendering from the model representation and introduced a completely new architecture to JavaScript.
 

It helps design complex and reusable mobile and web UI components using the component-based methodology.

 

In contrast to multi-page applications, single-page applications only render the necessary assets and update the page. The bandwidth is also reduced as all assets are not required to be re-fetched.  the logic is executed in the browser rather than the server. This makes the app faster.