The client-side is built using React, but instead of going with the traditional JavaScript ES6, we are using TypeScript as the language of choice.
Visit http://bit.ly/react-typescript to learn how TypeScript makes React components nicer. Also, visit https://github.com/piotrwitek/react-redux-typescript-guide to learn about how to make the most out of TypeScript + React.
This gives you the flexibility to use some of the concepts that you may already be familiar with if you come from a server-side background. It also provides static type checking, which makes development more efficient and less error-prone.