This section of the repo contains a site built with React.js and Semantic UI React that demonstrates how to communicate with PXT. You would mostly likely not use this if you're trying to integrate PXT into your application.
Follow these instructions to build the React site locally:
Install all the node dependencies in the /site directory.
npm install
Once you're done, simply run this command to build the main bundled js:
webpack
Follow these instructions to run PXT locally and you can test this React app by browsing to: http://localhost:3232/controller.html