Pay with Pi App is an example React app created with Create React App.
You will need a working NodeJS installation, and yarn. The Pay with Pi app frontend isn't meant to support npm.
In most cases, yarn will come along with your NodeJS installation.
Install dependencies by running yarn install.
Run the following command to start the app's development server: yarn start
This will open a browser window on http://paywithpi.com which you can close, since the Pay with Pi app mostly needs
to run inside of the Pi Sandbox environment in order to work correctly in development.
You've completed the frontend setup, return to doc/development.md to finish setting up the demo app