Issue the following once
npm install
Next start the dev server
npm start
This will do a full build then it will start the dev server. After it is done you can open the http://localhost:3001/app in your favourite browser
Issue the following once
npm install
Start watch mode for easier development
npm run watch
Do some development then you can instantly check your changes in the browser
open the app/index.html in your browser then just refresh the page