If you are developing a javascript only web application, then you do not need to build until you want to deploy on a production server. The build step will then optimize your javascript files and assets and provide you a much smaller app more suitable for deployment.
Building is performed using grunt. Your skeleton application provides already a suitable Gruntfile.js that you can just run by typing grunt.
The built application is placed in /build, and when the application is started in production mode it will serve it from there:
npm start --production
(Note, this only works as long as you do not manually specify a MODE environment, if so just start using: node server.js).
This minimal application example should help you with your first applications, you can find more information at the website.
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