We use webpack as buildtool and the included syntro-opensource/webpack-config helper package allows for a simple config. Have a look at the documentation of said module, if you need to build an additional bundle (eg. a map).
To start a process which watches for changes and rebuilds the bundles on change, run
npm run watch
To build assets for production use, run
npm run build
To override the the default favicon, place your icon (square pngs work best)
under app/client/files/favicon.png and run npm run favicon.