Just run:

npm start

This will kick off a rollup watch process that will create and place a crossed.js file into the build folder. At this point you can just visit the build\index.html to run the app locally. Notably, the crossed.js simply exports a Crossed object that is placed onto a global namespace in a browser enviorment, but can still be required like a normal module in a node enviorment. This export has a main function const main = (mountId: string, initialState: any) => void that run's the app on the given node. index.html handles this for you. It also exposes all of the other functions within the application though. This is used to test the compiled module node without having to have two seperate build processes. This could also easily be extended to run unit tests within a browser enviorment to analyize any JS engine differences in execution.