I suggest that, while the codebase is relatively small, we rewrite the code in ES6, separating the appropriate logic into individual files (modules) - making it cleaner and easier to debug and maintain. Plus we can take advantage of many awesome new ES6 features such as arrow functions, destructuring, default parameters, rest/spread etc.

Suggested tools:

• webpack
  • It would provide an automatic build system as well as a server, can support ES6 modules (awesome stuff) and also provides auto-reloading feature that would re-build the files and refresh the server on any change.
• babel
  • Provides ES6 to ES5 transpilation for wider browser support.

Using a build system would allow us to compile, minimize and combine all the code into a single js file that can be added into index.html. This would provide optimized execution, a single code loading request, and it would leave the actual logic in the files themselves nice & clean for the programmer to look at.