Comment 9 for bug 1733666

I think the solution is to add support for ES6 arrow functions. Users that are new to JS will just learn to use arrow functions in makeConnection and beginTimer because that's what we'll document in the wiki. On the other hand, seasoned JS developers will already know what a arrow function does, so they will know what's going on, because Mixxx will not bind "this" unexpectedly anymore.

How can we bring support for arrow functions?

1 - add a second QJSEngine in ControllerEngine, let's call it preprocessorEngine.
2 - bundle a browserified build of babel (https://github.com/babel/babelify) with only the arrow function plugin (https://babeljs.io/docs/plugins/transform-es2015-arrow-functions/)
3 - load babel in preprocessorEngine
4 - when a script is loaded, first pass through babel in preprocessorEngine, then evaluate the transformed script in engine.

I'd like to write tests for each JS extension we add, so I think it's not a good idea to bundle a full-blown babel build to write ES6 code. We can add specific JS extensions you fancy. I'd like to add just specific plugins for specific and justified purposes.

How can we keep compatibility for old scripts?

I think I'll add a ControllerEngine interface and then make a class for the current implementation and a class for the new implementation. Then add a engine version field in the xml file to choose which version to use (if no version is specified, use the new version). Then, specify old engine version for mappings that are not tested with the new engine when we are about to release Mixxx.
This also has the benefit that next person that needs to update the engine will have things a little bit easier.
This needs a little bit of work because there are uses of QScriptValue outside ControllerEngine.

What do you think of these ideas?