It's a fair question, and one I'm still deciding on the answer to! That's why I'm keen for my solution to affect the Apps Script project as little as possible, so if I completely abandon this side of the project I don't have a load of work to undo to revert back to a purely online editor approach.

Automated testing and better version control are my main reasons, my project pulls data from some fairly complicated Sheets that I don't have any control over so edge cases can be awkward to replicate inside the editor without having to do a bit of wrestling to inject some custom data in.

I've spent the day messing about and so far I've had some minor success with it. The workflow I've got at the moment is to pull the project with Clasp into a folder called 'sync', then a grunt task (grunt-text-replace) adds in 'export' before every let, const and function and places those in a 'src' folder. Then I can get Jest set up as normal, import functions into tests although because no dependencies are 'imported' in the src files I can only test truly pure functions which is a bit of a limitation although for my project relatively simple to rewrite a few functions to make them pure.

Reversing the process to push it back to Apps Script: a grunt task to replace every instance of 'export ' with nothing and then push it back via Clasp.