[nohuhu]

Ext JS is the framework in question. The last time I worked on it (Oct 2018), it was about that size shared between two major versions: Classic toolkit and Modern toolkit, including tests and examples. It could be more than that now but I haven't touched it since.

I used to do a lot of things on it, including _very_ deep refactorings with sweeping changes across the codebase. The secret sauce is focus on automated testing: when I left the company, we had ~70,000 test specs for the framework, and the test suite was executed on each commit to every PR pushed to Github. Depending on framework version (there were several in flight), and browser matrix (15+ supported browsers), each test run yielded 500,000-700,000 spec results (and finished in under 20 minutes).

There was a lot of custom tooling around this, of course. Including a test runner that I got open sourced right before leaving the company: https://github.com/sencha/jazzman. Sadly it looks like there were no new commits since I left; I hoped to pick it up later but so far I haven't encountered the need to run significantly sized test suites in massively parallel fashion. Nobody writes that many tests. :)

EDIT: I almost forgot that as a condition for open-sourcing the test runner they made me write a blog post about it: https://www.sencha.com/blog/ext-js-testing-story-under-the-h... :)