I have been making my test suite emit structured data for the API tests which is used to document the API. This eliminated the margin for error in manually keeping the API documentation up to date. This improved the test coverage a lot as complete coverage is required for the documentation to be complete. It looks great too -

https://github.com/userdashboard/organizations/blob/master/a... derived from https://github.com/userdashboard/organizations/blob/master/t...

Another thing that helped was moving all my UI page tests to Puppeteer which is a NodeJS API for browsing with Chrome and tentatively Firefox web browsers. This let me automatically generate screenshots for my entire UI to publish as documentation, while simultaneously testing the responsive design under different devices which surfaced many issues.

https://userdashboard.github.io/administrators/stripe-subscr... generated by https://github.com/userdashboard/userdashboard.github.io/blo...