Can you elaborate more? Im a frontend guy and sometimes I write JS plugins for the browser (that run after page load, and apply based on how things render on the page, or bawed on user interaction with the page) and non-frontend folks tell me I need tests for my code, or dont want to look at it until I have test. How would I build a tewt for this, other than a functional test that runs in-browser and the test is whether it works or not?
Here are two strategies. First, you can go "outside" what you've built, simulating a user's interaction and examining the resulting DOM. In general this is a tool-centric approach.
Another approach is to partition your code into what Gary Bernhardt calls an "imperative shell" of code that touches the outside world and a "functional core" that does not. Then unit test the functional core, which shouldn't require special testing libraries, and validate that the imperative shell works by the occasional manual or "outside" test.
First off, if your code don't have bugs, then that is preferable! In my experience with front web dev the issue has always been different (old) devices. Testing could be done by browser macros that simulate input, like mouse movement, click on x, y, z, etc. Then take screen-shots to see if something is off.
You can also make your widgets throw errors (1) and call home via XMLHttpRequest if an error is detected.
