I need to learn React, I need to learn Elm... I don't really know what I'm talking about, but on the internet nobody knows you're a dog.
We have some fairly sophisticated JavaScript that is part of our app, but we haven't drank any of the modern JS kool-aid yet, the only validation we're really using is a suite of feature tests written with Cucumber and Selenium/Chromedriver.
So if you have some JavaScript and it is functionally important to a feature that you've tested, it will show on the CI result if it's broken... but if there's a JavaScript error during the test that didn't stomp on the behavior tested by the feature, you'd never find out about it unless you happen to be watching the Inspector console at the time.