All JS I write in 2024 is ES6+ (usually more like ES2022+)

I think probably if you worked through line-by-line you’d find a lot of it is valid ES5 as well (and GoJa does actually support a lot of ES6, see https://github.com/dop251/goja/milestone/1?closed=1).

Also, what features do you need to code in JavaScript to extend PocketBase functionality?

Anything you want to write in JS. After all we are comparing Trailbase to Pocketbase, both of which allow you to write application logic in JS.

They do. My point is that for the vanishingly small number of use cases where you need to write application logic in PocketBase, and you need to write it in JavaScript, and you need to use unsupported super modern ES6 features, you might prefer Trailbase, but otherwise PocketBase is more polished at this point.

IMHO, this will greatly depend on your use-cases. If you have a rich client app, e.g. mobile or PWA, most of your logic will go into your client by default. However, there might be some code you want to run on the server in a trusted environment, or use some existing JS library for mobile, or have an otherwise mostly static web app, ... depends