Initial BangleJS Development questions

Hi! Sorry to hear about the customs issues - hopefully this will all go pretty much back to normal come July (and the new EU VAT rules).

Is there a way to add a custom "standard library"

Yes - it's worth checking out https://www.espruino.com/Modules#from-github-or-anywhere-on-the-internet-

You can have the module online, or can just write the module direct to the Bangle's Storage.

is there more info like this such as best-practices for UI and UX design for the watch?

Not specifically but if you haven't already it's worth checking out the example app/widget tutorials: https://www.espruino.com/Bangle.js#tutorials

There are also some hints about how to write reasonably efficient code: https://www.espruino.com/Bangle.js+Guidelines

Is there any concept of scrolling on the device?

Yes - you can scroll vertically with g.scroll(0, ...) but that's the whole screen - so you can't scroll if you have widgets. You can always write your data to an offscreen buffer, scroll that, and then write it to the screen.

Are there any details on what version of ECMAScript is the closest?

https://www.espruino.com/Features

Has anyone set up any toolchains to allow compiling from higher-level code?

If you do a quick search there are a few node.js modules to transpile. BangleRun for instance is written in TypeScript: https://github.com/espruino/BangleApps/tree/master/apps/banglerun

However it does make debugging more difficult (eg trying to track back stack traces) and may result in a bunch of polyfill code being added. Personally I'd start off just trying to write the JS that Espruino supports and see where that takes you before you try transpiling.