How to work GNOME's Looking Glass?

Question

I want to step into extension development for Gnome Shell, but I can't seem to understand how I do a few tasks in Looking Glass lg to actually debug my stuff.

1. When I type global.log("hi"); into LG, it throws me back r(...) = undefined. Why?
2. When I want to debug an extensions that doesn't load, how do I do that?
   It would be nice to see why an extension can't be loaded.
   I tried Main.ExtensionSystem.loadExtension() but it returns undefined no matter if I give it the extension's name string or the extension's object.
3. Where do I find documentation for classes, objects and member methods?
   (The LG inspector does not seem to be able to inspect functions.)

If I had these resources I could at least start to work on updating some extensions for newer Gnome versions. I would not ask these simple questions if they would be nicely explained somewhere but getting all the info seems really tough (I've googled for hours).

Answer 1

1: LookingGlass is basically a GJS console. It allows you to execute and test lines of GJS, grab and manipulate objects and use r(...) to use results in following commands. The reason you get r(...) = undefined is because log(String); does not return anything, thus the result is undefined.

2: Either log throughout init() and enable(), then check where it breaks and try to pinpoint the breaking line using log or commenting out lines of code (although the culprit is probably defined in the general logging, described in the section at the bottom).

3: There is no official documentation. They have some outdated tutorials for GNOME Shell 3.4 with a small difference explanation to 3.6, a few pages on the C(++) ends of the libraries you use (most of the functions, variables and constants use the same names though), but for any real info you'd need experience, digging through other GNOME and shell extension code or some unofficial sources. Unofficial sources aren't always up to date, however.

• Up to date generated docs at RooJS, although at least events are off as they are separated with - (minus) rather than _ (underscore).
• Old, but mostly useful documentation, MathematicalCoffee has more useful things, like in depth explained code that is quite a solid learning source.
• Generated docs, almost everything is there, very extensive, but sometimes the site drops offline for a few days. You could try to clone the git repo to run it locally, but the readme.md is a verbatim copy of what they forked...
• More recent reference made by a user, although this is quite extensive, it's not of much use if you're on an older version of GNOME Shell, as deprecated and old is more likely to work than next gen stuff. I recommend checking the official extensions website and go through whatever interesting extensions' source code you can get, as that is your best bet to learn anything.

Generally Debugging gnome can be done in several ways. - use the log() function and follow journalctl in a commandline, check either GNOME or JS, use journalctl -f | grep -i js (or use grep -i gnome). You will see some warnings no matter what, but most mention what extension they're from. This is nice and works on all distros implementing systemd - Or log a specific process that is a part of GNOME like journalctl /usr/bin/gnome-session -f -o cat, just beware that some distros and versions log to different files and old GNOME versions often log to whatever your display manager logs to. - use the official (albeit tricky) manner of logging: GNOME on debugging. I personally couldn't get this to work.