Hey all! This is not a question post, just wanted to contribute something I had been searching for quite frequently, but figured out myself.
Basically, if you want to get any website-related data on the client side, you can get it from the container object.
For eg, the code to get categories is
const container = Discourse.__container__;
const categories = container.lookup("controller:navigation/categories").site.categories;
These features aren’t documented anywhere, but you can figure it out via trial and error.
If you want to do anything more complex, you can also have a look at this post: How to reverse engineer the Discourse API