The Code Runner looks like it just runs a script and puts the output in a VSCode panel, which is not the same thing. VSCode is written in something that compile

The Code Runner looks like it just runs a script and puts the output in a VSCode panel, which is not the same thing. VSCode is written in something that compiles to Javascript, right? So, while you're running VSCode, there's a JS VM that's running the VSCode code. So what I'm missing is a way to be able to run your own code inside the same VM that the editor's code is running in, giving it full access to all the objects and state that the editor's code is in. You can use this to write ad-hoc extensions that disappear when you quit, or just experiment with the extension functionality. This is typically how you start and then iterate on extensions in Emacs.