Is compiler effectively a bunch of javascript running on a re-purposed browser engine?
Looks like TypeScript running on Node, yes. Is there a problem with that? A compiler is essentially a pure function from one string to another. You can write that in almost any system.
Well, it's slow. But that should be a problem for a later time.
Agreed. With first passes ay a project, better to get it up in the air first. The Wright brothers didn't start by making a 737.